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Copyright © 1994-2007
Reproduction of this document in whole or in part is permitted if both of the
following conditions are satisfied:
1.This notice is included in its entirety at the beginning.
We will not be responsible for damage to equipment, your ego, blown parts,
county wide power outages, spontaneously generated mini (or larger) black
holes, planetary disruptions, or personal injury that may result from the use
of this material.
Note: This document replaces the chapters relating to these topics in the
documents "Notes on the Troubleshooting and Repair of Small Household
Appliances and Power Tools" and "....Audio Equipment and Other Miscellaneous
Stuff".
Where another document is referenced, it is assumed to be at this site.
If the link doesn't work, find the document of the same name at the
Sci.Electronics.Repair FAQ or one
of its mirror sites.
Any internal overcurrent fuses or thermal fuses represent essential safety
features of an AC adapter. These must not be removed except during
testing. Where a fuse is found to be blown, use only an exact replacement.
I really don't recommend running a repaired cobbled together AC adapter
unattended in any case since even the sealed case provides some additional
amount of fire protection. Inexpensive replacements are generally available.
For power supplies inside equipment, the same basic precautions apply but
access and repair are generally much more easily accomplished.
The only real danger from an unplugged heavy iron transformer would be
accidentally dropping it on your foot. :(
The most common problems are due to failure of the output cable due to flexing
at either the adapter or output plug end. See section:
AC Adapter Testing.
If the output tested inside the adapter (assuming that you can get it
open without total destruction - it is secured with screws and is
not glued or you are skilled with a hacksaw - measures 0 or very low with no
load but plugged into a live outlet, either the transformer has failed or
the internal fuse had blown. In either case, it is probably easier to
just buy a new adapter but sometimes these can be repaired. Occasionally,
it will be as simple as a bad connection inside the adapter. Check the
fine wires connected to the AC plug as well as the output connections.
There may be a thermal fuse buried under the outer layers of the
transformer which may have blown. These can be replaced but locating
one may prove quite a challenge. Also see the section:
Comments on Importance of Thermal Fuses and
Protectors.
As above, you may find bad connections or a blown fuse or thermal fuse
inside the adapter but the most common problems are with the cable.
Again, cable problems predominate but failures of the switching power
supply components are also possible. If the output is dead and you have
eliminated the cable as a possible problem or the output is cycling on
and off at approximately a 1 second rate, then some part of the switching
power supply may be bad. In the first case, it could be a blown fuse,
bad startup resistor, shorted/open semiconductors, bad controller,
or other components. If the output is cycling, it could be a shorted
diode or capacitor, or a bad controller. See the document:
Notes on the Troubleshooting and Repair of Small
Switchmode Power Supplies for more info, especially on safety while
servicing these units.
Also see the chapter on "Equipment Power Supplies" in the document:
Notes on the Troubleshooting and Repair of Audio
Equipment and Other Miscellaneous Stuff.
There is no standard for rating AC adapters. When a particular adapter is
listed as, say, 12 V, 1 A max, there's a good chance the output will average
12 V when outputting 1 A but what it does at lower currents is not known.
In fact, lightly loaded, the output voltage may be more than double its
nameplate rating! This could be disastrous where a piece of equipment is
plugged into it that doesn't expect such a high voltage. The rating also
doesn't say anything about the ripple (for DC models) - it could be almost
anything.
The lifetime of an AC adapter (particularly those outputting DC) when run
at or near its nameplate rating may be very short. Why? Because they often
use low temperature (cheap!) components that can't take the heat. For AC
output models, the transformer itself may fail (or at least the thermal fuse).
For DC models, the electrolytic capacitor(s) may go bad very quickly. The
likely result will be that the output voltage will disappear entirely (AC
models) or drop in value with greatly increased ripple (DC models).
Where the adapter is used with its intended equipment, one can presume the
manufacturer did the proper testing to assure compatibility and adequate life
(though this isn't always the case!). However, where it is used in some other
application, the life of the adapter and the equipment may be much shorter
than expected, possibly failing almost immediately.
For example, I own 2 U.S. Robotics modems. One uses a 9 VAC adapter; the
other uses a 20 VAC adapter. The power jacks are identical and totally
unmarked. Guess what happens if I guess wrong? With too little voltage,
the modem may appear to work but be unreliable. With too much voltage, the
smoke will very likely be released instantly. :(
To save yourself a lot of hassle and possible damaged equipment, put a label
on each AC adapter powered device you own with the voltage, current, AC or
DC (with polarity), and model number of the adapter (make one up if nothing
is obvious and put it on the device and adapter). Then, if you misplace the
adapter, you'll know what to look for and if it is nowhere to be found, will
have enough information to purchase a replacement.
Line isolation is essential for safety with respect to electrical shock - no
part accessible to the user must be connected to either side of the power line.
A regular transformer provides this automatically. While combinations of
passive components can reduce the risk of shock, nothing quite matches the
virtually fail-safe nature of a simple transformer between the power line and
the low voltage circuitry. To achieve similar isolation without a line
transformer generally requires a switchmode power supply which actually
contains a small high frequency transformer to provide the isolation. Until
recently, such systems were much more expensive than a simple iron transformer
but that is changing and many modern devices do now use a wall adapter based on
this approach. These can be recognized by their light weight, DC (probably
regulated) output, and the required warnings NOT to cut them off and replace
them with an ordinary plug! I wonder how many people have ignored the
warnings when their equipment stopped working and replaced that fat "plug"?
What a scenario for disaster!
WARNING: DON'T attempt to disassemble or repair one of these unless you are
familiar with the safety and troubleshooting information for larger switchmode
power supplies - they can be quite deadly. See the document:
Notes on the Troubleshooting and Repair of Small
Switchmode Power Supplies.
(From: Mike Schuster (schuster@panix.com).)
For some reason I've been fascinated by tiny wall wart AC adaptors that use
switch mode power supplies, since they're light and can supply more
current than similar linear power cubes.
One type that keeps catching my eye is used a lot for "AC travel charger"
accessories for cellular phones. These things connect via a cable to the
bottom of a cell phone, much like the cigarette-lighter "charger/saver"
accessories, only these are driven by house current.
The typical wart is a small rectangular box, about the size of two 9V
transistor batteries side by side, manufactured in China or Taiwan. The
wall side is distinguished by the fact that the AC prongs line up with the
long axis of the box, rather than the other way around as with most wall
cubes. This makes it possible to put them side by side on an AC power
strip. The opposite face contains a tri-mode LED which may display red,
green, or orange under conditions I've yet to figure out.
Recently I noticed one of these thingies in K-Mart as part of a modular
power system for cell phones. There are several models of cigarette
lighter cords, however the actual 12VDC car plug in _interchangeable_ and
connected to the cable using a 4-pin modular telephone handset jack. Each
model comes with a cable constructed to mate with the phone it's sold for.
Next to these on the pegboard is a variant of the wall wart being
discussed, also having a 4-pin handset socket, and sold as an accessory to
the DC cords. Instead of using the cigarette lighter plug, you connect the
cable to the wall wart and create a new device which uses house current.
So I picked up the wall wart and started to play.
It's marked as being capable of 5-15 VDC at 750 mA. Playing with the 4
output pins; one is ground, two are tied together and supply 14.35 VDC
open circuit, and can deliver about 1.5 amps. The other reads about 13
volts between it and the ground. Unpowered there is a small leakage between
the ground and the "13 volt" pin.
Looking inside, there are two 8-pin DIPs on the PC board; both having
identifiers sanded off. One is near the transformer end and the other is
near the DC output end. All of the DC side output traces lead, directly or
indirectly, to the second IC.
My guess is that the "13 volt" pin is really used to program the output
voltage between ground and the other two pins that are tied together. The
cable sold for any specific phone has some passive components inside that
will cause the second IC to produce the required output voltage. Am I
warm?
I'd like to try programming this myself ... any ideas? Resistors?
If DC, the polarity must be the same. Even if the connectors are identical,
it's a coin toss as to whether the center is positive or negative.
The most common problem is one or both conductors breaking internally
at one of the ends due to continuous bending and stretching.
Make sure the outlet is live - check with a lamp.
Make sure any voltage selector switch is set to the correct position.
Move it back and forth a couple of times to make sure the contacts are clean.
If the voltage readings check out for now, then wiggle the cord as above in
any case to make sure the internal wiring is intact - it may be intermittent.
Although it is possible for the adapter to fail in peculiar ways, a
satisfactory voltage test should indicate that the adapter is functioning
correctly.
It's also possible that the power jack on the device itself is damaged from
use or abuse. If possible, confirm proper operation with a COMPATIBLE
adapter. With battery operated devices, there is usually a set of
contacts that should close when the adapter is removed to connect the internal
battery to the circuitry. If these don't operate properly, the device may not
work off batteries (they may appear to not be charged), the AC adapter, or
both. Check the jack for obvious signs of damage (cracked, loose, etc.). A
squirt of contact cleaner into the jack may clear up intermittent contact
problems not due to actual damage.
The most common problem (and the only one we will deal with here) is the
case of a broken wire internal to the cable at either the wall wart or
device end due to excessive flexing of the cable.
Usually, the point of the break is just at the end of the rubber cable
guard. If you flex the cable, you will probably see that it bends more easily
here than elsewhere due to the broken inner conductor. If you are reasonably
dextrous, you can cut the cable at this point, strip the wires back far
enough to get to the good copper, and solder the ends together. Insulate
completely with several layers of electrical tape. Make sure you do not
interchange the two wires for DC output adapters! (They are usually marked
somehow either with a stripe on the insulator, a thread inside with one
of the conductors, or copper and silver colored conductors. Before you
cut, make a note of the proper hookup just to be sure. Verify polarity
after the repair with a voltmeter.
The same procedure can be followed if the break is at the device plug end
but you may be able to buy a replacement plug which has solder or screw
terminals rather than attempting to salvage the old one.
Once the repair is complete, test for correct voltage and polarity before
connecting the powered equipment.
This repair may not be pretty, but it will work fine, is safe, and will
last a long time if done carefully.
If the adapter can be opened - it is assembled with screws rather than
being glued together - then you can run the good part of the cable inside
and solder directly to the internal terminals. Again, verify the polarity
before you plug in your expensive equipment.
WARNING: If this is a switching power supply type of adapter, there are
dangerous voltages present inside in addition to the actual line connections.
Do not touch any parts of the internal circuitry when plugged in and make
sure the large filter capacitor is discharged (test with a voltmeter)
before touching or doing any work on the circuit board. For more info on
switching power supply repair, refer to the document:
Notes on the Troubleshooting and Repair of Small
Switchmode Power Supplies.
If it is a normal adapter, then the only danger when open are direct
connections to the AC plug. Stay clear when it is plugged in.
A variety of types of protection are often incorporated into adapter
powered equipment. Sometimes these actually will save the day.
Unfortunately, designers cannot anticipate all the creative techniques
people use to prove they really do not have a clue of what they are doing.
The worst seems to be where an attempt is made to operate portable devices
off of an automotive electrical system. Fireworks are often the result,
see below and the section on: "Automotive power".
If you tried an incorrect adapter and the device now does not work there
are several possibilities (assuming the adapter survived and this is not
the problem):
I inherited a Sony Discman from a guy who thought he would save a few bucks and
make an adapter cord to use it in his car. Not only was the 12-15 volts
from the car battery too high but he got it backwards! Blew the DC-DC
converter transistor in two despite the built in reverse voltage protection
and fried the microcontroller. Needless to say, the player was a loss but the
cigarette lighter fuse was happy as a clam!
Moral: those voltage, current, and polarity ratings marked on portable
equipment are there for a reason. Voltage rating should not be exceeded,
though using a slightly lower voltage adapter will probably cause no harm
though performance may suffer. The current rating of the adapter should
be at least equal to the printed rating. The polarity, of course, must be
correct. If connected backwards with a current limited adapter, there may be
no immediate damage depending on the design of the protective circuits. But
don't take chances - double check that the polarities match - with a voltmeter
if necessary - before you plug it in! Note that even some identically marked
adapters put out widely different open circuit voltages. If the unloaded
voltage reading is more than 25-30% higher than the marked value, I would
be cautious about using the adapter without confirmation that it is acceptable
for your equipment. Needless to say, if you experience any strange or
unexpected behavior with a new adapter, if any part gets unusually warm, or if
there is any unusual odor, unplug it immediately and attempt to identify the
cause of the problem.
Or, a more dramatic result of the same principles:
(From: Don Parker (tazman@yournet.com).)
A guy brought a Johnson Messenger CB to my shop a few decades back. He had
been told it would run on 12 VDC *and* 115 VAC - so he tried it! I never saw
so many little leads sticking up from any PCB since - that once were capacitors
and top hat transistors. There was enough fluff from the caps to have the
chassis rated at least R-10 :->).
The little fellow made a stinky smell, so I assume that at least one
component is cooked."
The problem is that an auto battery has a very high current capacity and
any fuses respond too slowly to be of much value in a situation such as
this. Any capacitors and solid state components on the 12 V bus at the
time power was applied are likely fried - well done.
Well, based on that last statement ;-)
I would find and check any fuses, or components directly
in-line with or parallel to the power lines (the latter
might include the IC's unfortunately...)
If you are simply replacing a broken adapter with a universal
type, check the label on the old one - they almost always provide this
information. There are three issues: AC versus DC, the voltage, and polarity.
Unfortunately, fully determining these requirements experimentally can be
non-trivial. While many devices have built in protection for reverse polarity
(which would probably also include putting AC into a device requiring DC),
others do not and may be damaged or may at least blow an internal fuse. Few
devices protect against extreme overvoltage.
If you have a multimeter, there are also some tests you can perform without
opening the device but they are not foolproof. Here are some general
guidelines. The more of these you can confirm, the greater the confidence
of avoiding disaster.
Anything else will probably require you do (1) or (2). And, except for
manufacturer supplied information, even these are no guarantee of anything!
Once AC versus DC and polarity (if relevant) are determined, start low on
voltage to see at what point the device behaves normally. Depending on
design, this may be quite low compared to the recommended input voltage or
very near it - no way to really know. Devices with motors and solenoids
may appear to operate at relatively low voltage but fail to do the proper
mechanical things reliably if at all. RF devices capable of transmitting
may behave similarly when asked to transmit. Devices with more constant
power requirements may operate happily at these reduced voltages. However,
depending on the type of power supplies they use, running at a low voltage
may also be stressful (e.g., where DC-DC converters are involved).
NOTE: Some devices with microcontrollers and/or logic will require a fast
power turn-on so it may be necessary to switch off and then on for each
input voltage you try for proper reset.
Again, determining the requirements from the manufacturer is best!
The only cautions are that if one of them is unpowered for any reason (it falls
out of the AC outlet!) or the current rating of one of the adapters is
exceeded, then current may be forced through the other one in
the wrong direction possibly damaging its electrolytic capacitors or other
components. To prevent this possibility, place a rectifier like a 1N4002
(this is 1 A, use a larger one if your adapters are really huge) in REVERSE
across each output. This will bypass current safely around the internal
circuitry.
The idea of using multiple adapters can be extended to even more outputs but
this is left as an exercise for the student.
However, obtaining an AC adapter with the proper ratings for long term use
would be a good idea.
There are two cases:
WARNING: If one of the adapters is not plugged in, high voltage (possibly
even more than the normal line voltage) may appear on its exposed prongs
due to the AC from the other adapters present on its output (being stepped
up going the wrong way through the transformer). The voltage and available
current may be enough to be dangerous in some cases.
CAUTION: For the difference case, if one of the units isn't powered, you
may get a HIGHER voltage than expected at the output of the series
combination which may let the smoke out of your equipment. :(
The type we are considering in this discussion are plug-in wall adapter that
output a DC voltage (not AC transformers). This would be stated on the
nameplate.
The first major consideration is voltage. This needs to be matched to the
needs of the equipment. However, what you provide may also need to be well
regulated for several reasons as the manufacturer may have saved on the cost
of the circuitry by assuming the use of batteries:
Thus, the typical universal adapter found at Radio Shack and others may not
work satisfactorily. No-load voltage can be much higher than the voltage at
full load - which in itself may be greater than the marked voltage. Adding
an external regulator to a somewhat higher voltage wall adapter is best. See
the section: Adding an IC Regulator to a Wall Adapter or
Battery.
The other major consideration is current. The rating of the was adapter must
be at least equal to the *maximum* current - mA or A - drawn by the device
in any mode which lasts more than a fraction of a second. The best way to
determine this is to measure it using fresh batteries and checking all modes.
Add a safety factor of 10 to 25 percent to your maximum reading and use this
when selecting an adapter.
For shock and fire safety, any wall adapter you use should be isolated and
have UL approval.
To wire it in, you can obtain a socket like those used on appliances with
external adapter inputs - from something that is lying in your junk-box
or a distributor like MCM Electronics. Use one with an automatic disconnect
(3 terminals) if possible. Then, you can retain the optional use of the
battery. Cut the wire to the battery for the side that will be the outer
ring of the adapter plug and wire it in series with the disconnect (make
sure the disconnected terminal goes to the battery and the other terminal
goes to the equipment). The common (center) terminal goes to other side of
the battery, adapter, and equipment as shown in the example below. In this
wiring diagram, it is assumed that the ring is + and the center is -. Your
adapter could be wired either way. Don't get it backwards!
WARNING: if you do not use an automatic disconnect socket, remove the battery
holder or otherwise disable it - accidentally using the wall adapter with
the batteries installed could result in leakage or even an explosion!
A possibly simpler alternative is to fashion a 'module' the size and shape
of the battery or battery pack with screw contacts at the same locations and
connect your external power supply to it. For example, a couple of pieces
of wooden dowel rod about 2-1/4" long taped together with wood screws in the
appropriate ends would substitute for a pair of side-by-side AA batteries.
Then, you don't need to modify the Walkman or whatever at all (or at most
just file a slot for the wire to exit the battery door).
To convert such an adapter to DC requires the use of:
Depending on your needs, you may find a suitable wall adapter in your junk
box (maybe from that 2400 baud modem that was all the rage a couple of years
ago!).
The basic circuit is shown below:
Therefore, you will need to find an AC wall adapter that produces an output
voltage which will result in something close to what you need. However,
this may be a bit more difficult than it sounds since the nameplate rating
of many wall adapters is not an accurate indication of what they actually
produce especially when lightly loaded. Measuring the output is best.
Adding an IC regulator to either of these would permit an output of up to
about 2.5 V less than the filtered DC voltage.
The following is a very basic introduction to the construction of a circuit
with appropriate modifications will work for outputs in the range of about
1.25 to 35 V and currents up 1 A. This can also be used as the basis for a
small general purpose power supply for use with electronics experiments.
For an arbitrary voltage between about 1.2 and 35 V what you want is an IC
called an 'adjustable voltage regulator'. LM317 is one example - Radio Shack
should have it along with a schematic. The LM317 looks like a power
transistor but is a complete regulator on a chip.
Where the output needs to be a common value like +5 V or -12 V, ICs called
'fixed voltage regulators' are available which are preprogrammed for these.
Typical ICs have designations of 78xx (positive output) and 79xx (negative
output).
For example:
Here is a sample circuit using the LM317:
For the LM317:
However, note that a typical adapter's voltage may vary quite a bit
depending on manufacturer and load. You will have to select one that
isn't too much greater than what you really want since this will add
unnecessary wasted power in the device and additional heat dissipation.
Using 10,000 uF per *amp* of output current will result in less than 1 V
p-p ripple on the input to the regulator. As long as the input is always
greater than your desired output voltage plus 2.5 V, the regulator will
totally remove this ripple resulting in a constant DC output independent
of line voltage and load current fluctuations. (For you purists, the
regulator isn't quite perfect but is good enough for most applications.)
Make sure you select a capacitor with a voltage rating at least 25% greater
than the adapter's *unloaded* peak output voltage and observe the polarity!
Note: wall adapters designed as battery chargers may not have any filter
capacitors so this will definitely be needed with this type. Quick check:
If the voltage on the adapter's output drops to zero as soon as it is pulled
from the wall - even with no load - it does not have a filter capacitor.
If your equipment uses an AC adapter (wall wart), see the sections on those
devices.
The power supplies built in to consumer electronic equipment are usually
one of three types or a hybrid combination of these (There are no doubt
others):
First, make sure the outlet is live - try a lamp. Even a neon circuit
tester is not a 100% guarantee - the outlet may have a high resistance
marginal connection.
Check for blown fuses near the line cord input. With the unit unplugged,
test for continuity from the AC plug to the fuse, on/off switch and power
transformer. With the power switch in the 'on' position, testing across
the AC plug should result in a resistance of 1 to 100 ohms depending on
the size of the equipment:
If the fuse blew and the readings are too low, the transformer primary may
be partially or totally shorted. If the resistance is infinite even directly
across the primary of the power transformer, it may be open or there may be
an open thermal fuse underneath the outer layer of insulation wrapping.
Also see the section: Comments on Importance of Thermal
Fuses and Protectors.
If the fuse blew but resistance is reasonable, try a new fuse of the proper
ratings. If this blows instantly, there is still a fault in the power supply
or one of its loads. See the section: About Fuses, IC
Protectors, and Circuit Breakers.
If these check out, then the problem is likely on the secondary side.
One or more outputs may be low or missing due to bad regulator components.
A secondary winding could be open though is is less common than primary side
failure as the wire (in transistorized equipment at least) is much thicker.
Depending on the type of equipment, there may be a single output of several
outputs from the power supply. A failure of one of these can result in
multiple systems problems depending on what parts of the equipment use what
supply.
Check for bad fuses in the secondary circuits - test with an ohmmeter. (I
once even found an intermittent fuse!) Try a new fuse of the same ratings.
If this one blows immediately, there is a fault in the power supply or one of
its loads. See the section: About Fuses, IC Protectors,
and Circuit Breakers. The use of a series current limiting resistor - a
low wattage light bulb, for example - may be useful to allow you to make
measurements without undo risk of damage and an unlimited supply of fuses.
Locate the large electrolytic filter capacitor(s). These will probably be
near the power transformer connections to the circuit board with the power
supply components. Test for voltage across each of these with power on. If
they are in pairs, this may be a dual polarity supply (+/-, very common
in audio equipment). Sometimes, two or more capacitors are simply used
to provide a higher uF rating. If you find no voltage on one of these
capacitors, trace back to determine if the problem is a rectifier diode,
bad connection, or bad secondary winding on the power transformer (the
latter is somewhat uncommon as the wire is relatively thick, however).
Dried up electrolytic capacitors will result in excessive ripple leading
to hum or reduced headroom in audio outputs and possible regulation
problems as well. Test with a scope or multimeter on its AC scale (but
not all multimeters have DC blocking capacitors on its AC input and these
readings may be confused by the DC level). If ripple is excessive - as a
guideline if it is more than 10 to 20% of the DC level - then substitute
or jumper across with a good capacitor of similar uF rating and at least
the same voltage rating.
If you find voltages that are lower than expected, this could be due to bad
filter capacitors, an open diode or connection (one side of a full wave
rectifier circuit), or excessive load which may be either in the regulator(s),
if any, or driven circuitry.
Disconnect the output of the power supply from its load. If the voltage
jumps up dramatically (or the fuse now survives or the series light bulb
now goes out or glows dimly), then a short or excess load is likely.
If the behavior does not change substantially, the problem may be in the
regulator(s). Transistors, zener diodes, resistors, and other discrete
components, and IC regulators like LM317s or 7809s can be tested with an
ohmmeter or by substitution. The most common failures are shorts for
semiconductors, opens for resistors, and no or low output for ICs.
Where the supply uses a hybrid regulator like an STK5481, confirming
proper input and then testing each output is usually sufficient to
identify a failure. A defective hybrid regulator will likely provide
no or very low output on one or more outputs. Confirm by disconnecting
the load. Test with any on/off (logic) control in both states.
Note: inexpensive UPSs and inverters generate a squarewave output so don't
be surprised at how ugly the waveform appears if you look at it on a scope.
This is probably normal. More sophisticated and expensive units may use
a modified sinewave - actually a 3 or 5 level discrete approximation to a
sinewave (instead of a 2 level squarewave). The highest quality units
will generate a true sinewave using high frequency bipolar pulse width
modulation. Don't expect to find this in a $100 K-Mart special, however.
A UPS incorporates a battery charger, lead-acid (usually) storage battery,
DC-AC inverter, and control and bypass circuitry.
Note that if finding a UPS that provides surge protection is an important
consideration, look for one that runs the output off of the battery at all
times rather than bypassing the inverter during normal operation. The battery
will act as a nearly perfect filter in so far as short term line voltage
variations, spikes, and noise, are concerned.
A DC-AC power inverter used to run line powered equipment from an automotive
battery or other low voltage source is similar to the internal inverter in
a UPS.
For a unit that appears dead (and the power has not been off for more than
its rated holdup time and the outlet is live), first, check for a blown
fuse - external or internal. Perhaps, someone was attempting to run their
microwave oven off of the UPS or inverter!
(See the section on: "Fuse post mortems" to identify likely failure mode.)
If you find one - and it is blown due to a short circuit - then there are
likely internal problems like shorted components. However, if it is blown
due to a modest overload, the powered equipment may simply be of too high
a wattage for the UPS or inverter - or it may be defective.
Failures of a UPS can be due to:
Here is some additional information on the basics and troubleshooting of
uninterruptable power supplies. Note that the following is from someone
with a 230 VAC perspective so some aspects of the line circuitry may not
apply to U.S.A. models.
(From: Lex Cunningham (lextron@iinet.net.au).)
UPSs come in all shapes and sizes, from 300 VA units for PCs to 3 phase
units rated into the hundreds of kVA for use in industrial applications. The
most common type readers of the repair newsgroup will come across will be
single phase units with ratings up to 3 kVA. These mainly see use in
domestic and commercial applications to provide protection for a single PC,
a small network of PCs, or a server.
Types of UPS:
There are basically two types of UPS. With the standby type, the input
voltage is switched to the output under normal conditions and the control
electronics monitor the incoming line. Should the incoming line fail
altogether, fall below a set voltage, or rise above a set voltage, the
inverter is powered up to support the load. When the line returns to normal,
the inverter is turned off and the line is switched back to support the
load. The marketing people use some odd terms for the standby type such as
Line Interactive. What this means is that the UPS has Automatic Voltage
Regulation of the output voltage. This is achieved by using boost and buck
windings on the transformer. These windings are switched as necessary to
maintain a relatively constant output voltage, even though the input voltage
may be high or low.
The on-line type of UPS uses a dual conversion technique to deliver power to
the load. The incoming line is converted to DC and then converted back to
AC. The inverter runs continuously, hence the term on-line. These have the
advantage of removing all of the line noise and no changeover delay. When
the incoming line fails, a DC to DC converter is powered up to provide the
DC rail required for the DC to AC inverter.
Power Conversion Techniques:
The standby UPS has two sub classes to describe the method of converting the
battery supply to AC. The cheapest method is called quasi sine wave which
uses pulse width modulation at the line frequency (50/60 Hz) to maintain the
inverter output voltage. The output is basically a square wave of variable
duty-cycle depending on the load. This square wave is applied to a transformer
to obtain the required output voltage. To a switch mode power supply, the wave
shape is not all that important.
The more expensive method is known as true sine wave. Pulse width modulation
at 15 to 20 kHz is used to reconstruct a sine wave. This type of UPS is used
in situations where square waves would cause overheating of electric motors.
These are more complex than quasi sine wave units, and hence come at a
higher price.
The on-line UPS uses high frequency PWM techniques and provides a sine wave
output. The difference from the standby sine wave type is that the modern
on-line inverter works directly at the line output voltage, rather than
through a step up transformer.
Safety Considerations:
It can not be stressed enough that all UPS's are potential death traps. The
line voltage in any country is lethal. Any voltage higher than around 50 V
is considered hazardous.
DC Safety:
UPS's using a single 12 V battery can cause injury. The batteries used can
supply large currents if short circuited. It is easy to lose a finger if a
ring shorts the battery supply. The risk is that the ring becomes hot,
causing cauterisation of the blood vessels. With no blood supply, the only
solution is amputation.
The other risks of shorted batteries is the potential for the battery case
to split open releasing electrolyte, and flying molten metal.
The DC supply of units in the 2 to 3 kVA range from 48 to 96 V. The 96 V
units float the battery bank at 110 V, which will electrocute.
On-line units work at high voltages. In Australia, the nominal line voltage
is 240 VAC which when rectified, results in power rails of plus and minus
350 VDC. That is a total of some 700 V.
AC Safety:
When working on UPS's, the use of an earth leakage circuit breaker/residual
current detector, or whatever they are known as in your part of the world,
has to be considered mandatory.
Other Safety:
Some manufacturers, for cost reasons, do not use line monitoring
transformers. Instead, high value resistors are used to divide the line
voltage to a value suitable for the control electronics. This results in a
machine that is technically totally live. Keep a watch out for these ones
and test before touching.
Be aware that the ground clip of an oscilloscope probe is earth. Depending
on wiring rules, earth may be bonded to neutral at the main switch/fuse
panel. This is true in Australia. Use an oscilloscope with differential
inputs designed for the job. A suitable alternative is an add on
differential input unit in conjunction with a standard scope. The design
presented in Elektor Electronics around 1994 works very well and with a
bandwidth of 15 MHz, nothing will be missed.
Battery Changing Procedures:
There are a couple of types of UPS that have a strict battery change
procedure. Failure to follow correct procedure results in the destruction of
the UPS. Both types come from Lantech of Taiwan. This is not to say that
only Lantech machines behave in this way. The ALi range from the mid 90's
require the battery to be disconnected, and the reservoir capacitors on the
main board discharged using a 220 ohm, 5 W resistor. Before reconnecting the
battery, the unit must be plugged into the line to precharge the capacitors.
Failure to discharge or precharge the capacitors causes destruction of the
inverter output devices. This procedure is noted on a sticker inside the
unit.
The current AI-UPS range requires reservoir capacitor precharging using a 1K
ohm, 1 W resistor between the battery and the main board, however it is good
practice to do the discharge part as well. This procedure is only noted on
19 inch rack mount units. There is no warning on stand alone machines.
Common UPS Faults
The most common fault ever seen is sulphated batteries. The constant
charging causes the electrolyte to dry out and the lead plates to become
lead sulphate. This results in a battery that has a greatly reduced capacity
resulting in the UPS shutting down on line failure. To determine a poor
battery, check for swelling of the battery case, or do a load test while
monitoring the battery voltage. If the battery voltage falls rapidly to
below 12 volts, then it is faulty. Also, if the battery charges rapidly from
below 12 volts to 13.8 volts, it is faulty. Always replace all batteries in
a bank, or the new one(s) will fail quickly.
Most UPS's will not start unless there is sufficient input voltage. Assuming
the fuse is not blown or circuit breaker is good, check the input cutout
relay. The coils have been known to burn out.
Even with an input voltage, the UPS will not start if the battery is flat.
This can be caused by battery age, a faulty charger, or running the UPS
until low voltage shutdown. If possible program the UPS to shut down at 30%
remaining capacity as this increases the life of the battery and ensures
there is sufficient power to restart when the line is restored.
The Best Power (Sola in Oz) range of machines will go into fault mode at
power up if an internal fault is detected. Two of the causes are flat
batteries or the output relay not switching over. The 510 range has a
problem with the output relay not switching over. This is caused by an
electrolytic in the battery charger circuit failing. This is associated with
the auxiliary winding on the transformer. The capacitor fails leaving the
relay with insufficient voltage to switch, the microprocessor detects no
output voltage and enters fault mode. Replacing the capacitor with a bipolar
type capable of handling the high frequency ripple currents solves the
problem.
Some UPS's will not work from their front panel controls as they have been
programmed into a certain mode to suit shutdown software. This requires the
use of a programming utility from the manufacturer to reprogram the unit.
Check the line monitoring transformer windings. In units without
transformers, check the RF chokes and resistors. This particularly applies
to Lantech units.
All faults can be traced using standard fault finding techniques. If the
fault simply can not be found, take the unit to your friendly UPS service
tech for an opinion. You will be charged for the work, but then the
technician likes to eat as well!
Fuses use a fine wire or strip (called the element) made from a metal which
has enough resistance (more than for copper usually) to be heated by current
flow and which melts at a relatively low well defined temperature. When the
rated current is exceeded, this element heats up enough to melt (or vaporize).
How quickly this happens depends on the extent of the overload and the type
of fuse.
Fuses found in consumer electronic equipment are usually cartridge type
consisting of a glass (or sometimes ceramic) body and metal end caps. The
most common sizes are 1-1/4" mm x 1/4" or 20 mm x 5 mm. Some of these have
wire leads to the end caps and are directly soldered to the circuit board but
most snap into a fuse holder or fuse clips. Miniature types include: Pico(tm)
fuses that look like green 1/4 W resistors or other miniature cylindrical or
square varieties, little clear plastic buttons, etc. Typical circuit board
markings are F or PR.
IC protectors are just miniature fuses specifically designed to have a
very rapid response to prevent damage to sensitive solid state components
including intergrated circuits and transistors. These usually are often
in TO92 plastic cases but with only 2 leads or little rectangular cases
about .1" W x .3" L x .2" H. Test just like a fuse. These may be designated
ICP, PR, or F.
Circuit breakers may be thermal, magnetic, or a combination of the two.
Small (push button) circuit breakers for electronic equipment are most
often thermal - metal heats up due to current flow and breaks the circuit
when its temperature exceeds a set value. The mechanism is often the
bending action of a bimetal strip or disc - similar to the operation of
a thermostat. Flip type circuit breakers are normally magnetic. An electro-
magnet pulls on a lever held from tripping by a calibrated spring. These
are not usually common in consumer equipment (but are used at the electrical
service panel).
At just over the rated current, it may take minutes to break the circuit.
At 10 times rated current, the fuse may blow or circuit breaker may open
in milliseconds.
The response time of a 'normal' or 'rapid action' fuse or circuit breaker
depends on the instantaneous value of the overcurrent.
A 'slow blow' or 'delayed action' fuse or circuit breaker allows instantaneous
overload (such as normal motor starting) but will interrupt the circuit
quickly for significant extended overloads or short circuits. A large thermal
mass delays the temperature rise so that momentary overloads are ignored. The
magnetic type breaker adds a viscous damping fluid to slow down the movement
of the tripping mechanism.
A fuse which has an element that looks intact but tests open may have just
become tired with age. Even if the fuse does not blow, continuous cycling
at currents approaching its rating or instantaneous overloads results in
repeated heating and cooling of the fuse element. It is quite common for
the fuse to eventually fail when no actual fault is present.
A fuse where the element is broken in a single or multiple locations blew
due to an overload. The current was probably more than twice the fuse's
rating but not a dead short.
A fuse with a blackened or silvered discoloration on the glass where the
entire element is likely vaporized blew due to a short circuit.
This information can be of use in directly further troubleshooting.
Even with circuit breakers, a short circuit may so damage the contacts or
totally melt the device that replacement will be needed.
Four parameters characterizes a fuse or circuit breaker:
However, as long as the other specifications are met, soldering a normal
1-1/4" (3AG) fuse across a 20 mm fuse is perfectly fine, for example.
Sometimes a fuse will have wire leads and be soldered directly onto the
circuit board. However, your own wires can be carefully soldered to the
much more common cartridge type to create a suitable replacement.
For testing, it is perfectly acceptable to temporarily short out the device
to see if the equipment then operates normally without overheating. However,
while these fuses do sometimes just fail on their own, most likely, there was
another cause. If you know what it was - you were trying to charge a shorted
battery pack, using your window fan to mix cement, or something was shorted
externally, then the fuse served its protective function and the equipment is
fine. IT SHOULD BE REPLACED WITH THE SAME TYPE or the entire transformer,
motor, or whatever it was in should be replaced! This is especially critical
for unattended devices. Otherwise, especially with unattended devices, you
have a situation where if the overload occurred again or something else
failed, the equipment could overheat to the point of causing a fire - and your
insurance company may refuse to cover the claim if they find that a change was
made to the circuit. And even for portable devices like blow dryers and
portable power tools, aside from personal safety should the device
malfunction, the thermal protector is there to prevent damage to the
equipment itself - don't leave it out!
Transformers are used in nearly every type of electronic equipment both for
power and signals, and throughout the electrical distribution network to
optimize the voltage/current used on each leg of the journey from the power
plant to the user.
The types we are interested in with respect to household appliances, power
tools, and consumer electronic equipment are most often use to convert the AC
line voltage to some other value, lower or higher:
First, identify all connections that have continuity between them. Except for
the possible case of a water soaked transformer with excessive leakage, any
reading less than infinity on the meter is an indication of a connection. The
typical values will be between something very close to 0 ohms and 100 ohms.
Each group of connected terminals represents one winding. The highest reading
for each group will be between the ends of the winding; others will be lower.
With a few measurements and some logical thinking, you will be able to
label the arrangement ends and taps of each winding.
Once you do this, applying a low voltage AC input (from another power
transformer driven by a Variac) will enable you to determine voltage ratios.
Then, you may be able to make some educated guesses as to the primary and
secondary. Often, primary and secondary windings will exit from opposite
sides of the transformer.
For typical power transformers, there will be two primary wires but
international power transformers may have multiple taps as well as a
pair or primary windings (possibly with multiple taps) for switching
between 110/115/120 VAC and 220/230/240 VAC operation. Typical color
codes for the primary winding(s) will be black or black with various
color stripes. Almost any colors can be used for secondary windings.
Stripes may indicate center tap connections but not always.
Note: for safety, use the Variac and another isolated transformer for this.
Here is a more specific example:
Here is a suggestion:
There will be two primary windings (resistance between the two will be
infinite). Each of these may also have additional taps to accommodate
various slight variations in input voltage. For example, there may
be taps for 110/220, 115/230, 120/240, etc.
For the U.S. (110 VAC), the two primary windings will be wired in parallel.
For overseas (220 VAC) operation, they will be wired in series. When
switching from one to the other make sure you get the phases of the two
windings correct - otherwise you will have a short circuit! You can test
for this when you apply power - leave one end of one winding disconnected
and measure between these two points - there should be close to zero voltage
present if the phase is correct. If the voltage is significant, reverse
one of the windings and then confirm.
A multimeter on the lowest resistance scale should permit you to determine
the internal arrangement of any taps on the primaries and which sets of
secondary terminals are connected to each winding. This will probably
need to be a DMM as many VOMs do not have low enough resistance ranges.
It is best to test with a Variac so you can bring up the voltage gradually
and catch your mistakes before anything smokes.
You can then power it from a low voltage AC source, say 10 VAC from your
Variac or even an AC wall adapter, to be safe and make your secondary
measurements. Then scale all these voltage readings appropriately.
Where multiple output windings are involved, this is more difficult since
the safe currents from each are unknown.
(From: Greg Szekeres (szekeres@pitt.edu).)
Generally, the VA rating of individual secondary taps can be measured. While
measuring the no load voltage, start to load the winding until the voltage
drops 10%, stop measure the voltage and measure or compute the current. 10%
would be a very safe value. A cheap transformer may compute the VA rating with
a 20% drop. 15% is considered good. You will have to play around with it to
make sure everything is OK with no overheating, etc.
(From: James Meyer (jimbob@acpub.duke.edu).)
With the open circuit voltage of the individual windings, and their DC
resistance, you can make a very reasonable assumption as to the relative
amounts of power available at each winding.
Set up something like a spread-sheet model and adjust the output current to
make the losses equal in each secondary. The major factor in any winding's
safe power capability is wire size since the volts per turn and therefore the
winding's length is fixed for any particular output voltage.
Since the primary is open, the transformer is totally lifeless.
First, confirm that the transformer is indeed beyond redemption. Some have
thermal or normal fuses under the outer layer of insulating tape or paper.
The transformer may now blow the equipment fuse and even if it does not,
probably overheats very quickly.
First, make sure that it isn't a problem in the equipment being powered.
Disconnect all outputs of the transformer and confirm that it still has
nearly the same symptoms.
Remove the case and frame (if any) and separate and discard the (iron) core.
The insulating tape or paper can then be pealed off revealing each of the
windings. The secondaries will be the outer ones. The primary will be the
last - closest to the center. As you unwind the wires, count the number of
full turns around the form or bobbin.
By counting turns, you will know the precise (open circuit) voltages of each
of the outputs. Even if the primary is a melted charred mass, enough of the
wire will likely be intact to permit a fairly accurate count. Don't worry,
an error of a few turns between friends won't matter.
Measuring the wire size will help to determine the relative amount of
current each of the outputs was able to supply. The overall ratings of
the transformer are probably more reliably found from the wattage listed
on the equipment nameplate.
Where an open thermal fuse is not the problem, aside from bad solder or crimp
connections where the wire leads or terminals connect to the transformer
windings, anything else will require unwrapping one or more of the windings to
locate an open or short. Where a total melt-down has occurred and the result
is a charred hunk of copper and iron, even more drastic measures would be
required.
In principle, it would be possible to totally rebuild a faulty transformer.
All that is needed is to determine the number of turns, direction, layer
distribution and order for each winding. Suitable magnet (sometimes called
motor wire) is readily available.
However, unless you really know what you are doing and obtain the proper
insulating material and varnish, long term reliability and safety are unknown.
Therefore, I would definitely recommend obtaining a proper commercial
replacement if at all possible.
See the section: Rewinding Power Transformers.
However, DIY transformer construction is nothing new:
(From: Robert Blum (rfblum@worldnet.att.net).)
I have a book from the Government Printing Office . The title is: "Information
for the Amateur Designer of Transformers for 25 to 60 cycle circuits" by
Herbert B. Brooks. It was issued June 14, 1935 so I do not know if it is still
in print. At the time I got it it cost $.10.
(From: Mark Zenier (mzenier@netcom.com).)
"Practical Transformer Design Handbook" by Eric Lowdon. Trouble is, last I
checked it's out of print. Published by both Sams and Tab Professional Books.
(From: Paul Giancaterino (PAULYGS@prodigy.net).)
I found a decent article on the subject in Radio Electronics,
May 1983. The article explains the basics, including how to figure what
amps your transformer can handle and how to size the wiring.
(From: colin@rowec.screaming.net.)
DISCLAIMER: There is a safety aspect of mains transformers. Use this
information entirely at your own risk.
I have wound and re wound several transformers. When I was first into
Electronics (at about 12), I rewound a line output transformer of a colour
TV. I reused the wire but I had to re insulate it by suspending it all around
the garage and painting it with a special paint I had found. I would never do
this again or suggest anyone else do it like this either! but it outlasted the
tube.
Since then as an electronics engineer I have wound many SMPS transformers and
rewound some working mains transformers to get different voltages.
If you do wind a transformer yourself you need a lot of patience and to be
able to keep count of the number of turns (not as easy as it sounds) and
strong fingers.
However, the mains transformers that I have come across that have blown up
have been beyond repair. This is because the plastic former or bobbin usually
melts with the heat that is generated by the fault current that flows when the
insulation on the windings gives up. I would not attempt to try and wind a
small mains transformer without the coil former as it would be too difficult
to SAFELY keep the windings insulated from each other and get the required
amount of wire to fit.
If the windings are severely shorted it would seem as though your transformer
has suffered this fate. You would definitely have to replace all the windings.
There is of course the problem of finding out what voltage/current the
windings were in the first place.
If the machine is only used at one input voltage you may be able to get away
with one primary winding (where there were two before - a slight
simplification but the wire will need to be slightly thicker - lower by 3 AWG
numbers).
Apart from obtaining a direct replacement the best bet would be to find a
transformer that has outputs that are the right voltages and sufficient
current. This may be tricky and it may not fit inside the case. there are many
places that sell of the shelf transformers. maybe you would need two
transformers to get the right combination of voltages.
If you are very luck you might get just what you want from a junk shop. or
from a piece of junk equipment.
However if you are determined to try to wind a transformer there are several
possibilities.
The most critical aspect of winding a mains transformer is the primary winding
as the wire used is incredibly fine on small transformers and is easily
damaged or even broken and good insulation is of the utmost importance. Also
there is a heck of a lot wire and it becomes impracticable unless you are
prepared to set up some sort of rig. I would suggest that you consider the
alternatives to winding this yourself which are :-
You may well be able to use one or more of the existing windings but you must
bear in mind that each winding takes up an amount of space proportional to
its current X voltage.
To work out the number of turns and size of wire in the windings you need to
know the turns per volt of the new transformer. this can be found by counting
one of the secondary windings and dividing by its rated voltage. The number of
turns you need is this number times the voltage you want. The size of wire is
determined by the current rating. use the wire with the same area per amp as
the existing winding. The ends of the windings must be terminated
properly. Use enameled copper wire. the enamel might need to be scraped of to
enable soldering unless it is the self fluxing type and you have a very hot
soldering iron. usually there are tags to solder the ends to.
Also, if it is in something like a tape recorder it most probably needs
shielding.
It is up to you to ensure that the finished transformer is safe. The best way
to test the insulation is to test with a high voltage (a few kV) between
primary and secondary and then between the core and each winding and check
there is no leakage current. with mains applied check that there is correct
voltages at the outputs. check that the transformer does not get too hot. All
transformers get hot, some too hot to touch, but if after several hours its so
hot that you skin sticks to it when you touch it it wont last very long !!
There are various places to get the EC wire and junk transformers, a search on
the internet would be a good place to start.
(From: Bill Rothanburg (william.rothanburg@worldnet.att.net).)
I've done this. It was more of an intellectual challenge, rather than
something practical, but it can be done. Some requirements:
I had a relatively easy transformer to work with - single primary, dual
secondaries. The windings had not been saturated with varnish, so I was able
to unwind them COUNTING THE TURNS. Did I mention that this required a great
deal of patience? I was able to determine the wire gauge from the old
windings.
The transformer had overheated to the point the plastic bobbin was garbage. I
was able to fabricate a replacement using fish paper and lots of varnish.
To assist in rewinding I built a "tool" to help - Actually a crank through a
piece of wood. The bobbin was held in place by a couple of nuts and spacers.
The actual rewinding was the easiest part of the process.
If I were to try this again, I would definitely use a thermal protector in the
transformer.
Two of the hottest areas in engineering these days are in developing
higher capacity battery technologies (electrochemical systems) for
rechargeable equipment and in the implementation of smart power management
(optimal charging and high efficiency power conversion) for portable devices.
Lithium and Nickel Metal Hydride are among the more recent additions to
the inventory of popular battery technologies. A variety of ICs are now
available to implement rapid charging techniques while preserving battery
life. Low cost DC-DC converter designs are capable of generating whatever
voltages are required by the equipment at over 90% efficiency
However, most of the devices you are likely to encounter still use pretty
basic battery technologies - most commonly throwaway Alkaline and Lithium
followed by rechargeable Nickel Cadmium or Lead-Acid. The charging circuits
are often very simple and don't really do the best job but it is adequate
for many applications.
For more detailed information on all aspects of battery technology, see
the articles at:
Many major battery manufacturers have extensive technical information on
their Web sites, though not all of it may be unbiased.
There is more on batteries than you ever dreamed of ever needing. The
sections below represent just a brief introduction.
Four types of batteries are typically used in consumer electronic equipment:
In most cases, trickle charging at a slow rate - C/100 to C/20 - is easier on
batteries. Where this is convenient, you will likely see better performance
and longer life. Such an approach should be less expensive in the long run
even if it means having extra cells or packs on hand to pop in when the others
are being charged. Fast charging is hard on batteries - it generates heat and
gasses and the chemical reactions may be less uniform.
Each type of battery requires a different type of charging technique.
Rapid chargers for portable tools, laptop computers, and camcorders, do at
least sense the temperature rise which is one indication of having reached
full charge but this is far from totally reliable and some damage is
probably unavoidable as some cells reach full charge before others due
to slight unavoidable differences in capacity. Better charging techniques
depend on sensing the slight voltage drop that occurs when full charge
is reached but even this can be deceptive. The best power management
techniques use a combination of sensing and precise control of charge
to each cell, knowledge about the battery's characteristics, and state
of charge.
While slow charging is better for NiCds, long term trickle charging is
generally not recommended.
Problems with simple NiCd battery chargers are usually pretty easy to
find - bad transformer, rectifiers, capacitors, possibly a regulator.
Where temperature sensing is used, the sensor in the battery pack may
be defective and there may be problems in the control circuits as well.
However, more sophisticated power management systems controlled by
microprocessors or custom ICs and may be impossible to troubleshoot for
anything beyond obviously bad parts or bad connections.
A simple charger for a lead-acid battery is simply a stepped down rectified
AC source with some resistance to provide current limiting. The current
will naturally taper off as the battery voltage approaches the peaks
of the charging waveform. This is how inexpensive automotive battery
chargers are constructed. For small sealed lead-acid batteries, an IC
regulator may be used to provide current limited constant voltage charging.
A 1 A (max) charger for a 12 V battery may use an LM317, 3 resistors,
and two capacitors, running off of a 15 V or greater input supply.
Trickle chargers for lead-acid batteries are usually constant voltage and
current tapers off as the battery reaches full charge. Therefore, leaving
the battery under constant charge is acceptable and will maintain it at the
desired state of full charge.
Problems with lead-acid battery chargers are usually pretty easy to
diagnose due to the simplicity of most designs.
(From: Dave Martindale (davem@cs.ubc.ca).)
The simple way is to build a power supply that outputs 13.8 volts regulated,
with a current limit of 0.5 A. 13.8 V can be left connected to the
battery forever without damage - this is called a float charge. The
0.5 A current limit protects the battery from drawing too much current
and overheating if it's been deeply discharged. This sort of charger
will get the battery back up to 80% charge within a few hours, so it's
fine for most uses.
However, when designing it, make sure the charger doesn't self-destruct
if the input voltage goes away (due to AC power failure) while still
connected to the battery. With a standard series regulator, when the
input power fails the whole battery voltage gets applied to the base-
emitter junction of the output transistor in reverse. Many transistors
are only specified to withstand about 6 V reverse base-emitter voltage,
so with this design your charger will be toast at the first power failure.
If you want higher-performance charging, there are special charge
controller chips that provide 3 or more charge phases. They are:
On the other hand, NiCd batteries can safely be charged in less than an
hour with suitable electronics. Lead-acid simply can't be recharged
that fast.
For many toys and games, portable phones, tape players and CD players, and
boomboxes, TVs, palmtop computers, and other battery gobbling gadgets, it
may be possible to substitute rechargeable batteries for disposable primary
batteries. However, NiCds have a lower terminal voltage - 1.2V vs. 1.5V - and
some devices will just not be happy. In particular, tape players may not
work well due to this reduced voltage not being able to power the motor
at a constant correct speed. Manufacturers may specifically warn
against their use. Flashlights will not be as bright unless the light
bulb is also replaced with a lower voltage type. Other equipment may
perform poorly or fail to operate entirely on NiCds. When in doubt, check
your instruction manual. And, there is a slight, but non-zero chance that
some equipment may actually be damaged. This might occur if its design
assumed something about the internal resistance of the batteris; the resistance
is much lower for NiCds than Alkalines.
Furthermore, even a SuperCap cannot begin to compare to a small NiCd for
capacity. A 5.5 V 1 F (that's Farad) capacitor holds about 15 W-s of energy
which is roughly equivalent to a 5 V battery of 3 A-s capacity - less than
1 mA-h. A very tiny NiCd pack is 100 mA-h or two orders of magnitude larger.
When laying eggs, start with a chicken. Actually, you have to estimate
the capacity so that charge and discharge rates can be approximated.
However, this is usually easy to do with a factor of 2 either way just be
size:
Then discharge at approximately a C/20 - C/10 rate until the cell voltages
drops to about 1 V (don't discharge until flat or damage may occur). Capacity
is calculated as average current x elapsed time since the current for a NiCd
will be fairly constant until very near the end.
(The next section is from: Bob Myers (myers@fc.hp.com) and are based on a
GE technical note on NiCd batteries.)
The following are the most common causes of application problems wrongly
attributed to 'memory':
To close with a quote from the GE note:
This information should dispel many of the myths that exaggerate the idea of
a 'memory' phenomenon."
(Portions of the following guidelines are
from the NiCd FAQ written by: Ken A. Nishimura (KO6AF))
All of which tends to support my basic operating theory about the charging of
nickel-cadmium batteries:
NiMHs have slightly higher capacity and no memory effect but have higher
initial cost and are more sensitive to overcharging. Must be used with
compatible charger.
(From: Randolph Miller (randolph.miller2@verizon.net).)
Many "name brand" camcorder and other similar battery packs contain two or
even 3 thermal switches (those rectangular, un-identifiable, wired
between the cells). They contain a bimetal strip operating a set of contacts
which open at a preset temperature. Often only one of these will fail,
resulting in a $40 NiCad that won't charge. Since these little suckers are
pricey if ya kind find them, a safe and cheap fix, is to test the thermal
switches for continuity (they should be closed at room temp) and remove the
defective one. If needed move the other, or at least one, to the mid-point
of the cells series. If a battery pack has 8 separate cells, (i.e.: a 9.6 V
VHS-C camera pack) the thermal switch should be wired between the 4th and 5th,
and as far away from the charging contacts as possible. The extra switches
were added as a safety factor but since the average one is designed to open
at 87°C, there is no fire hazard so long as the pack is re-sealed after
working on it.
A quick fix for a NiCad pack left on the dashboard. Since good ol' solar
power can heat a battery pack to the point where the thermal protection can
open (and even warp a case) you can be stuck at the soccer game with what
seems like dead batteries. The trick is to drop the temp below 87°C. Wrap
the battery in plastic so the contacts won't get wet, and stick it in the
cooler with the kids lunch and your six-pack. A few minutes and the
thermistor should close. letting the batter work normally. Also, if the
cord is long enough, never recharge a NiCad inside the car. Place the
battery and charger under the car, in the shade, so it doesn't heat quickly
and will get a full charge.
Therefore, it is generally easy to tell what kind of technology is
inside a pack even if the type is not marked as long as the voltage
is marked. Of course, there are some - like 6 V that will be ambiguous.
The specifications for LEDs you see in electronics distributor's catalogs
may look the same as those for incandescent lamps but they are not.
Incandescent lamps provide their own current limiting; LEDs do not.
It's possible to luck out and happen to have a given LED work without
current limiting with a particular set of batteries but it hardly an
acceptable design approach. Slight variations in battery parameters will
result in gross changes in light intensity and possible shortening of life
or outright destruction of the LED.
If the voltage drops when the device is turned on or the batteries are
installed - and the batteries are known to be good - then an overload
may be pulling the voltage down.
Assuming the battery is putting out the proper voltage, then a number of
causes are possible:
What is most likely happening is that several of the NiCd cells have
high leakage current and drain themselves quite rapidly. If they are
bad enough, then a substantial fraction of the charging current itself is
being wasted so that even right after charging, their capacity is less
than expected. However, in many cases, the pack will deliver close to
rated capacity if used immediately after charging.
If the pack is old and unused or abused (especially, it seems, if it
is a fast recharge type of pack), this is quite possible. The cause
is the growth of fine metallic whiskers called dendrites that partially
shorts the cell(s). If severe enough, a dead short is created and no
charge at all is possible.
Sometimes this can be repaired temporarily at least by 'zapping' using
a large charged capacitor to blow out the whiskers or dendrites that
are causing the leakage (on a cell-by-cell basis) but my success on
these types of larger or high charge rate packs such as used in laptop
computers or camcorders has been less than spectacular. See the section:
Zapping NiCds to Clear Shorted Cells.
If it is a little rectangular silver or plastic box in series with one of the
positive or negative terminals of the pack, it is probably a thermostat and
is there to shut down the charging or discharging if the temperature of the
pack rises too high. (The manufacturer name "Klixon" would be a dead giveaway
to identity. Izuzu also makes these things.) If it tests open at room
temperature, it is bad. With care, you can safely substitute a low value
resistor or auto tail light bulb and see if the original problem goes away
or at least the behavior changes. However, if there is a dead short
somewhere, that device may have sacrificed its life to protect your equipment
or charger and going beyond this (like shorting it out entirely) should be
done with extreme care. These may be either mechanical (bimetal
strip/contacts) or solid state (Polyfuse(tm) - increases resistance
with overcurrent).
If it looks like a small diode or resistor, it could be a temperature
sensing thermistor which is used by the charger to determine that the
cells are heating which in its simple minded way means the cells are being
overcharged and it is should quit charging them. You can try using a
resistor in place of the thermistor to see if the charger will now
cooperate. Try a variety of values while monitoring the current or
charge indicators. However, the problem may actually be in the charger
controller and not the thermistor. The best approach is to try another pack.
It could be any of a number of other possible components but they all serve
a protective and/or charge related function.
Of course, the part may be bad due to a fault in the charger not shutting
down or not properly limiting the current as well.
The cause of these bad NiCd cells is the formation of conductive filaments
called whiskers or dendrites that pierce the separator and short the positive
and negative electrodes of the cell. The result is either a cell that will
not take a charge at all or which self discharges in a very short time. A
high current pulse can sometimes vaporize the filament and clear the short.
The result may be reliable particularly if the battery is under constant
charge (float service) and/or is never discharged fully. Since there are
still holes in the separator, repeated shorts are quite likely especially if
the battery is discharged fully which seems to promote filament formation,
I have used zapping with long term reliability (with the restrictions
identified above) on NiCds for shavers, Dustbusters, portable phones,
and calculators.
WARNING: There is some danger in the following procedures as heat is
generated. The cell may explode! Take appropriate precautions and don't
overdo it. If the first few attempts do not work, dump the battery pack.
Attempt sapping at your own risk!!!
You will need a DC power supply and a large capacitor - one of those 70,000
uF 40 V types used for filtering in multimegawatt geek type automotive audio
systems, for example. A smaller capacitor can be tried as well.
Alternatively, a you can use a 50 to 100 A 5 volt power supply that doesn't
mind (or is protected against) being overloaded or shorted.
Some people recommend the use of a car battery for NiCd zapping. DO NOT be
tempted - there is nearly unlimited current available and you could end with
a disaster including the possible destruction of that battery, your NiCd,
you, and anything else that is in the vicinity.
OK, you have read the warnings:
Remove the battery pack from the equipment. Gain access to the shorted
cell(s) by removing the outer covering or case of the battery pack and
test the individual cells with a multimeter. Since you likely tried
charging the pack, the good cells will be around 1.2 V and the shorted
cells will be exactly 0 V. You must perform the zapping directly across
each shorted cell for best results.
Connect a pair of heavy duty clip leads - #12 wire would be fine - directly
across the first shorted cell. Clip your multimeter across the cell
as well to monitor the operation. Put it on a high enough scale such
that the full voltage of your power supply or capacitor won't cause any
damage to the multimeter.
Wear your eye protection!!!
If the dendrites have blown, the voltage on the cell should have jumped
to anywhere from a few hundred millivolts to the normal 1 V of a
charged NiCd cell. If there is no change or if the voltage almost
immediately decays back to zero, you can try zapping couple more times
but beyond this is probably not productive.
If the voltage has increased and is relatively stable, immediately
continue charging the repaired cell at the maximum SAFE rate specified
for the battery pack. Note: if the other cells of the battery pack are
fully charged as is likely if you had attempted to charge the pack, don't
put the entire pack on high current charge as this will damage the other
cells through overcharging.
One easy way is to use your power supply with a current limiting resistor
connected just to the cell you just zapped. A 1/4 C rate should be
safe and effective but avoid overcharging. Then trickle charge at
the 1/10 C rate for several hours. (C here is the amp-hour capacity of
the cell. Therefore, a 1/10 C rate for a 600 mA NiCd is 50 mA.)
This works better on small cells like AAs than on C or D cells since the
zapping current requirement is lower. Also, it seems to be more difficult
to reliably restore the quick charge type battery packs in portable tools
and laptop computers that have developed shorted cells (though there are
some success stories).
My experience has been that if you then maintain the battery pack in
float service (on a trickle charger) and/or make sure it never discharges
completely, there is a good chance it will last. However, allow the
bad cells to discharge to near 0 volts and those mischievous dendrites
will make their may through the separator again and short out the cell(s).
In most cases, the actual stuff that leaks from a battery is not 'battery
acid' but rather some other chemical. For example, alkaline batteries
are so called because their electrolyte is an alkaline material - just the
opposite in reactivity from an acid. Usually it is not particularly
reactive (but isn't something you would want to eat).
The exception is the lead-acid type where the liquid inside is sulfuric acid
of varying degrees of strength depending on charge. This is nasty and should
be neutralized with an alkaline material like baking soda before being
cleaned up. Fortunately, these sealed lead-acid battery packs rarely
leak (though I did find one with a scary looking bulging case, probably
due to overcharging - got rid of that is a hurry).
Scrape dried up battery juice from the battery compartment and contacts
with a plastic or wooden stick and/or wipe any liquid up first with a dry
paper towel. Then use a damp paper towel to pick up as much residue as
possible. Dispose of the dirty towels promptly.
If the contacts are corroded, use fine sandpaper or a small file to remove
the corrosion and brighten the metal. Do not an emery board or emery paper
or steel wool as any of these will leave conductive particles behind which
will be difficult to remove. If the contacts are eaten through entirely,
you will have to improvise alternate contacts or obtain replacements.
Sometimes the corrosion extends to the solder and circuit board traces as
well and some additional repairs may be needed - possible requiring
disassembly to gain access to the wiring.
When I was about 10 years old I was sitting in my dad's driveway in a '65
Plymouth Fury III station wagon while he disconnected the trickle charger from
the '67 Fiat in the garage. I heard a pop and saw my dad throw his hands over
his face, run to the back door and start kicking it to get someone to open it.
Fortunately he wasn't injured. But it was an eye opener. It was probably 30
or below, there was no flame present, and the double garage door was open (this
happened in Connecticut). Also in a Fiat 850 sport coupe the battery is in the
trunk (front) so there really isn't anything up there that would cause a spark
(engine & gas tank in back). So it must have been a spark off of the charger
when he pulled it off the terminal (he hadn't unplugged the charger).
I use a high power Weller (140 W) soldering gun. Use fine sandpaper to
thoroughly clean and roughen up the surface of the battery cell at both
ends. Tin the wires ahead of time as well. Arrange the wire and cell
so that they are in their final position - use a vise or clamp or buddy
to do this. Heat up the soldering gun but do not touch it to the battery
until it is hot - perhaps 10 seconds. Then, heat the contact area on the
battery end while applying solder. It should melt and flow quite quickly.
As soon as the solder adheres to the battery, remove the heat without
moving anything for a few seconds. Inspect and test the joint.
A high power soldering iron can also be used.
Here is a novel approach that appears to work:
(From: Clifford Buttschardt (cbuttsch@slonet.org).)
There is really no great amount of danger spot welding tabs! They usually
are made of pure nickel material. I put two sharp pointed copper wires in
a soldering gun, place both on the tab in contact with the battery case
and pull the trigger for a short burst. The battery remains cool.
(From: mcovingt@ai.uga.edu (Michael Covington).)
Of course! A soldering gun is a source of about 1.5 V at 100 A RMS.
Should make a fine spot-welder. You should write that up for QST
("Hints and Kinks") or better yet, send it in a letter to the editor
of "Electronics Now" (the magazine I write for).
Furthermore, there is essentially unlimited current available from the
battery (cigarette lighter) - 20 A or more. This will instantly turn
your expensive CD player to toast should you get the connections wrong.
No amount of internal protection can protect equipment from fools.
My recommendation for laptop computers is to use a commercially available
DC-AC inverter with the laptop's normal AC power pack. This is not the
most efficient but is the safest and should maintain the laptop's warranty
should something go wrong. For CD players and other audio equipment, only
use approved automotive adapters.
Incidentally, since the current is significant, repeated 'testing' will drain
the batteries - as with any proper under-load battery test! This isn't an
issue for occasional testing but if the kids figure how to do this....
Personally, I would rather use a $3 battery checker instead of paying for
throw-away frills!
One alternative is to substitute a regulated power supply with an output equal
to the the battery voltage and current capacity found by dividing the VA
rating of the normal wall adapter by the battery's nominal terminal voltage
(this will be worst case - actual requirements may be less). Connect this
directly in place of the original battery pack. Unless there is some other
sort of interlock, the equipment should be perfectly happy and think it is
operating from battery power!
Also see the other parts of this document dealing with AC Adapters and
Transformers.
-- end V1.14
All Rights Reserved
2.There is no charge except to cover the costs of copying.
DISCLAIMER
AC adapters, transformers, and even batteries, are critical safety components.
Replacement with an improperly rated or incompatible device can result in
damage or destruction of the powered equipment as well as the risk of shock or
electrocution in certain cases.
Introduction
Scope of this Document
This collection of information deals with the troubleshooting, repair, and
use (normal or unconventional) of AC (wall) adapters, transformers, equipment
power supplies (non-switching type), and batteries used in portable electronic
devices and power tools.
SAFETY
For the common transformer based AC adapter, there is no danger anywhere inside
the device once unplugged. For the switchmode variety, see the document:
Notes on the Troubleshooting and Repair of Small
Switchmode Power Supplies for information beyond what is covered in this
document.
General AC Adapter Information
AC Adapter Basics
It seems that the world now revolves around AC Adapters or 'Wall Warts'
as they tend to be called. There are several basic types. Despite the
fact that the plugs to the equipment may be identical THESE CAN GENERALLY
NOT BE INTERCHANGED. The type (AC or DC), voltage, current capacity, and
polarity are all critical to proper operation of the equipment. Use of an
improper adapter or even just reverse polarity can permanently damage or
destroy the device. Most equipment is protected against stupidity to a
greater or lessor degree but don't count on it.
About AC Adapter Ratings
The following mainly applies to AC adapters using transformers. Those based
on switchmode power supplies adapters have tended to be well designed with
decent regulation and realistic ratings. Of course, they are generally also
much more expensive!
Protect Yourself from "Unknown AC Adapter Syndrome"
Apparently, manufacturers of equipment powered by AC adapters have discovered
that they can improve their bottom line by not printing the AC adapter
ratings on the device itself, and possibly not even in the user manual. I
don't know whether this is actually done for liability reasons (so you aren't
tempted to actually use an AC adapter other than their own exorbitantly
priced replacement) or just to same 3 microcents on printing ink but the net
result is that the owner has no idea what adapter in that drawer that collects
adapters is the correct one. They could at least specify a particular model
adapter if they don't think the average consumer has an intelligence greater
than a carrot.
Why do AC Adapters Usually Use Heavy Transformers?
The main reasons are safety and cost.
Compact AC Adapters
These use switchmode power supply technology and can therefore be quite small
and light weight. In addition to the applications noted below, they are
turning up on a variety of other high tech gadgets from shavers to Personal
Digital Assistants.
Substituting AC Adapters
This relates to replacing a missing or broken adapter for which the
specifications are known.
AC Adapter Troubleshooting and Repair
AC Adapter Testing
AC adapters that are not the switching type (1) and (2), above, can easily
be tested with a VOM or DMM. The voltage you measure (AC or DC) will
probably be 10-25% higher than the label specification. If you get no
reading, wiggle, squeeze, squish, and otherwise abuse the cord both at
the wall wart end and at the device end. You may be able to get it to
make momentary contact and confirm that the adapter itself is functioning.
Pocket Wall Adapter Tester/Polarity Checker
This handy low cost device can be built into an old ball point pen case or
something similar to provide a convenient indication of wall adapter type,
operation, and polarity:
Probe(+) o-----/\/\-----+----|>|----+---o Probe(-)
1K, 1/2 W | Green LED |
+----|<|----+
Red LED
Getting Inside an AC Adapter
Manufacturers come up with all sorts of creative ways of making access a
challenge:
For those that are glued:
After the repair, the two halves (or pieces!) can be glued back together
using something like Duco Cement or windshield sealer.
AC Adapter Repair
Although the cost of a new adapter is usually modest, repair is often
so easy that it makes sense in any case.
AC Adapter Substitution and Equipment Damage
Those voltage and current ratings are there for a reason. You may get
away with a lower voltage or current adapter without permanent damage but
using a higher voltage adapter is playing Russian Roulette. Even using
an adapter from a different device - even with similar ratings, may
be risky because there is no real standard. A 12 V adapter from one
manufacturer may put out 12 V at all times whereas one from another
manufacturer may put out 20 V or more when unloaded.
Some devices are designed in such a way that they will survive almost anything.
A series diode would protect against reverse polarity. Alternatively, a large
parallel diode with upstream current limiting resistor or PTC thermistor, and
fuses, fusable resistors, or IC protectors would cut off current before the
parallel diode or circuit board traces have time to vaporize. A crowbar
circuit (zener to trigger an SCR) could be used to protect against reasonable
overvoltage.
Power Reversal - Better Pray
"That's right, I reversed power and ground on a Sony XR-6000 AM/FM cassette
car stereo. (12V negative ground).
If it had not been turned on before you discovered your error, the damage
may have been limited to the display and some filter caps. Then again...
"Is there any hope of my repairing it? (This assumes I show more ability
than I did when installing it.) Which part(s) are likely damaged?"
(From: Onat Ahmet (onat@turbine.kuee.kyoto-u.ac.jp).)
If not, join the happy crowd, and gut the good old stereo for parts!
Determining Voltage and Polarity of AC Adapter Powered Devices
This is often required when the original adapter is lost or misplaced or isn't
labeled so you are not sure if it is the correct one for your device. It's
amazing how many things like modems and phone answering machines don't list
the voltage and polarity on the case - it's not like the extra printing would
cost anything! While I would stop short of calling this a conspiracy, there
does appear to be an industry-wide practice of leaving out key information
to encourage replacement of the equipment rather than the much less costly
and much less profitable repair or replacement of only the wall adapter.
Information on voltage, current, and AC or DC polarity, is often missing
on the equipment itself. And, absolutely totally incompatible wall adapters
having similar plugs can be attached with the possible result being instant
destruction of the device. This even applies to equipment from the
same manufacturer! At least wall sockets are standardized - wall adapters
are not.
DC 5V ---- AC 12 V ~
____ _
AC Adapter Modifications or Enhancements
Using AC Adapters in Series for Multiple Voltages
Where a bipolar DC power supply is needed, it is possible to create this
with a pair of DC output adapters in series. Each adapter must have voltage
and current ratings adequate for your application. They can be used with or
without external regulators (see the section: Adding an IC
Regulator to a Wall Adapter or Battery. Since they are fully isolated
from the AC line and each other, they can be tied together with any desired
polarity and common point.
Using AC Adapters in Series for Higher or Lower Voltage
Wall adapters are totally isolated from everything (except possibly for a
very high value resistor to one side of the AC line which for this purpose
can be ignored) so using one set of wires as a common for the series
connection won't blow anything.
Replacing Batteries With an AC Adapter
While most appliances that run off of internal batteries also include a socket
for an wall adapter, this is not always the case. Just because there is no
hole to plug one in doesn't necessarily mean that you cannot use one.
+--+
X V | (Inserting plug breaks connection at X)
Battery (+) o------- |
Adapter (+) o---------+------------------o Equipment (Ring, +)
\______
o===+
Battery/ |
Adapter (-) o-----------------------+----o Equipment (Center, -)
Converting an AC Output Wall Adapter to DC
Where a modest source of DC is required for an appliance or other device,
it may be possible to add a rectifier and filter capacitor (and possibly
a regulator as well) to a wall adapter with an AC output. While many wall
adapter output DC, some - modems and some phone answering machines, for
example - are just transformers and output low voltage AC.
Bridge Rectifier Filter Capacitor
AC o-----+----|>|-------+---------+-----o DC (+)
~| |+ |
In from +----|<|----+ | +_|_ Out to powered device
AC wall | | C ___ or voltage regulator
Adapter +----|>|----|--+ - |
| | |
AC o-----+----|<|----+------------+-----o DC (-)
~ -
Considerations:
The following examples illustrate some of the possibilities.
Limiting your load to the VA ratings of the transformer should keep it from
overheating. Whether you will get a decently smooth output will depend on how
much filtering you have AND on the peak current available from the transformer
to recharge the filter capacitors on each half-cycle. A high quality
transformer (e.g. something from a manufacturer like Stancor or Thorderson
that is designed with much more copper) will be much much better in this
respect. A wall adapter is likely to have limited peak current and
significant droop.
Adding an IC Regulator to a Wall Adapter or Battery
For many applications, it is desirable to have a well regulated source of
DC power. This may be the case when running equipment from batteries as
well as from a wall adapter that outputs a DC voltage or the enhanced adapter
described in the section: Converting an AC Output Wall
Adapter to DC.
Positive Negative
Voltage Regulator Voltage Regulator
----------------------- -----------------------
7805 +5 V 7905 -5 V
7809 +9 V 7909 -9 V
7812 +12 V 7912 -12 V
7815 +15 V 7915 -15 V
and so forth. Where these will suffice, the circuit below can be simplified
by eliminating the resistors and tying the third terminal to ground. Note:
pinouts differ between positive and negative types - check the datasheet!
I +-------+ O
Vin (+) o-----+---| LM317 |---+--------------+-----o Vout (+)
| +-------+ | |
| | A / |
| | \ R1 = 240 |
| | / | ___
_|_ C1 | | +_|_ C2 |_0_| LM317
___ .01 +-------+ ___ 1 uF | | 1 - Adjust
| uF | - | |___| 2 - Output
| \ | ||| 3 - Input
| / R2 | 123
| \ |
| | |
Vin(-) o------+-------+----------------------+-----o Vout (-)
Note: Not all voltage regulator ICs use this pinout. If you are not using an
LM317, double check its pinout - as well as all the other specifications.
More information on this topic can be found in the document:
Various Schematics and Diagrams.
Equipment Power Supplies
Types of Power Supplies
See the document:
Safety Guidelines for High Voltage and/or Line Powered
Equipment before tackling any power supply problems!
Some comments for each type:
Power Supply Troubleshooting
Totally Dead Power Supply (Non-Switching Type)
Don't overlook the possibility of bad solder connections or even a
bad line cord or plug. Maybe Fido was hungry.
Low or Missing Power Supply Outputs (Non-Switching Type)
Once the line input and primary circuits have been found to be good (or at
have continuity and a resistance that is reasonable, the problems is
most likely in the secondary side - fuses, rectifiers, filter capacitors,
regulator components, bad connections, excess load due to electronic
problems elsewhere.
Uninterruptible Power Supplies (UPSs) and Power Inverters
CAUTION: reread safety guidelines as portions of these devices can be nasty.
Protection Devices
About Fuses, IC Protectors, and Circuit Breakers
The purpose of fuses and circuit breakers is to protect both the wiring
from heating and possible fire due to a short circuit or severe overload
and to prevent damage to the equipment due to excess current resulting
from a failed component or improper use (i.e., excess volume to loudspeakers).
Fuse Post Mortems
Quite a bit can be inferred from the appearance of a blown fuse if the
inside is visible as is the case with a glass cartridge type. One
advantage to the use of fuses is that this diagnostic information is
often available!
Fuse or Circuit Breaker Replacement
As noted, sometimes a fuse will blow for no good reason. Replace fuse,
end of story. In this situation, or after the problem is found, what are
the rules of safe fuse replacement? It is inconvenient, to say the least,
to have to wait a week until the proper fuse arrives or to venture out to
Radio Shack in the middle of the night.
Comments on Importance of Thermal Fuses and Protectors
Like a normal fuse or circuit breaker, a thermal fuse or thermal protector
provides a critical safety function. Therefore, it is extremely ill advised
to just short it out if it fails. Some designs even make this option extra
tempting by providing an easy way to bypass even one buried inside a power
transformer - using an additional, normally unused terminal.
Transformers
Common Types of Transformers
A transformer consists of a laminated iron or ferrite core and 2 or more
insulated windings that are most often not connected to each other directly.
If one set of windings is used as the input for AC power or an audio signal
(the 'primary' winding), the voltage appearing on each of the other windings
(the 'secondary' winding(s)) will be related by the ratio of the number of
turns on each of the windings. However, you don't get something for nothing:
The current is related by the inverse of this ratio so the power doesn't
change (except due to unavoidable losses).
There are also a couple of other common types of AC line operated
transformers used in servicing:
See the document: Troubleshooting of Consumer Electronic
Equipment for more information on these types of transformers.
Testing a Power Transformer
Here are some simple tests to perform where you want to determine if a used
(or new) power transformer with known specifications is actually good:
Identifying the Connections on an Unknown Power Transformer
Start with a good multimeter - DMM on the lowest ohms scale or VOM on the
X1 resistance range. (You will need to be able to measure down to .1 ohms
for many of these.) This will permit you to map the windings.
"I recently purchased at a local electronics surplus store at 35volt center
tap 2A transformer for a model railroad throttle (power supply). The
secondary wires are red-red/yellow-red and I understand how to hook up the
secondary in order to get two 17.5 volt sources. My dilemma is the
primary. There are SIX black wires (black, black/red, black/blue,
black/green, black/yellow, black/grey). Two of the wires were already
stripped and I hooked these up to 115 VAC but no voltage on the secondary
side. Does anyone have any ideas? I don't know the manufacturer, the
transformer is in an enclosed case (no open windings). I also don't know
if it has multiple primaries that must be connected or if it has five taps
for different input voltages. Any ideas????"
Of course, I assume you did measure on the AC scale on the secondary! :-)
Sorry, have to confirm the basics. My natural assumption would also be
that the striped wires were the ones you needed.
Using a combination of the above procedures should enable you to pretty
fully determine what is going on. I suspect that you have a pair of
primary windings that can be connected either series (for 220) or parallel
(110) and a tap but who knows. Do the tests. If in doubt, don't just
connect it to 110 - you could end up with a melt-down. Post your findings.
Determining Unknown Connections on International Power Transformers
Most likely, you can figure this out if you can identify the input connections.
Determining Power (VA) Ratings of Unknown Transformers
For a transformer with a single output winding, measuring temperature rise
isn't a bad way to go. Since you don't know what an acceptable temperature
is for the transformer, a conservative approach is to load it - increase
the current gradually - until it runs warm to the touch after an extended
period (say an hour) of time.
Determining the Ratings of a Fried Power Transformer
A power transformer can die in a number of ways. The following are the most
common:
There are several approaches to analyzing the blown transformer and/or
identifying what is needed as a replacement:
If you cannot do this for whatever reason, some educated guesswork will be
required. Each of the outputs will likely drive either a half wave (one
diode), full wave (2 diodes if it has a centertap), or bridge (module or
4 diodes). For the bridge, there might be a centertap as well to provide
both a positive and negative output.
Transformer Troubleshooting and Repair
Transformer Fault Diagnosis
Some power transformers include a thermal fuse under the outer layers of
insulation. In many cases, an overload will result in a thermal fuse opening
and if you can get at it, replacement will restore the transformer to health.
Also see the section: Comments on Importance of Thermal
Fuses and Protectors.
Rewinding Power Transformers
Here is some information and recommendations from people who have successfully
repaired power transformers that have self destructed:
You must of course use a transformer that is big enough and has the correct
primary voltage. given that the original failed and hand made transformers are
never as compact as manufactured ones it would be best to use the biggest
transformer that you can possible get to fit. For safety sake only use a
modern transformer that is in good condition.
Batteries and Battery Packs
Battery Technology
The desire for portable power seems to be increasing exponentially with
the proliferation of notebook and palmtop computers, electronic organizers,
PDAs, cellular phones and faxes, pagers, pocket cameras, camcorders and
audio cassette recorders, boomboxes - the list is endless.
Battery Basics
A battery is, strictly speaking, made up of a number of individual cells
(most often wired in series to provide multiples of the basic cell voltage
for the battery technology - 1.2, 1.5, 2.0, or 3.0 V are most common).
However, the term is popularly used even for single cells.
Battery Chargers
The (energy storage) capacity, C, of a battery is measured in ampere hours
denoted a A-h (or mA-h for smaller types). The charging rate is normally
expressed as a fraction of C - e.g., .5 C or C/2.
Dave's Comments on Building Charger for Small Lead-Acid Batteries
The following applies to the sort of lead-acid batteries found in some
camcorders and other portable equipment:
By using the 14.5 V instead of 13.8 V for the initial charge voltage, this
type of charger gets the battery back up to 90% charged in considerably
less time. But if you only care about charging overnight, you don't need
the extra complexity.
Substituting NiCds for Alkalines
First note that rechargeable batteries are NOT suitable for safety critical
applications like smoke detectors unless they are used only as emergency
power fail backup (the smoke detector is also plugged into the AC line) and
are on continuous trickle charge). NiCds self discharge (with no load) at
a rate which will cause them to go dead in a month or two.
Can a Large Electrolytic Capacitor be Substituted for a NiCd?
The quick answer is: probably not. The charger very likely assumes that
the NiCds will limit voltage. The circuits found in many common appliances
just use a voltage source significantly higher than the terminal voltage
of the battery pack through a current limiting resistor. If you replace
the NiCd with a capacitor and the voltage will end up much higher than
expected with unknown consequences. For more sophisticated chargers, the
results might be even more unpredictable.
Determining the Actual Capacity of a NiCd Battery Pack
When a battery pack is not performing up to expectations or is not marked
in terms of capacity, here are some comments on experimentally determining
the A-h rating.
Size of cells Capacity range, A-h
---------------------------------------------
AAA .2 - .4
AA .4 - 1
C 1 - 2
D 1 - 5
Cordless phone .1 - .3
Camcorder 1 - 3+
Laptop computer 1 - 5+
First, you must charge the battery fully. For a battery that does not
appear to have full capacity, this may be the only problem. Your charger
may be cutting off prematurely due to a fault in the charger and not the
battery. This could be due to dirty or corroded contacts on the charger
or battery, bad connections, faulty temperature sensor or other end-of-charge
control circuitry. Monitoring the current during charge to determine if the
battery is getting roughly the correct A-h to charge it fully would be a
desirable first step. Figure about 1.2 to 1.5 times the A-h of the battery
capacity to bring it to full charge.
NiCd Batteries and the Infamous 'Memory Effect'
Whether the NiCd 'memory effect' is fact or fiction seems to depend on
one's point of view and anecdotal evidence. What most people think is
due to the memory effect is more accurately described as voltage
depression - reduced voltage (and therefore, reduced power and capacity)
during use.
"To recap, we can say that true 'memory' is exceedingly rare. When we see
poor battery performance attributed to 'memory', it is almost always certain
to be a correctable application problem. Of the problems noted above,
Voltage Depression is the one most often mistaken for 'memory'.....
Memory Effect in NiMH Batteries?
The party line is that Nickel-Metal-Hydride batteries do not have any
memory effect. Perhaps, perhaps not.
See HiMH Batteries, Memory,
and Thermal Runaway for one person's test results and other information.
Care and Feeding of NiCds
Here are six guidelines to follow which will hopefully avoid voltage
depression or the memory effect or whatever:
Author's note: I refuse to get involved in the flame wars with respect
to NiCd battery myths and legends --- sam.
Why There Will Never Actually be Closure on This Topic
(From: Mark Kinsler (kinsler@froggy.frognet.net).)
Nickel Cadmium Versus Nickel-Metal-Hydride in a Nutshell
NiCds are inexpensive, reliable, and easy to charge, but may suffer from
voltage depression (what people call the memory effect) from repeated shallow
discharge cycles.
Randy's Notes on First Aid for NiCd Battery Packs
CAUTION: Opening these battery packs will of course void any warranty but
you knew that. Also, make notes of exactly how the cells and anything else
inside is arranged. Improper reassembly can result in damage to equipment
and/or risk of overheating should cells short inside the pack due to lack of
or misplaced insulation. Under no circumstances should all thermal switches
be removed - not only are they a safety device to prevent excessive
temperatures but may also be part of the charging circuit. So, if they are
removed, your next charge may be your last! I'd highly recommend that all
of them be replaced (from another pack as a last resort) and installed
in exactly the same positions they were originally.
Identifying Technology of Unmarked Battery Packs
Since the nominal (rated) voltages for the common battery technologies
differ, it is often possible to identify which type is inside a pack
by the total output voltage:
Note that these are open circuit voltages and may be very slightly higher
when fully charged or new.
Powering LEDs with Batteries
LEDs look like diodes with a high forward voltage drop. Above the that
voltage, the incremental resistance is very low and without current limiting,
the current would be critically dependent on the exact voltage of the power
source. Most of the time, they are spec'd at a particular maximum current
and need some means to limit the current to that value based on the input
voltage. Some devices may depend on the internal resistance of the batteries
to provide the current limiting - this is a poor approach and depends greatly
on the type and capacity of the batteries being used. Most common is just a
resistor but this provides no regulation and poor efficiency. Better designs
(used in LED flashlights) will use a DC to pulse inverter with regulation
achieving constant light output regardless of battery state-of-charge and high
efficiency. LEDs can usually withstand short high current pulses and this
allows the circuit to be designed with low losses.
Battery Problem Troubleshooting and Repair
Problems with Battery Operated Equipment
For primary batteries like Alkalines, first try a fresh set. For NiCds,
test across the battery pack after charging overnight (or as recommended
by the manufacturer of the equipment). The voltage should be 1.2 x n V
where n is the number of cells in the pack. If it is much lower - off by
a multiple of 1.2 V, one or more cells is shorted and will need to be
replaced or you can attempt zapping it to restore the shorted cells. See
the section: Zapping NiCds to Clear Shorted Cells.
Attempt at your own risk!
NiCd Battery Pack Will Not Hold a Charge
This applies if the pack appears to charge normally and the terminal
voltage immediately after charging is at least 1.2 x n where n is the
number of cells in the pack but after a couple of days, the terminal
voltage has dropped drastically. For example, a 12 V pack reads only 6 V
48 hours after charging without being used.
What is This Thing in my NiCd Battery Pack?
In addition to the NiCd cells, you will often find one or more small parts
that are generally unrecognizable. Normally, you won't see these until
you have a problem and, ignoring all warnings, open the pack.
Zapping NiCds to Clear Shorted Cells
Nickel-Cadmium batteries that have shorted cells can sometimes be rejuvenated -
at least temporarily - by a procedure affectionately called 'zapping'.
Now check the voltage on the (hopefully previously) shorted cell.
Tom's Comments on NiCd Care and Feeding
(From: Tom Lamb (tlamb@gwe.net).)
Battery Juice and Corroded Contacts
Unless you have just arrived from the other side of the galaxy (where
such problems do not exist), you know that so-called 'leak-proof' batteries
sometimes leak. This is a lot less common with modern technologies than with
the carbon-zinc cells of the good old days, but still can happen. It is
always good advice to remove batteries from equipment when it is not being
used for an extended period of time. Dead batteries also seem to be more
prone to leakage than fresh ones (in some cases because the casing material
is depleted in the chemical reaction which generates electricity and thus
gets thinner or develops actual holes).
Exploding Batteries - These Things Really Happen
(From: Greg Raines (ghr@ix.netcom.com).)
Soldering Tabs Onto NiCd Batteries
When replacing NiCd batteries in packs or portable tools, it is often
necessary to attach wires to the individual cells. It may be possible to
obtain NiCds with solder tabs attached (Radio Shack has these) but if
yours do not, here are two ways that work. They both require a (Weller)
high wattage soldering gun.
Battery Related Information
Automotive Power
While it is tempting to want to use your car's battery as a power source
for small portable appliances, audio equipment, and laptop computers,
beware: the power available from your car's electrical system is not
pretty. The voltage can vary from 9 (0 for a dead battery) to 15 V under
normal conditions and much higher spikes or excursions are possible.
Unless the equipment is designed specifically for such power, you are
taking a serious risk that it will be damaged or blown away.
How Do Those On-Battery or On-the-Package Battery Testers Work?
There is a graded width resistance element that gets connected when you pinch
those two points. It heats up - substantially, BTW. Some sort of liquid
crystal or other heat sensitive material changes from dark to clear or yellow
at a fairly well defined temperature.
Battery Eliminator for Laptop or Appliance with Dead NiCds
Even where you have the AC adapter, it is quite likely that simply removing
the (shorted) battery pack will not allow you to use it. This is because
it probably uses the battery as a smoothing capacitor. You cannot simply
replace the battery with a large electrolytic capacitor because the battery
also limits the voltage to a value determined by the number of cells in the
pack. Without it, the voltage would be much too high, possibly resulting in
damage. You could use N power diodes in series (i.e., N=Vb/.7) to drop the
approximate voltage of the battery pack AND a large capacitor but you would be
wasting a lot of power in the form of heat.