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Copyright © 1994-2007
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In most cases, this will identify bad silicon transistors. Gain, frequency
response, etc. are not addressed here. While the tests can be applied to
germanium devices, these are more likely to change characteristics, it would
seem, without totally failing.
It is also possible to determine the lead arrangements of identified diodes
and bipolar transistors as well as breakdown voltage ratings.
Curve tracers are pieces of electronic test equipment similar to an
oscilloscope. They can not only test transistors and other devices but
evaluate the functional specifications as well. The chapter: "Curve Tracer
Design" includes information on their basic principles of operation and
provides details on some very additions to conventional scopes to add
some basic curve tracer capability.
This document evolved from a posting on the USENET newsgroup: sci.electronics
(no longer active - closest replacement in the sci.electronics hierarchy is
probably
sci.electronics.components)
from Randy Fromm
(Randy@randyfromm.com)
who maintains a
Technical Department with an
extensive collection of repair related information.
Before touching, probing, or unsoldering any component, make sure the equipment
is unplugged and any large capacitors have been safely discharged. See the
document:
Capacitor Testing,
Safe Discharging, and Other Related Information or the specific document
dealing with your equipment for details. Not only can coming in contact with
a live circuit or charged capacitor ruin your entire day, your test equipment
could be damaged or destroyed as well.
Caution: An analog VOM on the lowest resistance range may put out too much
current for smaller devices possibly damaging them. Ironically, this is more
likely with better meters like the Simpson 260 which can test to lower ohms
(X1 scale). Use the next higher resistance range in this case or a DMM as
these never drive the device under test with significant current. However,
this can result in false readings as the current may be too low to adequately
bias the junctions of some power devices or devices with built in resistors.
For the VOM, you are measuring the resistance at a particular (low current)
operating point - this is not the actual resistance that you will see in
a power rectifier circuit, for example.
On a (digital) DMM, there will usually be a diode test mode. Using this,
a silicon diode should read between .5 to .8 V in the forward direction
and open in reverse. For a germanium diode, it will be lower, perhaps
.2 to .4 V or so in the forward direction. Using the normal resistance
ranges - any of them - will usually show open for any semiconductor
junction since the meter does not apply enough voltage to reach the value
of the forward drop. Note, however, that a defective diode may indeed indicate
a resistance lower than infinity especially on the highest ohms range. So, any
reading of this sort would be an indication of a bad device but the opposite
is not guaranteed.
Note: For a VOM, the polarity of the probes is often reversed from what
you would expect from the color coding - the red lead is negative with
respect to the black one. DMMs usually have the polarity as you would
expect it. Confirm this using a known diode as a reference. Also,
'calibrate' your meter with both silicon and germanium semiconductors so
you will know what to expect with an unknown device.
One exception to this occurs with some power transistors which have built in
diodes (damper diodes reversed connected across C-E) and resistors (B-E,
around 50 ohms) which will confuse these readings. If you are testing
a transistor of this type - horizontal output transistors are the most common
example - you will need to compare with a known good transistor or check the
specifications to be sure. There are some other cases as well. So, if you
get readings that do not make sense, try to confirm with a known good
transistors of the same type or with a spec sheet.
Before testing an unknown device, it is best to confirm and label
lead polarity (of voltage provided in resistance or diode test mode) of
your meter whether it be an analog VOM or digital DMM using
a known good diode (e.g., 1N4007 rectifier or 1N4148 signal diode) as
discussed below. This will also show you what to expect for a reading
of a forward biased junction. If you expect any Germanium devices, you
should do this with a Ge diode as well (e.g., 1N34).
The assumption made here is that a transistor can be tested for shorts,
opens, or leakage, as though it is just a pair of connected diodes.
Now move lead "B" to the collector. You should get nearly the same reading.
Now try the other 4 combinations and you should get a reading of
infinite Ohms (open circuit). If any of these resistances is wrong,
replace the transistor. Only 2 of the 6 possible combinations should
show a low resistance; none of the resistances should be near 0 Ohms (shorted).
As noted above, some types of devices include built in diodes or resistors
which can confuse these measurements.
As noted, some transistors will have built in diodes or resistors which
can confuse these readings.
In many ways, a Darlington configuration behaves like a single transistor
where:
This doesn't prove that the device is good - only that it isn't blown up. A
more complete test requires a simple circuit and some means of detecting an
audio output signal.
For the UJT:
For the PUT (Programmable Unijunction Transistor), an additional voltage
divider (R3 and R4) is needed to set the threshold:
(From: Spehro Pefhany (speff@interlog.com).)
A PUT is essentially an SCR with a large reverse gate breakdown voltage
(G can be more positive than A by maybe 40 V) and a sensitive gate.
When the voltage at A exceeds the voltage at G by a diode drop, and
assuming enough voltage from A to K, the SCR turns on (conducts from A to
K) and stays that way until the current drops below the holding current
(typically around 100 uA, but it drops with increasing resistance in series
with the gate).
Symbol and example:
If you connect your meter from A to K, it should measure open both ways.
If you connect the positive lead (which may be red or black, depending on
the meter design) to A and the negative lead to K, and then momentarily
short G to K it should change to a relatively low resistance reading
(meter dependent). It will most likely stay latched when the G lead is
returned to being open, because the meter measuring current will exceed
the "holding current" of the PUT (called "valley current" in PUT specs).
If your meter has a "diode" range (in the ohms group), using that would
assure there is enough open-circuit voltage to make this work, but it
works this way in the half-dozen or so meters I have checked, using
reasonable ohms ranges.
Measurements between A and G, with K open, should be similar to a silicon
diode (fairly low in one direction, open in the other). Between G and K,
with A open, should be open in both directions.
PUTs are pretty sensitive (less than 1 uA trigger current) so be sure to keep
fingers away from the G lead.
The following assumes a silicon photodiode which is the most common type
with a useful spectral range from near-UV to near-IR, typically from 400 to
1,150 nm at the 10 percent response points. See the chart in:
Typical Silicon Photodiode Spectral Response.
The simplest electrical test is to check it like a normal diode. The results
should be similar - a forward voltage drop of 0.5 to 0.7 V, and open in the
reverse direction. For GaAs and other types, the forward voltage drop
will differ.
To test for functionality, connect the photodiode to a multimeter set to
its mA current range (1 mA full scale optimal). This is operating the
photodiode in photovoltaic mode - like a solar cell. A laser pointer
or helium-neon laser is the ideal light source to use for testing, but
the Sun, a light bulb, a flashlight, or even an LED will work fine as
well but will not provide any useful sensitivity information. For the
laser source, the sensitivity should be between 0.2 and 0.5 mA/mW of laser
power depending on wavelength. So, using a typical cheap Far East import
red laser pointer (typically 3 mW at 650 nm), the current will be about 1 mA.
For a more accurate measurement, reverse bias the photodiode with a few
volts with a current limiting resistor (for protection) and repeat the
light measurement. This is operating the photodiode in photoconductive
mode, which is probably the way it is used in your equipment. The results
should be similar but the response is more linear at higher current than
in photovoltaic mode.
For most applications, photodiodes either work or they don't. But in some
cases, performance degradation may occur from age or abuse. Substitution
of a known good device is the easiest confirmation where the photodiode appears
to behave properly based on the tests above, but doesn't perform properly
in-circuit.
A Silicon Controlled Rectifier is one type of thyristor used where the power
to be controlled is unidirectional. The Triac is a thyristor used where AC
power is to be controlled. (There are exceptions in both cases but for this
simple discussion these can be ignored).
Both types are normally off but may be triggered on by a low current pulse to
an input called the Gate. Once triggered on, they remain on until the current
flowing through the main terminals of the device drops below a hold value which
is very close to zero. It is usually not possible (at least not easy) to turn
thyristors off while current is flowing. However, there are special types
called Gate Turnoff Thyristors which enable this type of control as well.
Both SCRs and Triacs are 4 layer PNPN structures.
If we connect the positive terminal of a supply to say, a light bulb, and then
to the emitter of the PNP transistor and its return to the emitter of the NPN
transistor, no current will flow as long as the breakdown voltage ratings of
the transistor are not exceeded because there is no base current to either
transistor. However, if we provide some current to the base of the NPN
(IG(+).)ransistor, it will turn on and provide current to the base of the PNP
transistor which will turn on providing more current to the NPN transistor.
The entire structure is now in the solid on state and will stay that way even
when the input to the NPN's base is removed - until the power supply goes to
zero and the load current goes below the hold value.
The same scenario is true if we reverse the power supply and use the IG(-)
input for the trigger.
For a light dimmer or motor speed control, for example, the exact time when
the thyristor is triggered relative to the zero crossings of the AC power is
used to determine the power level. Trigger the thyristor early in the cycle
and the load is driver an high power. Trigger the thyristor late in the cycle
and there is only a small amount of power delivered to the load. The thermal
or mechanical inertia is generally counted to smooth out the power and results
in smooth continuous operation (i.e., a light bulb controlled by a dimmer does
not flicker.)
The advantage of thyristors over simple variable resistors is that they
(ideally) dissipate very little power as they are either fully on or fully off.
There are a wide variety of other types of thyristor and thyristor-like
devices. In particular, are diacs and sidacs which have no gate input but
simply turn on when a specified threshold voltage is exceeded across their main
terminals. See the section: Testing Diacs and Sidacs.
These are often used to trigger other thyristors in phase control applications.
For more information on thyristors, see Horowitz and Hill or any thyristor
databook.
Note: Some thyristors will have a low G-K/MT1 resistance but it should not read
as a short.
The real test is quite simple but will require a low voltage DC power supply
and two resistors. For triacs, a negative output from the supply is desireable
as well to test the triggering when the gate is negative).
R1 will be used to limit current through the device and R2 will be used to
limit current to the gate. A 12 VDC supply of at least 200 mA capacity with
a 100 ohm 2 W resistor for R1 and 1 K 1/4 W resistor for R2 should work for
most small to medium power SCRs. Check the 'minimum gate current' and
'holding current' specs to be sure. For larger devices, R1 and/or R2 may
need to be smaller.
If the device passes these tests, it is behaving properly and is probably
functional. However, without applying full voltage or current, there is no
way of knowing if it will meet all specifications.
You can replace the DC supply with a low voltage power transformer (say, 12
VAC). Use a scope to monitor the voltage across the DUT or R1. Then, when
the gate is connected to R2, you should see the voltage across the DUT drop
to nearly zero when it switches on part way through the positive cycle. This
phase will be determined by the voltage and value of R2. It should remain off
for the entire negative cycle (SCRs only) with the gate connected and remain
off all the time with the gate connected to the cathode.
(From: T. O. Prellwitz (timilen@halcyon.com).)
If you have a semiconductor curve tracer you can configure a small audio
transformer circuit to drive the gate. I did this with my B&K and it works
well. The secondary should provide enough voltage to drive the gate of
the SCR and the negative swing of the AC will cycle the scr off while
the positive phase turns it on. I drive the transformer with an audio
generator. Hope this offers some ideas.
However, you can test a diac or sidac with a resistor, variable power supply
(you will need at least the rating of the device), and a DMM. Hook them in
series and monitor across the device. With care, your variable supply can be
a Variac, 1N4007, and 1 uF, 200 V capacitor. Use a 47 K resistor to limit the
current:
As you increase the input, the voltage on the DUT will track it until the
rated voltage at which point it will drop abruptly to zero and stay there
until the voltage is reduced below its holding current. Repeat with the
opposite polarity.
With a scope it is even easier as you can use an AC supply directly (remove
D1 and C1) and observe that the DUT will turn on at the proper voltage on
both polarities of the AC waveform and stay on until the voltage crosses 0.
Use an isolation transformer for safey.
It is soooo easy: just use a DC current to drive the gate of the triac. Even
the polarity of the current doesn't matter, although most triacs are more
sensitive for a negative input current (flowing out of the gate to a negative
supply). A large triac may require some 50 mA.
There will be some applications where there is no 50 mA supply available.
That's where you would want to drive with short pulses. But these pulses would
have to occur around the expected instant of the zero-crossing of the load
current, which is a bit tricky with an inductive load.
As an alternative you could look for a more sensitive triac, for not too large
load currents there are types down to 5 mA or so. If you have 50 mA of DC to
spare, go for it, it will work.
By the way, never switch an inductive load like a power transformer ON at the
zero crossing of the mains voltage. That's guaranteed to drive the transformer
into saturation and create the worst possible current transient. Try and
switch on at maximum mains voltage, at +/- 90 degrees delay. Do not use a
voltage differentiator to generate +90 degrees phase shift, as it will be too
sensitive to mains disturbances. Instead, use a double integrator to give
2 * -45 degrees and a low-pass filter. Using only 1 integrator to approach -90
degrees gives too much attenuation of the voltage, hence 2 are recommended.
Any thyristor will have a maximum change in current vs change in time dI/dt.
If this is exceeded, then current flowing through the thyristor will find
the path of least resistance through the silicon. Unfortunately, for us, this
can be thought of as a molecular sized lightning bolt streaking through the
doped layers of silicon - finding the path of least resistance from individual
molecule to individual molecule. This soon results in an 'avalanche' of
electrons streaming through a very small path and this process feeds on itself
until the thyristor dies. This whole process probably takes only microseconds
to happen.
I don't know if fast blow fuses will help this situation if the current changes
too rapidly. A fuse is a very analog device with mass and it seems like it
would be a slow, lumbering giant compared to almost instantaneous current
change.
The solution for this problem? I am guessing putting an appropriately sized
inductor in series with the light bulb, but just be sure to add the correct
over voltage snubbing network. The inductor will keep the current from
changing too rapidly.
To determine the lead arrangement, label the pins on the unknown device
1, 2, and 3. Put the positive probe (as determined above) of you multimeter
on pin 1. Now, measure the resistance (VOM) or diode drop (DMM) to the other
two pins. If the positive probe is on the base of a good NPN transistor,
you should get low resistance readings or a low diode drop to the other
two leads. The B-C resistance or diode drop will be just slightly lower
than the B-E reading.
If one or both measurements to the other two pins is high, put the positive
probe on pin 2 and try again. If still no cigar, try pin 3.
If this still doesn't work, you may have a PNP transistor - repeat with the
negative probe as the common pin.
If none of the six combinations yields a pair of low readings - or if more
than one combination results in a pair of low readings, your transistor is
likely bad - or it is not a bipolar transistor!
As noted, some power transistors have built in base resistors or damper
diodes and will confuse these measurements. However, the lead arrangement
of these types of transistors is usually self evident (standard TO3, TOP3,
or TO220 cases). There are also some transistors with series base resistors
which may prove confusing. There are relatively rare, however.
Voltage ratings are more difficult and require a low current variable DC
power supply with a maximum voltage output greater than the expected (or
desired) breakdown rating of the transistors being tested. A fixed DC
supply with a suitable potentiometer is also satisfactory. For tests up
to 100 V, a 100K ohm pot would be satisfactory. Put a current limiting
resistor of about 100 K ohms in series with the output. For higher voltage
transistors, use an appropriate power supply and increase the value of the
potentiometer (if used) and current limiting resistor. It should be
possible to determine approximate values for Breakdown Voltages such as:
BVcbo - collector to base, emitter open.
BVceo - collector to emitter, base open.
BVces - collector to emitter, base shorted to emitter.
BVebo - emitter to base, collector open.
Apply your variable voltage across the appropriate leads and monitor
at the transistor with your VOM or DMM. The breakover point should be
easily detectable. The current limiting resistor should prevent damage
to the part from power dissipated in the reverse biased junction.
This approach also works for signal, rectifier, zener diodes, and other
similar devices.
The B-C junction voltage drop is always very slightly lower than the E-B
junction drop. The drop is given by the equation:
This has been confirmed below on a selection of common transistors using an
El-cheapo DMM:
(From: Lance (cast@iafrica.com).)
Using an analog (VOM - a DMM will not work), on its highest resistance range
I test across the collector and the emitter one way and then change the leads
around. The reading that is lower reading is the one to note (the one
with the most leakage on a uA meter). Sometimes the needle only just barely
moves. For a PNP the positive lead is on the emitter and for a NPN the
positive lead is on the collector. Now you know the base collector
and emitter, this has helped me work out how a circuit works by finding the
legs of the working transistors and then repairing it. I found this in a
very old mag more than fifteen years ago. If I can't remember which way
is what I use a known transistor. I then find out the hard way. (Note:
for a VOM, the polarities of the leads are often opposite of the color code
as noted above --- sam).
(From: Richard Torrens (4qd@argonet.co.uk).)
A lot of common multimeters have a diode range: you can use this to measure
a MOSFET out of circuit and get a good idea of whether it is OK. Meter negative
on the source, you should get no reading (open circuit) on the drain. Not on
the gate but if you measure the drain AFTER measuring the gate you will find
it conducts. A finger between source and gate will bleed away the charge and
the MOSFET stops conducting.
You really need a 'scope to check the drive circuit. What it does will
depend on the circuit configuration, whether there is current limiting etc.
(From: E. Wolsner (interser@algonet.se).)
My way of testing a power MOSFET is indeed simple and normally sufficient:
One ohmmeter is connected to the drain and the source, measuring the
resistance between drain and source, which should be very high. Another
ohmmeter is connected between gate and source. This ohm-meter should have a
high resistance capability (maybe 20 M ohms) and thus have a relatively high
test voltage (more than 5 volts). Now this voltage, when connected with the
proper polarity, will turn the mosfet on, which will be indicated by the first
ohm-meter. It will show zero resistance. To turn the transistor off, you
reverse the gate-source voltage, and the drain-source ohm-meter will again
indicate high resistance.
(Portions from: Egon Wolsner (interser@algonet.se).)
The multimeter must be able to provide at least 5 volts output on the
resistance measuring range (this usually means that a DMM will not work).
If it does, here is the procedure:
First you measure the resistance between the drain and source terminals, it
should be infinity. Then connect the plus to the gate and the minus to the
source pin. That should turn the MOSFET on. Then you measure the resistance
between the drain and source pins, which should verify that the resistance is
indeed near zero. (The gate capacitance will hold the device in the on-state
long enough for this test.) Turn the mosfet off by shorting the gate and
source pins (for a n-channel MOSFET)
(From: Bruce (reglarnavy@aol.com).)
You can get a pretty good idea about the condition of a MOSFET with some
quick & simple bench tests. The first thing you can do with a meter is measure
the parasitic substrate diode that connects the drain to the source. In an
NMOS part, this diode's cathode will be at the drain, and the anode at the
source. It will meter out similar to any conventional diode in both fwd /
reverse directions. You can see this diode in the schematic representation of
the FET in some databooks and a few schematics. The FET should show infinite
resistance, gate - source and gate - drain. If it does not, then the gate
oxide may be blown.
A second simple test can be done with a meter and a 9 V battery. First, short
the gate to the source to discharge any stored charge there. Then put your
meter on ohms and connect it across the drain - source. It should measure as
an open. Briefly connect the 9 V across the gate (+) to source (-) , again,
NMOS polarities, and the meter resistance should fall to a very low resistance,
on the order of an ohm or less. Removing the battery will not change the
reading, because in a good FET, Ciss will remain charged for a long time and
keep the FET on. Most FETs come on at Vgs=2 volts or so.
If these two tests work, then the FET is off to a good start. Substituting a
power supply and a proper load resistor for the meter, and a variable voltage
(a pot across the 9v will work) for the Vgs supply, in the aforementioned test,
will obviously be a more realistic test, and will also let you measure Vds,
Id, etc.
BEWARE ESD WITH FETS! Wear a wrist strap, keep the parts away from insulators
like plastics, and make sure your soldering iron tips are grounded. If you do
not have any of the black ESD foam to keep your parts in, then look around for
an anti-static bag that once may have contained a computer board, SIMMs, etc.
A curve tracer should be able to be configured for IGBT testing to determine
more complete behavior.
Typical values are:
IR: 1.2 V, Red: 1.85 V, Yellow: 2 V, Green: 2.15 V. The new blue LEDs will
be somewhat higher (perhaps 3 V). These voltages are at reasonable forward
current. Depending on the actual technology (i.e., compounds like GaAsP, GaP,
GaAsP/GaP, GaAlAs, etc.), actual voltages can vary quite a bit. For example,
the forward voltage drop of red LEDs may range at least from 1.50 V to 2.10 V.
Therefore, LED voltage drop is not a reliable test of color though multiple
samples of similar LEDs should be very close. Obviously, if the device is
good, it will also be emitting light when driven in this way if the current is
high enough.
So, test for short and open with a multimeter (but it must be able to supply
more than the forward voltage drop to show a non-open condition).
An LED can be weak and still pass the electrical tests so checking for output
is still necessary.
Therefore, if these tests don't find a problem, drive the LED from a DC supply
and appropriate current limiting resistor. For the IR types, you will need a
suitable IR detector. See the document: "Notes on the Troubleshooting and
Repair of Hand Held Remote Controls" for a variety of options.
Refer to an optoelectronics databook or the catalog of a large electronics
distributor for specific pinouts and specifications.
Assuming a photodiode or phototransistor type (most common), these can be
tested for basic functionality pretty easily:
Wire up a test circuit as follows:
Most problems will be obvious - like the entire thing was smashed by your pet
elephant or melted down due to applying too much power. :) However,
a hairline crack in one of the interior junctions could be undetectable
without testing.
The best way to test a TEC is to apply a controlled current and monitor the
voltage across the device as a function of current and measure the temperature
difference between the hot and cold surfaces. Then, compare the readings
with the device's specifications. If these aren't known, it may be possible
to match up your device with one of similar dimensions. One major supplier
is Melcore.
Testing for continuity can be done with an ohmmeter but really only if the
temperature of the two sides is exactly equal - otherwise there will be
a voltage offset (the junctions also generates voltage when a temperature
difference is present - and this can also serve as a test of sorts).
The I-V characteristics should be fairly linear over a relatively wide
range of current within the device's specified operating range. However,
the voltage for a given current does vary slightly with the temperature
difference between the hot and cold surfaces.
There are a variety of alternatives: fast, super-fast, ultra-fast (and so
forth) recovery diodes, schottky diodes, and others that must be used in
high frequency signal, switching, and power supply circuits:
Thus, if you find a bad diode in a piece of electronic equipment, don't
assume it is just an ordinary diode because the case looks the same. Replacing
a fast recovery diode with a 1N4007 will very likely just result in more
confusion. A proper device must be used even for testing. In most cases,
a faster part can be substituted without problems. However, there are
occasional situations where the specific characteristics of a slow part
(a reverse pulse due to its long recovery time or high capacitance) are
depended upon for the circuit to operation properly!
(From: John Popelish (jpopelish@rica.net).)
Fast recovery diodes have a little gold added to the silicon (and perhaps
other process changes) that make the minority carriers (holes in n type and
electrons in p type sides of the junction) have shorter lifetimes, so that in
addition to sweeping them out by applied voltage, the carriers spontaneously
disappear. This makes the diodes turn off faster. Other tricks help the
diodes to turn off with less of a snap, to reduce high frequency noise
generation. These changes usually compromise other properties of the diode,
like reverse leakage, forward drop or breakdown voltage, so there are lots of
different combinations of trade-offs.
Schottky diodes are really just half diodes. A metal intimately bonded to a p
type semiconductor. Holes have a zero lifetime in metal, so the minority
lifetime is just about zero for schottky diodes. They also have about half a
diode drop in the forward direction, and so are twice as efficient even for
low frequency rectification. The trade off here is that they can only be made
to handle low reverse voltages and even there, they have more reverse leakage
than junction diodes.
Motorola published a diode handbook that goes into a lot more detail on these
things and I recommend it.
Note that those with a built in damper diode may read around 50 ohms between
B and E (near 0 on the diode test range) - this is normal as long as the
resistance is not really low like under 10 ohms.
Yep! We "hit our head" on this one while diagnosing a sick scope.
Electrically, the parts are exactly the same. The "R" stands for 'reverse'.
The 'R' pinout is a mirror image of the normal one.
With a VOM, a good zener diode should read like a normal diode in the forward
direction and open in the reverse direction unless the VOM applies more than
the zener voltage for the device. A DMM on its diode test range may read the
actual zener voltage if it is very low (e.g., a couple V) but will read open
otherwise. The most common failure would be for the device to short - read
0.0 ohms in both directions. Then, it is definitely dead. :)
Some zeners are marked with a JEDEC (1N) part number, others with a couple of
colored bands (e.g., for 18 V, brown/gray), or a house number or house color
code.
You can easily test a zener and identify its voltage rating with a DC power
supply, resistor, and multimeter. You will need a power supply (a DC wall
adapter or AC wall adapter with a rectifier and filter capacitor is fine)
greater than the highest zener voltage you want to test. Select a resistor
that will limit current to a few mA. For example, for zeners up to about
20 V, you can use:
They are supposed to be located beyond the line fuse (though possibly not
always). In this case, where the line fuse blows but there is no visible
damage to the MOV(s), the simplest test may be to just temporarily remove the
MOV(s) and see if your problem goes away.
A multimeter can be used to test for leakage (there should be none) but the
best option is to remove the device. Since the proper functioning of the
equipment doesn't depend on any MOVs (in 99.9999 percent of the cases - the
exception being where the MOV is used as a high voltage triggering device
or something like that rather than a surge suppressor), remove the MOV(s),
test the equipment, and just replace the MOV(s) if in doubt.
(From: Brad Thompson (Brad_Thompson@pop.valley.net).)
Usually, the manufacturers specify a maximum leakage current (usually one
milliampere) at a AC specified voltage. You'd need a Variac adjustable AC
source, an isolation transformer (for safety), an AC voltmeter and an AC
milliammeter to make the measurement.
An MOV works as follows: It's essentially a batch of metallic-oxide grains
separated by insulating layers. Repeated voltage surges break down the
insulating layers, lowering the overall resistance and eventually causing the
device to draw too much current and trip whatever overcurrent protection is
inherent in the system.
I've seen MOVs exuding tiny metallic "teardrops" through their epoxy coatings,
which remained bright and shiny. These devices needed replacement!
(From: Kevin Carney (carneyke@us.ibm.com).)
This is not a valid test for breakdown voltage but these devices read a few
megohms when damaged. The new replacements read open on my meter that has a
20 Megohm range of a DMM.
To those how enjoy the theory!!
When is a Zener not a Zener??
If is voltage is over 6 volts, technically it's an avalanche diode.
Here's an engineering text quote that explains rather well:
The avalanche effect is different. In this case, when the diode is reverse
biased, minority carriers are flowing. For higher reverse
voltages these minority carriers can attain sufficient velocity to knock bound
electrons out of there outer shells. These released
electrons then attain sufficient velocity to dislodge more bound electrons, etc.
The process is well named, since it is suggestive
of an avalanche."
As noted, the voltage where the zener effect leaves off and the avalanche effect
takes over is approximately 6 volts. More accurately, the zener effect dominates
below 4 volts and the avalanche effect dominates above 6 volts and a combination
of the two between 4 and 6 volts.
Now, which one is in effect significantly affects the temperature characteristics
of the device.
For the horizontal (collector supply) you need a variable ramp generator.
If your scope has a sweep output, then you can derive it from this - if
you are not interested in frequency response, an audio amplifier may be
adequate with a volume control to adjust the amplitude.
For the base drive you need a programmable current source capable of putting
out a series of constant currents for the base drive. Here, a counter driving
a D/A set up for a current output mode. Use the trigger output or sweep
output of the scope to increment the counter so that it sequences through
a set of say, 10 current settings.
Then, you need some way of sensing collector current to drive the vertical
channel - a small series resistor in the emitter circuit, for example.
For simple diode tests, you can just use a variable AC voltage source like a
variable isolation transformer (with a current limiting resistor) across the
diode. The X (horizontal) input of the scope goes across the device under
test. The Y (vertical) input of the scope goes across the current limiting
resistor or a separate series current sense resistor. See the section:
Quick and Dirty Curve Tracer.
Then, you can jazz it up with microprocessor controlled on-screen display.
The following are circuits that are only a bit more complex than
the minimal solution:
And, here's a MAXIM application note on one that is driven from your
PC's parallel port:
Curve tracers can be big expensive things (e.g., multi-$K) or little add-ons
to regular scopes. Here one company selling a curve tracer kit or assembled.
I have no idea how good it is but check out
Gootee Systems for more
info.
Popular Electronics, May 1999, has complete plans for a "Semiconductor Tester"
which can handle NPN and PNP bipolar transistors, JFETs and MOSFETs, all sorts
of diodes including zeners, and a variety of other devices. This is basically
a curve tracer adapter for an oscilloscope. With a little ingenuity, it can
be enhanced to test virtually all the semiconductors discussed in this
document.
Therefore, if you want a sophisticated piece of test equipment, one of these
would be suitable. Or, get yourself a used Tektronix 575 curve tracer. This
will do just about everything you could possibly want (including the testing
of vacuum tubes with the addition of a bit of external circuitry.)
However, to just test 2 terminal devices - or to just get a feel for device
characteristics, there are much simpler, cheaper, alternatives.
I used a 12 VAC transformer just because it was handy. You can use anything
you like as long as you understand the safety implications of higher voltages
and make sure the components you use can withstand the power that might be
dissipated in them if the Device Under Test (DUT) is a dead short. In
addition, it is bad form to blow out the DUT while testing it! A signal
generator driving a small audio transformer could also be used if it is
desired to test components at frequencies other than 60 (or 50) Hz.
CAUTION: turn down the intensity of the scope so the spot is just barely
visible so that when there is no input, you don't end up drilling a hole
in the face of the CRT!
For higher power or higher voltage devices, substitute a suitable larger
transformer.
Modify these (selector switches might be nice) for your needs. A Variac
provides a convenient method of adjusting the voltage applied to the DUT.
The very complicated circuit is shown in: curve.gif
and below in ASCII.
CAUTION: Use at your own risk. I cannot absolutely guarantee that there
won't be certain devices in use today that didn't exist in 1975 that might
be unhappy with this approach.
(From: Wern Thiel (wern@zoo.toronto.edu).)
In the August 1975 issue of Popular Electronics author John T. Fyre wrote in a
story called "A simple On-Board Tester" about this fairly simple piece of test
equipment.
The device can be used with any type of oscilloscope and consists of a 6 volt
filament transformer, three 1/4 watt resistors and two test probes. Half of
the filament voltage is applied to a voltage divider consisting of 220 ohm and
100 ohm resistors, yielding 1 volt ac on top of the 1 K ohm resistor. This
voltage can be applied to any component or combination of components across
which the test leads are placed. The current is limited to one milliampere
by the 1 K ohm resistor.
The voltage across the probes is connected to the horizontal input of a scope
while the voltage across the 1 K ohm resistor as a result of the current
through it is connected to the vertical input.
What we see on the scope is a voltage across a component under test versus the
current through the component:
Capacitor: .1 uF Shallow ellipse.
2.6 uF Circle.
50. uF Narrow vertical.
Transformer: Ellipse depending on impedance.
Diodes (Germanium): Right angle display.
Diodes (Silicon): Right angle one side longer
(any leakage showing less sharp angle).
Transistors: Test as two diodes (B to E and B to C).
Integrated Circuits: Input for gates and counters show a certain signature
display.
Outputs display a different signature.
A short will show a vertical line.
An open will show a horizontal line
With some experience one is able to test components in and out of circuit and
troubleshoot without danger of a damage to components.
For FETs, just leave off the transformer.
(From: Michael Covington (mcovingt@ai.uga.edu).)
Get an old Tektronix 575 (mine cost $25 at a hamfest). That is a transistor
curve tracer that goes back to the 1950s and goes up to 200 volts.
It doesn't have FET settings, but you can control the 'base' and 'collector'
polarity independently. So what you do is put a 1 K resistor from 'base'
to ground, so that you can read milliamps as volts. Then put a positive-going
voltage on the 'collector' and a negative-going current into the 'base'.
For tubes, emitter, collector, and base are cathode, plate, and grid,
respectively. Naturally you also need a filament supply; I use a lab-type
DC supply because it's handy and can't introduce hum.
I also test FETs that way (without the filament supply, of course). Then,
emitter, collector, and base become source, drain, and gate respectively.
-- end V2.45 --
All Rights Reserved
2.There is no charge except to cover the costs of copying.
DISCLAIMER
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.
Introduction
Scope of This Document
The first part of this note describes procedures for testing of diodes
(signal, rectifier, and zener); bipolar (NPN or PNP, small signal and power)
transistors; SCRs, and MOSFETs for catastrophic failures like shorts and opens.
Safety Considerations
None of the tests described in this document require probing live circuits.
However, should you need to do so, see the document:
Safety Guidelines for
High Voltage and/or Line Powered Equipment first.
Testing Semiconductor Devices with a VOM or DMM
VOMs and DMMs
Analog and Digital meters behave quite differently when testing
nonlinear devices like diodes and transistors. It is recommended that
you read through this document in its entirety.
Testing Diode Junctions with a Multimeter
On an (analog) VOM, use the low ohms scale. A regular signal diode or
rectifier should read a low resistance (typically 2/3 scale or a couple
hundred ohms) in the forward direction and infinite (nearly) resistance
in the reverse direction. It should not read near 0 ohms (shorted) or
open in both directions. A germanium diode will result in a higher scale
reading (lower resistance) due to its lower voltage drop.
Transistor Testing Methodology
As with diode junctions, most digital meters show infinite resistance for
all 6 combinations of junction measurements since their effective resistance
test voltage is less than a junction diode drop (if you accidentally get your
skin involved it will show something between 200K and 2M Ohms). The best way
to test transistors with a DMM is to make use of the "diode test" function
which will be described after the analog test. For both methods, if you
read a short circuit (0 Ohms or voltage drop of 0) or the transistor
fails any of the readings, it is bad and must be replaced. This
discussion is for OUT OF CIRCUIT transistors *ONLY*.
C C
o o
| +--|>|---o C | +--|<|---o C
|/ | |/ |
B o---| = B o---+ B o---| = B o---+
|> | |< |
| +--|>|---o E | +--|<|---o E
o o
E E
NPN Transistor PNP Transistor
Obviously, simple diodes can be tested as well using the this technique.
However, LEDs (forward drop too high more most meters) and Zeners (reverse
breakdown - zener voltage - too large for most meters) cannot be fully
tested in this manner (see the specific sections on these devices).
Testing with a (Analog) VOM
For NPN transistors, lead "A" is black and lead "B" is red; for
PNP transistors, lead "A" is red and lead "B" is black (NOTE: this
is the standard polarity for resistance but many multi-meters have the
colors reversed since this makes the internal circuitry easier to design;
if the readings don't jive this way, switch the leads
and try it again). Start with lead "A" of your multi-meter on the base
and lead "B" on the emitter. You should get a reasonable low resistance
reading. Depending on scale, this could be anywhere from 100 ohms to
several K. The actual value is not critical as long as it is similar to
the reading you got with your 'known good diode test', above. All Silicon
devices will produce somewhat similar readings and all Germanium
devices will result in similar but lower resistance readings.
Testing with a (Digital) DMM
Set your meter to the diode test. Connect the red meter lead to the
base of the transistor. Connect the black meter lead to the emitter. A
good NPN transistor will read a JUNCTION DROP voltage of between .45v
and .9v. A good PNP transistor will read OPEN. Leave the red meter
lead on the base and move the black lead to the collector. The reading
should be the same as the previous test. Reverse the meter leads in
your hands and repeat the test. This time, connect the black meter lead
to the base of the transistor. Connect the red meter lead to the
emitter. A good PNP transistor will read a JUNCTION DROP voltage of
between .45v and .9v. A good NPN transistor will read OPEN. Leave the
black meter lead on the base and move the red lead to the collector.
The reading should be the same as the previous test. Place one meter
lead on the collector, the other on the emitter. The meter should read
OPEN. Reverse your meter leads. The meter should read OPEN. This is
the same for both NPN and PNP transistors.
Testing Power Transistors
Power transistors without internal damper diodes test just about like
small signal transistors using the dual diode model, high in one direction
B-E or B-C. If there is a built in damper diode, it is across C-E back
biased under normal operating conditions. Therefore, a reading between
C-E will also test low in one direction and B-C will show a double diode drop
in the reverse direction. Also, there is often a low value resistor - about
50 ohms - between B-E when there is a built in damper. This will show up
as a nearly zero volt junction drop on the diode test scale of a DMM but
such a reading does not indicate a bad part. Use the resistance scale
to confirm.
Testing Darlington Transistors
A Darlington is a special type of configuration usually consisting of 2
transistors fabricated on the same chip or at least mounted in the same
package. Discrete implementations as well as Darlingtons with more than
2 transistors are also possible.
Darlingtons are used where drive is limited and the high gain - typically over
1,000 - is needed. Frequency response is not usually that great, however.
C
o
|
+-------+
| |
B1 |/ C1 |
B o-----| |
|\ E1 |
| B2 |/ C2
+-----|
|\ E2
|
o
E
Testing with a VOM or DMM is basically similar to that of normal bipolar
transistors except that in the forward direction, B-E will measure higher than
a normal transistor on a VOM (but not open and 1.2 to 1.4 V on a DMM's diode
test range due to the pair of junctions in series. Note, 1.2 V may be too
high for some DMMs and thus a good Darlington may test open - confirm that the
open circuit reading on your DMM is higher than 1.4 V or check with a known
good Darlington.
Testing Digital or Bias Resistor Transistors
Occasionally you may find a transistor that includes an internal bias
resistor network attached to the base and emitter so that it can be driven
directly from a digital (e.g., TTL) source. These may be used in consumer
electronic equipment where space is critical or for no good reason other than
to make it difficult to locate a suitable replacement device!
C
o
|
R1 |/
B o---/\/\---+----| Typical R1, R1: 47K.
| |\
/ |
R2 \ |
/ |
| |
+------+
|
o
E
The addition of R1 makes testing with a multimeter other than for shorts
more difficult. With a VOM, you should see a difference in the B-E and B-C
junctions in the forward and reverse directions. However, a DMM will probably
read open across all pairs of terminals.
Testing Unijunction and Programmable Unijunction
Transistors
Unijunction Transistors (UJTs) and Programmable Unijunction Transistors (PUTs)
are used in similar sorts of circuits though the UJT is all but extinct.
They both exhibit a negative resistance characteristic and can be used easily
in low to medium frequency free running relaxation oscillators and other
trigger type circuits.
For an initial test, check between B1 and B2 (UJT) or A and K (PUT) with an
ohmmeter. The resistance should be the same in both directions and typically
a few K ohms or more. A short or wildly different readings would indicate a
bad device.
+5 VDC o--------+---------+
| |
/ |
R1 \ |
100K / |
\ |
| |B2
+-----. |-+
| \| Q1 UJT
| E|-+--------o
| |B1
C1 _|_ / To scope or
.01uF --- R3 \ audio amp
| 1K / ~1K Hz
| \
| |
Gnd o---------+---------+--------o
+10 VDC o--------+-----------------+
| |
/ /
R1 \ R3 \
100K / 1K /
\ \
| |
| +---+
+---------+ | |
| |A .G |
| Q1 __|__/ |
| PUT _\_/_ |
| |K |
| +-------|------o
| | |
C1 _|_ / / To scope or
.01uF --- R2 \ R4 \ audio amp
| 1K / 1K / ~1K Hz
| \ \
| | |
Gnd o---------+---------+-------+------o
_
/ \
A G -----
| / | |
----- | |
\ / PUT ----- 2N6028
----- | | |
| K A G K
Testing a Photodiode
Photodiodes are used in all sorts of equipment from PC mice (those with a
ball) to high power lasers (for monitoring the output power). They are
generally very reliable and rarely fail on their own. However, some types
are susceptible to damage from ESD and other abuse.
Thyristors - SCRs and Triacs
What are Thyristors
Thyristors are used to control power in numerous applications including
light dimmers and motor speed controls, solid state relays, some microwave
ovens, photocopiers, traction motors for electric locomotives and electric
cars, power inverters for transmission of electric power over long distances,
frequency converters, other DC-DC or DC-AC or AC-AC inverters, AC-DC regulated
power supplies, and many other applications where efficient power control
is required.
How Does a Thyristor Work?
The usual way an SCR is described is with an analogy to a pair of cross
connected transistors - one is NPN and the other is PNP. The base of the NPN
is connected to the collector of the PNP and the base of the PNP is connected
to the collector of the NPN.
+------+
+ >------------+ LOAD +----------------+
+------+ |
|
E \|
PNP |---+-------< IG(-)
C /| |
| |
| |/ C
Gate IG(+) >-----+---| NPN
|\ E
|
|
- >------------------------------------------+
A Triac works in a basically similar manner except that the polarity of the
Gate can be either + or - during either half cycle of an AC cycle.
Testing SCRs and Triacs
R1
+ o----+-----/\/\---------+-----o Test+
| 100, 2W __|__
| _\/\_ Device Under Test - DUT
12 VDC | R2 / | (SCR or triac).
+---/\/\---o <--' |
1K o |
| |
- o----------------+------+-----o Test-
Testing Diacs and Sidacs
Diacs and Sidacs are thyristors without any gate terminal. They depend on the
leakage current to switch them on once the voltage across the device exceeds
their specified ratings. With an ohmmeter, they can be tested only for shorts.
Resistance should be infinite in both directions.
D1 R1
~ o----|>|----+-----/\/\---------+------o Test+
1N4007 | 47K |
_|_ C1 __|__
Variable AC --- 1 uF _\/\_ Device Under Test - DUT
0 to 140 VRMS | |
| |
~ o-----------+------------------+------o Test-
CAUTION: this is not isolated from the power line. Use an isolation
transformer for safety. If the DUT is rated more than about 180 V, you will
need to use a doubler and higher voltage capacitor but testing is otherwise
similar.
Thyristors Driving Inductive Loads
"I am trying to turn on a triac which is driving an inductive load (solenoid)
using a digital signal without using an opto triac. I get limited success."
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
Burning Up of Thyristors
(From: Neill Means (means@expert.cc.purdue.edu).)
Additional Semiconductor Tests
Identifying Unknown Bipolar Transistors
The type (NPN or PNP) and lead arrangement of unmarked transistors can be
determined using a multimeter based on similar considerations. This, again,
assumes the back-to-back diode model. The collector and emitter can then be
identified based on the fact that the doping for the B-E junction is
always much higher than for the B-C junction. Therefore, the forward
voltage drop will be very slightly higher - this will show up as a couple
of mV (sometimes more) difference on a DMM's diode-test scale or a slightly
higher resistance on an analog VOM.
Luke's Comments on Junction Voltage Drops and
Doping
(From: Luke Enriquez VK3DLE (ecsclfe@lux.latrobe.edu.au).)
Vdrop = Vt * ln ( Na*Nd/ni2 )
where:
Vt = kT/q = 26 mV at 300 degrees K
ni = intrinsic carrier concentration in a pure sample of silicon
(ni = 1.5 * 1010 cm-3 at 300 deg K for silicon)
Nd = doping density atoms/cm3 in the n-type material
Na = doping density atoms/cm3 in the p-type material
This equation means that if the doping density at the Base-Emitter junction
is higher than the Base-Collector junction, the Vdrop of the Base-Emitter
junction will be higher than that of the Base-Collector junction.
Transistor B-C Voltage B-E Voltage
--------------------------------------------------
TIP3055 0.640 0.642
TIP2955 0.668 0.668
BD140 0.697 0.699
2N2369A 0.682 0.710
PN3563 0.752 0.753
BC108 0.715 0.716
CAUTION: Do not hold the transistor under test in your hand. For every degree
the transistor increases in tempreture, the Base-Emitter Diode Drop (commonly
called Vbe) decreases by 2 mV. This is a significant amount when determining
the B-E and B-C junctions.
Lance's Method for Determining C and E on an Unmarked
Bipolar Transistor
(Slightly edited for readability --- sam)
Testing MOSFETs
(From: Paul Mathews (optoeng@whidbey.com).)
The usual failure mode: GS short AND DS short. In other words, everything
connected together.
Testing IGBT
Basic testing IGBTs (Insulated Gate Bipolar Transistors) should be similar
to an enhancement mode N-channel MOSFET except that the threshold voltage
may be larger than a typical MOSFET (e.g., 8 V instead of 4 V).
Testing LEDs
Electrically, LEDs (and IR emitting diodes, strictly speaking called IREDs)
behave like ordinary diodes except that their forward voltage drop is higher.
Testing Opto-Isolators and Photo-Interrupters
Both these classes of components are basically similar: a light source (usually
an IR LED) and photodetector together in a single package.
For both types, the photodetector can be a photodiode, phototransistor,
photothyristor, or other more complex device or circuit.
+5 o-----+------------------+
| |
/ /
\ 500 \ 5K
/ /
\ \
| _|_ S1 |
+--- ----+ +-------------o Out
- - | - - - - | -
: __|__ _|_ :
LED : _\_/_ ---> /_\ : Phododiode
: | | :
- - | - - - - | -
Gnd o--------------+---------+-------------o Gnd
Depressing S1 should result in the Output dropping from +5 V to close to 0 V.
For monitoring on a scope, drive the LED with a pulse generator and current
limiting resistor instead of S1. With a photo-interrupter type, blocking
or adding a reflector to the optical path (as appropriate) should result in
similar behavior.
Testing Thermistors
There are two types of thermistors:
For a small thermistor, put an ohmmeter on it and the heat it up with a blow
dryer, heat gun, or the tip of a soldering iron - the resistance should change
smoothly (up or down depending on whether it is PTC or NTC type). If the
resistance changes erratically, or goes to infinity or zero, the device is
bad. However, you will need specifications, temperature measuring sensors,
etc. to really determine if it is operating correctly.
Testing Thermo-Electric (Peltier) Devices
While these are often called Thermo-Electric Coolers (TECs), they are equally
good (or poor) at heating. The typical TEC uses a series connected string
of thermocouple-like junctions sandwiched between a pair of ceramic plates.
They are generally specified in terms of maximum temperature difference
(typically between 60 to 70 °C); maximum current, voltage, and power
dissipation; and physical dimensions.
Miscellaneous Information
Differences Between Ordinary, Fast Recovery, and Schottky
Diodes
When the polarity reverses on a diode, it takes finite time for the charge
carriers to be cleared from the area of the junction. During this time,
reverse current flows. For high frequency applications - i.e., switching
power supplies, horizontal deflection circuits, etc. - a normal diode would
act more like a short circuit and result in poor performance or even burn
out.
Horizontal Output Transistor Pinouts
You will nearly always find one of two types of horizontal output transistors
in TVs and monitors:
Some other transistor types use the same pinout (TO66 for metal can, TO218
and TO220 for plastic tab) but not all. However, for horizontal output
transistors, these pinouts should be valid.
_
/ O \ View from bottom (pin side).
/ o o \
( B E ) B = Base, E = Emitter, C = Collector.
\ /
\ O / C The metal case is the Collector.
TO3Pn
TO220 _____
_____ / \
| o | | O | View from front (label side).
|-----| |-------|
|Label| | | B = Base, E = Emitter, C = Collector.
|_____| | Label |
| | | |_______| If there is an exposed metal tab, that is
| | | | | | the Collector as well.
B C E | | |
B C E
Difference Between Normal and 'R' Marked Parts
"Does anyone know the difference between transistor BDY58R and BDY58 (if any
at all)?"
(From: Paul Grohe (grohe@galaxy.nsc.com).)
C C
____|_|____ ____|_|____
| | | |
|___________| |___________|
| | | | | | | |
B E E B
Top view of 'normal' SOT Top view of 'R' SOT
This makes layout of high-frequency pairs easier because traces do not have to
cross over one another, and the layout is 'cleaner' but bites you if you are
unaware!
Testing Zener Diodes
The following applies to both testing of zeners for failure and determining
the ratings of an unknown device. Zeners most often fail short or open
with short probably more likely. However, it is also possible, though a lot
less common, for the zener voltage to change (almost always to a lower voltage)
and/or for the shape of the I-V curve to change dramatically (e.g., become
less sharp cornered).
R=2K
24VDC o----------/\/\----------+----------o
_|_. +
'/_\ ZD VOM/DMM
| -
Gnd o------------------------+----------o
This same approach applies to other devices that exhibit a similar behavior
such as the B-E junction of a bipolar transistor.
Testing MOVs
MOVs are used mostly for surge suppression in power strips and the front-ends
of the power supplies of TVs, VCRs, and other consumer electronic equipment.
They are those brightly colored things that look like Epoxy dipped capacitors.
At least, that's what they look like when new. A common failure mode is for
the MOV to be totally obliterated by a surge or from old age. Then testing
is not needed! :)
More Gory Details on Zeners and Similar Diodes
(From: Gord Neish (gord.neish@sk.sympatico.ca).)
"The zener effect refers to removing bound electrons from outer shells by means
of an electric field. In other words, as a reverse
voltage is applied to a diode, an electric field appears at the junction. When
this field is intense enough, outer-shell electrons are
dislodged, resulting in a significant increase in reverse current.
Introduction to Curve Tracers
Curve Tracer Design
A curve tracer is a piece of test equipment or an add-on to an oscilloscope
which provides a graphical display of the V-I (or other parameters) of an
electronic component. The design of a curve tracer is simple in principle
(description here for bipolar transistors):
Quick and Dirty Curve Tracer
I threw the following circuit together in about 10 minutes. With minor
modifications, it is capable of displaying V-I curves for diodes, zeners,
transistors, thyristors, resistors, capacitors, inductors, etc.
o-------+ X R1
)|| +------/\/\/\------+----o Horizontal Scope Input
)||( | (Voltage Display)
)||( o
Adjustable AC )||( +
from Variac )||( DUT
)||( -
)||( o
)||( Y R2 |
)|| +--+---/\/\/\------+----o Scope Ground
o-------+ |
T1 +--------------------o Vertical Scope Input Inverted)
(Current Display)
In-Circuit Tester
The following is along the same lines as the "Quick and Dirty Curve Tracer"
but is suitable for in-circuit testing as the current and voltage are limited
to safe values for most devices (less than 1 VAC and than 1 mAAC respectively).
220
o-----/\/\-------+---------+-------------o Vertical Scope Input
| |
| \
To the 3 V | / 1 K
winding of a | \
6 V center / /
tapped power \ 100 |
transformer / o-------------o Ground
\ |
| o
| Red component test lead
|
| Black component test lead
| o
| |
o----------------+---------+-------------o Horizontal Scope Input
Resistors: Open Horizontal line.
10 K 10 degree.
1 K 45 degree.
0 Vertical line.
In circuit testing is done with *no* power applied to the equipment under test.
Testing Vacuum Tubes (or FETs) on a Bipolar Curve
Tracer
A transistor curve tracer can be easily adapted to test vacuum tubes (OK,
valves for those of you on the other side of the lake) if it has an adequate
voltage range for the collector (now plate) drive and independent control of
base and collector polarity. All that is needed is to add a separate
transformer to power the tube's filament(s) and a resistor to convert base
current to voltage.
