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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.
For really high voltage equipment, also see:
Tesla Coils Safety Information.
WARNING: The microwave oven is perhaps the most dangerous equipment you are
likely to encounter around the house. The high voltage (up to 5,000 V) along
with the high current (1 A or more) availability make this an instantly lethal
combination. It is highly recommended that NO measurements be made on a
powered microwave oven. Only after the plug has been pulled and its high
voltage capacitor has been safely discharged should you even think about
touching or probing anything. Most troubleshooting can be done with at most
an ohmmeter. See the document:
Notes on the
Troubleshooting and Repair of Microwave Ovens for more information.
By comparison, TVs, monitors, and even large helium-neon lasers, are tame.
While still very dangerous, they don't have quite the deadly quality of the
microwave oven!
This document provides information on constructing very basic high voltage
probes suitable for measuring the high voltages found in consumer electronic
equipment like TVs, monitors, and microwave ovens (though the latter is not
recommended for safety reasons).
These simple approaches will work for DC and low frequency AC voltages but
no effort is made to compensate for stray capacitance - which will seriously
limit high frequency response. However, some of the issues are discussed.
If you will be making HV measurements regularly, by all means invest in a
real HV probe for your multimeter. A commercial HV probe will still be a
far better long term investment than some cobbled-together unit. However,
for occasional HV testing, what is described below can be built and used
safely but probably won't have the accuracy, consistency, or frequency
response of a good commercial probe. Aside from purchasing a HV probe new,
these do show up surplus as well as on eBay, possibly at greatly reduced
prices. Even if a model isn't available for your particular multimeter
(which is likely), it should be possible to adapt almost any commercial
probe to work with it, requiring at most a scaling factor when taking a
reading.
A simple high voltage probe for a DMM or VOM may be constructed from a pair of
resistors. This is suitable for DC measurements but without compensation,
will have a unknown AC response due to the very high impedance and stray
capacitance forming a filter - low pass or high pass depending on the amount
of stray capacitance and input capacitance of your meter or scope. However,
this simple design is sufficient for the majority of consumer electronics work
which are mostly DC measurements. I have not characterized the AC response of
this probe design. However, if there is AC riding on your high voltage, it
may mess up your readings if there is no compensation provided as it may act
as a high pass filter.
To design the voltage divider, the input impedance of the meter must be taken
into account. There is a minor but significant difference between DMMs and
VOMs.
While R2 is not strictly needed, it is recommended that it be included and
approximately equal to the Z-in of the meter on the scale you will be using.
The reason to include R2 is to insure that high voltage never can reach the
meter. The ground clip should be securely connected to the metal chassis of
the device being tested - the frame of a microwave oven or CRT
grounding/mounting strap of a TV or monitor - before it is powered up. Both
R1 and R2 should be located in the probe head.
The only difficult part is locating a suitable resistor for R1 that has high
enough resistance and physically is long enough such that arc-over is avoided.
The only difficult part is locating a suitable resistor for R1 that has high
enough resistance and physically is long enough such that arc-over is avoided.
Caddock,
OhmCraft,
Victoreen, and
Vishay
are among the major companies that manufacture suitable resstors.
But don't expect them to pay much attention to you for an order of 5
resistors! However, it may be possible obtain free samples if you explain
what you're doing - and their lawyers don't get involved! If this doesn't
work out, electronics surplus outfits occasionally come up with odd lots of
strange components such as these and they even show up on eBay from
time-to-time.
The high value high voltage resistor can also be constructed from several equal
lower value resistors in series if they are all approximately the same size.
Another possibility is salvaging the focus divider networks from dead flybacks
or TV/monitor voltage multiplier assemblies. Even if the unit was discarded
as being faulty, where there are no internal shorts in the HV rectifier or
resistive network itself, the entire unit can be used intact.
In addition to basic safety precautions when working around high voltages,
some form of equipment protection should be considered in provide an arc-over
path to ground should there be arcing over the surface of the resistor as well
as if the resistor should somehow decrease in value. There is no telling what
can happen under less than ideal damp or dirty conditions.
A 'corona', 'arc', or 'discharge' ring could be placed around the resistor near
the low voltage end securely connected to the ground cable. The idea is that
any arcing over the surface should find this as its destination before
obliterating your meter.
A variety of devices could be placed across R2 to limit the maximum voltage
present in the event of a breakdown. Suitable devices include neon light
bulbs (NE2s without resistors); zener, avalanche, or ordinary diodes; or other
semiconductor junction devices. Traditional surge suppressors like MOVs and
Tranzorbs may work but their off-state impedance may be too low compared to
R2). The neon bulb is good since its impedance is essentially infinite until
its breakdown of 90 volts or so is reached. In some cases, these devices will
be destroyed (semiconductors may short) but they will have served their
protective function and are a small price to pay to prevent you and your
meter from being blown.
If you are only interested in DC measurements, putting a .1 uF capacitor
across R2 should smooth out any 50/60 Hz or higher frequency ripple.
The implementation of full probe compensation is left as an exercise for the
motivated student.
To minimally load the circuit under test, R1 should be as high as practical.
Practical here means (1) low enough so that leakage over its surface is not a
problem, (2) low enough that a reasonable voltage can be developed across
R2||R3, and high enough so that loading of equipment being tested will not
change the readings by more than a few percent.
R2||R3 is 5M ohm. Selecting R1 to be 4,995M ohm will give a 1000:1 ratio so
that 50,000 volts will read out as 50 V on the DMM. 4,995M ohm is high
enough that loading of a 250M ohm focus network should not be an issue (5%).
1000:1 is a nice easy to remember ratio. You could go to something higher if
loading is still a concern but then leakage current over the surface of R1
becomes an even greater concern. Even 5,000M ohm is about as close to an
open circuit as you can get - any contamination whatsoever will change the
calibration significantly. You may find that using a resistance around
1,000M will result in less of a problem and accept the circuit loading that
this value implies.
For all practical purposes, you can use 5,000M instead of the exact value of
4,995M. The error of about 0.1 percent will be less than the error spec of
most portable DMMs and much less than the error spec of any VOM. And, you
probably aren't going to risk your expensive precision bench multimeter on
this lunacy anyhow!
It is a simple matter to determine a scale and an R2 such that the actual high
voltage measurement is easily calculated from the meter reading. What you
want is the ratio of R1 to R2||R3 to be a nice round number. Note that
switching ranges will produce some peculiar behavior due to this current
division between R2 and R3. A unique R2 must be selected for each range of
interest. You are already using nearly the maximum sensitivity of the meter
and switching to a lower range will only slightly change the position of the
needle unless you construct a range switch box as shown below.
This circuit uses only a 203M ohm high voltage resistor. Since the internal
resistance of a typical focus divider network is 200-300M ohm, this probe
would obviously load such a circuit excessively.
Modifications to use a higher value R1 are straightforward.
This detail is important for safety reasons. If the connection to the scope
becomes disconnected then their will not be a dangerous shock hazard as
would be the case if the scope was in series.
You can do this with a high voltage resistor divider network. That is what is
in a high voltage probe you would buy. This can be very dangerous to you and
your equipment in the event of a failure. Please be very careful. I suggest a
fuse at the probe input and an MOV across the resistor to ground that will
connect to the scope and use a plexiglass tube to put it all in to contain the
bits if anything blows up.
(From: Kevin Astir (kferguson@aquilagroup.com).)
With respect to preventing high voltage arcing and corona, *do not* use RTV.
Places that carry the GC line will have some 'anti corona discharge dope'
often called 'Q-dope'. This is *the* stuff to use at HV. You can clean it
off with acetone when you discover that you didn't clean flux off good enough
and have an arc underneath. Epoxy and RTV have no such advantage, and RTV
releases corrosive acid while curing to boot.
Heed the warnings of other respondents WRT resistor voltage. As they said 100
V per for garden variety resistors will yield a safe margin. 200V is typical
max rating.
There are special HV resistors (up to 10 kV or so) made, available into the G
Ohm range. I don't know of a hobbiest source however. If you know anyone who
works in nuclear instrumentation field they may be able to snag one for
you. (HV used as detector and PMT bias in radiation detectors). This is what
will be inside "real" HV probe from Fluke, or Tektronix.
Finally, I have a lot of experience, and am fairly blase' around HV, but in
addition to "normal" 115V AC rules, (no rings, one hand in pocket, etc.) I
*never* work on HV stuff (not even a TV or hi-pot test) alone. And, I make
sure the 'observer' knows CPR, even if I have to wait 2 days to fix TV, so
girl friend can 'help'.
You can calculate this for yourself. The parasitic lead-to-lead capacitance
of a typical small resistor is 0.05 to 0.2pF. The capacitance from the
*middle* of the resistor and from any connection node between series resistors,
to ground, may range from 1pF to 5pF or more depending upon your choice of a
shielding scheme. Longer glass resistors intended for high voltages have
lower lead-to-lead capacitance, but higher distributed parallel capacitance.
As a worst case, imagine a 1000M ohm probe made with a 2-inch long resistor.
To start, place the capacitance to ground from the midpoint. If you assume
5pF of parallel capacitance, you'll see you're in trouble even at 60Hz!
One solution invokes the capacitance from a few carefully-placed concentric
sleeves connected to the input and the signal output, plus an overall shield
or guard connected to ground.
One solution is to make C1 very large, but it's just a matter of specs - if
you want 1% performance over the whole range, C1 is a severe load. There is
a good overall solution, which I think is fairly clever (after thinking of it,
I discovered the experts had beat me to it!).
The usual method is simply to use a capacitive divider, a small 1 kV
capacitor, etc., or make the HV capacitor yourself for really high voltages,
like 5 to 20 kV, use an air neutralizing capacitor, etc.
Say for example, its a 3pF capacitor. With shields. With another more
conventional capacitor, say 3000pF for the bottom of the attenuator, followed
with a voltage buffer if desired, and you've got a nice wide-band 1000:1 HV
probe installed in the system, good for mucho kV.
First let me strongly say that designing HV AC probes which include
a resistor for DC measurements is not trivial.
Read the rest of this document including my other comments on this
subject in the previous sections.
You can see details of an impressive 500 kV five-foot probe design,
Rob's
High Voltage Probe Page. Lacking a shield, this probe is suitable
only for DC or low-frequency AC use.
For your purposes a simple ac-only probe should suffice. Happily
they are relatively easy to make. The basic principle is to make
a capacitive divider. For example, a 3pF HV input capacitor with a
3000 pF load capacitor will make a 1000:1 divider with perfect high-
frequency response. You can use a home-made 3 pF, 30 kV air capacitor.
Used with a standard 1M scope input the probe will have a 3 ms droop
time constant and low-frequency roll-off of 53 Hz. This is suitable
for measuring all kinds of fast HV signals like auto ignition pulses,
camera-flash triggers and discharges, TV flyback transformer outputs
(before the HV diodes), Tesla-coil primary voltages, atomic-trap ring
voltages, etc. A 30 kV input will present a safe 30 V to the scope.
Because your 3 pF capacitor must be able to withstand 30 kV, it'll be
much larger than you would at first imagine. For example, I have
used two 1/4-inch thick 4-inch diameter discs placed 1/2-inch apart,
IIRC, held in place by metal bars to a long ceramic rod located off
to one side. Both discs had 1/4" rounded sides to prevent corona
discharge. The "lower" disc was grounded to act as a shield. The
low-voltage sense electrode was a thin copper shim-stock disc with
a slightly smaller diameter, stuck to the ground disc with double-
sided mylar tape. A hole in the ground disc allowed a coax cable
connection to the copper sensing disc.
A smaller design might use two concentric metal tubes, with an outer
shield and an inner low-voltage electrode. The HV would be presented
on a wire held in place in the center, making only 3pF of capacitance
to the inner tube. Holding the wire in place with a sturdy tip mount
without unduly increasing the capacitance would be a design issue.
Calibrate the probes with low voltage AC signals and an AC RMS DVM.
A second issue is protecting the scope. You have to absolutely sure
your homemade 30 kV capacitor will not have a small breakdown event
and destroy your scope! One solution is to make a 1:3000 divider so
the output is limited to 10 V and use a diode-protected opamp follower.
Also, with the 10M resistor the droop time constant is better, 100 ms
and the low-frequency -3dB point is lower, 1.6 Hz.
Both Tektronix and Hewlett Packard sell HV probes rated at 5kV and up.
Bandwidths are (at least) into the 100kHz area, probably more. I imagine
there are others.
The older Tek probes even had ports to refill with now-banned chemicals. The
newer ones don't, but are more expensive.
Frequency response is a significant concern. Designing and manufacturing a
decent HV probe is definitely non-trivial if you need flat frequency response.
Many parts have significant voltage coefficients, too, as well as breakdown
voltages.
(From Winfield Hill (hill@rowland.org).)
A significant part of the design effort (and cost) deals with, the problem of
how to go smoothly from a resistive divider at low frequencies, to a
capacitive divider at high frequencies, while keeping a constant attenuation
value at mid-frequencies. This isn't easy. Consider for example, that an
overall shield is clearly needed and must properly prevent the high-Z end of
the probe from simply acting as an antenna (as some HV probes do! i.e. ground
the tip of the probe and *still* see large signals at the output). This
shield acts as a capacitance to ground for the HV resistor, routing some of
the high-frequency current which is supposed to go to the output, to ground.
Hence at some middle frequency there's a dip! This is solved in various ways
- with shields connected to the probe tip (but inside the ground), capacitors
bypassing the resistor, special resistor construction, etc. Most solutions
can just as easily cause a region with a response hump, as well as a dip, or
even both. BTW, these problems are much harder if one seeks to make a probe
with very low capacitive loading and high frequency response. The Tek P6015A
probe is 3pF, and you'll also note it has a veritable raft of response
adjustments on the scope-input end.
Much of the cost of the probe is knowing how to do all this!
Incidentally, a low-cost intermediate-range HV probe is the Fluke PM9100,
which is a 4kV 100:1 probe with a 200MHz bandwidth. Also the Tek P5100 is
rated to 2.5kV. Most of these probes also have a derating above some d
frequency.
Most of this mess you can avoid entirely by not attempting to make the probe
measure DC (or at least not the whole frequency range).
(From: Sam.)
The person who contributed the following comments may not be totally unbiased
but the information is still valid.
(From: Cicel Clenci (cicel@cic-research.com).)
I used many different probes on high voltage measurements and found out that
their performance is terrible when exposed to even relatively low common mode
voltage transients (100 V or more). Even when using differential probes like
Tektronix's P5200 or P5205 measurements can be influenced by common mode
voltage transients. You will get glitches on the output that are not there,
these will confuse the engineers. One big problem is the high input
capacitance of the probe. In order to get the best common mode rejection of
the various transients and an accurate representation of the input waveform,
you must reduce the input capacitance. 4 pF or 3 pF input capacitance is too
high, when dealing with high voltage fast transients, and the compensation is
not as easy as it might seem. Look for 0.5 pF or lower input capacitance.
There are many issues that need to be addressed when designing high voltage
(differential probes). Just take a look at
CIC Research HV Probes
Page for probes that outperform Tektronix's or LeCroy's probes.
-- end V1.28 --
All Rights Reserved
2.There is no charge except to cover the costs of copying.
DISCLAIMER
The devices, equipment, circuits, and other gadgets described in this document
may be dangerous. Much of it deals with potentially lethal voltages. Getting
electrocuted could ruin your whole day. Using an inadequate or improperly
designed or fabricated high voltage probe to measure high voltage can be
equally dangerous.
Introduction
Scope and Purpose of This Document
There are all sorts of times when being able to determine the value of a
high voltage DC source is desirable. Most multimeters have a maximum range
of 750 or 1,000 V. (One exception is the workhorse Simpson 260 which has a
5,000 V range). Whether testing a TV with a dim picture, a helium-neon
laser power supply that does work quite right, or troubleshooting some
home-built high voltage project, the ability to measure 10, 20, 30, or more
kV can come in handy.
SAFETY
Read the associated document: Safety Guidelines for High
Voltage and/or Line Powered Equipment before attempting to work with high
voltage systems. High voltage can jump amazing distances when you least
expect it. The direct or indirect consequences of this can ruin your entire
day or a whole lot more.
High Voltage Probe Design
Basic Considerations
CAUTION: DMMs may not be particularly forgiving of voltages on their inputs
exceeding their specifications. Autoranging DMMs may be even more likely to
blowout as they are selecting the correct range - if there even is one.
Depending on your electrical and mechanical components, the chance of excess
voltage due to arc-over, leakage, or component breakdown may be a major
consideration. My analog VOM has survived many close encounters with HV. You
should not assume the same for the typical low cost or even expensive DMM.
There is a reason for the high cost of commercial HV probes - these kinds of
factors are incorporated (hopefully) in their design.
Here is the basic circuit:
High Voltage <------/\/\/\/\/\---------+-------------> + to DMM/VOM
R1 | |
\ \
R2 / R3 /
\ \
/ /
| |
Ground Clip <-------------------------+-------------> - to DMM/VOM
R1 together with R2||R3 form a voltage divider where R3 is the internal
resistance of the DMM or VOM on the scale for which the probe is designed.
Frequency Response
Probe compensation similar to that used on oscilloscope probes can be
implemented. However, the determination of the capacitor values is beyond the
scope of this note. To put it simply, the ratio of the capacitance C1:C2
(where C1 is across R1 and C2 is across R2||R3) needs to be equal to the ratio
of R2||R3 to R1 (or equivalently, to the inverse of the voltage divider
ratio). C2 includes the stray capacitance and input capacitance of the meter
or scope probe. The capacitor across R1 would need to sustain the HV so that
is another complication. Since a 10x scope probe usually has an input
impedance of 10M, the same design as used for the DMM would work with a
scope. Although I have not pursued this issue, it sounds like based on the
ratio (1000:1 would mean that C1 would need to be extremely small, probably
smaller than the stray capacitance of the R1 and the associated wire) you
would need to add a capacitance for C2 and that there will be enough stray
capacitance such that no physical C1 will be needed.
Simple High Voltage Probe Design Examples
50,000 V Maximum Using a 10Mohm Z-in DMM
By my rule above, I will select R2 to be 10M ohms. Fine adjustment of
calibration could be made by making R2 out of a combination of a fixed
resistor and a multiturn pot.
50,000 V Maximum Using a 30kohm/V VOM
This is a little more complicated because you need to pick a range and then
calculate the Z-in for that range. for example, for the 100 V range of a
30kohm/V VOM, the Z-in will be 3M ohms. For the same 5,000M ohm R1 and 10 M
ohm R2, you would get a reading of 23 V (roughly) on the 100 V scale for a 50
kV input. The divide ratio in this case is about 218.
Sample Circuit
I have constructed a high voltage probe from the surplus bleeder resistor from
a defunct video terminal. For the probe tip, I used a discarded probe from a
VOM. The resistor and probe tip were mounted inside an insulating plastic
tube with R2 included at its base. A ground cable with an allegator clip
provided the connection to the chassis. A second pair of wires with banana
plugs connected to the meter via a switchbox which could select between a DMM
or a couple of different scales on a VOM. Potting the entire HV head is a
good idea to minimize the possibility of arc-over. Remember that 50,000 V can
jump several inches (2 inches in dry air approximately). See above text for
other suggestions on equipment/you protection (which is not shown).
.
High Voltage <----/\/\/\/\-----+--------+-------------+----o + to DMM/VOM
R1 | . | |
203M | . \ |
(15W HV rated) | . / R4 |
\ . \ 360K SW1 o
R2 / . / /
1M \ . | 3 o o o 1
/ . | | 2 |
| . \ \ \
| . +->/ R5 R6 / / R7
| . | \ 25K 1M \ \ 810K
| . | / Adj / /
| . | | | |
Ground Clip <-----------------+-----+--+-----------+---+--o - to DMM/VOM
.
Probe Head . Range Switch Box
.
Calibration
Unless you have a calibrated HV supply, a working TV for which you have the
service manual makes a good starting point. The proper high voltage is
usually specified to within 5-10%. If you have a line-transformer based HV
supply (e.g., neon sign transformer, rectifier, capacitor), then this would be
pretty accurate based on your power line voltage. For a DMM with a constant
input resistance, you can use a low voltage (like a few hundred V) on a lower
range and extrapolate for the HV range. However, for a VOM, you cannot use
this technique since changing ranges also changes the parallel resistance of
R2||R3. You are already using nearly the full sensitivity of the meter.
More Information on High Voltage Probes
Construction of High Voltage Probes
(From: Duane C. Johnson (redrok@pclink.com).)
(From: Larry G. Nelson Sr. (nr@ma.ultranet.com).)
High Voltage Probe Frequency Response
(From: Winfield Hill (hill@rowland.org).)
The classic low-voltage probe architecture of a pair pf RCs doesn't work for
HV scope probes, unless (1) you're willing to have an overly high capacitive
loading, or (2) you don't care about mid-frequency or pulse-shape response
accuracy. This is because the RES1 value will be very high, 100M or more
likely 1000M ohms, and physically long and large. So the real circuit is
like:
+------------ CAP1 -------------+
| |
Probe tip o--------- Rs -*- Rs -*- Rs -*- Rs -------- etc
| | |
Cs Cs Cs
| | |
Gnd Gnd Gnd
Because the Rs are so high, the probe becomes a good antenna, and a shield is
mandatory. Therefore the Cs "stray" capacitance is higher than you might
think. I think you see the problem.
High Voltage Probes for AC Measurements
(From Winfield Hill (hill@rowland.org).)
More on Measuring High Frequency High Voltage
(The following was prompted by a request to measure the pulses in a capacitive
discharge ignition system.)
(From: Winfield Hill (hill@rowland.org).)
You want to measure the voltage pulse or spike, so you'll need a high voltage
high-frequency probe. Many popular HV DC probes are not suitable, such as the
Fluke 80K-40 probe. Fluke does have a high-frequency HV probe, the PM9100,
which is a 200 MHz, 4 kV probe. For voltages higher than 4 kV, use a
Tektronix P6015A probe (buy one on eBay), or you can make a probe yourself.
3pF 3000pF
O---||--+--||-- GND
\ | ______________
''--> '--)_____________)-- scope
oops!
Sounds simple, but there are a few problems. A big one is labeled
oops! on the drawing above, namely unplanned capacitive paths from
your HV signal to the divider junction. To prevent this you'll need
some cleverly designed shields.
,------------- HV
/
,--------------------------------------+--,
| |
'-----------------------------------------'
_____________________________________
,------ | --------------------------------,
| | |
'------ | -+------------------------------'
| |_________
'--)_________ cable
Keep in mind that you should get zero output with the HV probe tip
grounded - with the wrong shield layout this is a tough requirement.
3.33pF 0.01uF
O---||--+--||-- GND pair of diodes to +/-15V
| __________ | FET opamp
'--)_________)- 10k -+-+---|\ follower
coax | | | >--+-- 50 ---o To scope
GND 10M ,-|/ |
| '------'
GND
Commercial High Voltage Probes
(From: Frank Miles (fpm@u.washington.edu).)