for testing of any high voltage component
vacuum tubes, high voltage capacitors, vacuum relays,
components BEFORE circuit operation prevents equipment
just hook up component, adjust the high voltage
with panel variac, then read leakage current on
make at home. Requires a block of wood for the base
and Plexiglass for the panel and enclosure.
components used in Amateur Radio home projects are used
or may have been in stock for years prior to purchase.
It is important to have a method to test these parts prior
to installation in equipment. This is especially true
for components that will be used in high voltage circuitry,
where the potential for catastrophic component failure
is highest. During an amplifier project, it became necessary
to test new old stock Russian military surplus vacuum
Since commercially available high voltage testers are
relatively expensive, I decided to build a low cost version
suitable for home workshop use. Testing a variety of high
voltage components like vacuum tubes, vacuum capacitors,
vacuum relays, high voltage diodes and transistors would
then be possible. This project can be described as a junk
box effort. To increase the chances of successfully completing
this project, two different circuits are offered that
generate the variable DC high voltage.
Always test newly acquired tubes and high voltage components
with a tester before use. A simple Ohmmeter test is not
complete one page schematic
Voltage Tester Basics
breakdown is where a voltage increase causes an "insulator-to-conductor"
transition in a material. It typically is an irreversible
effect permanently damaging the element, unless current
limited by the circuit. If conduction takes place in a
gas (air) then this post-breakdown conduction is called
an electrical "arc" or "spark". 1
basic idea of a breakdown voltage tester is to apply current
limited high voltage to an insulator, and raise the voltage
until the desired voltage test level is reached, or until
a small amount (1 to 2 micro Amps) of leakage current
flows. If the insulation does break down, the test instrument
limits the current flow, and the process can be stopped
without destroying the item under test.
commercial test units have an automatic circuit that will
cease application of the high voltage when a selected
leakage current occurs. In a Go-No Go production test
environment, this helps relatively untrained operators
decide if a component is good or bad. Typically a relay
circuit will trip, a loud buzzer will sound, and the test
is stopped. A front panel reset button starts everything
commercial units have a "burn" mode. This allows
a current limited arc to continue. This is helpful to
physically determine where a fault is located. For amateur
use, the automatic trip circuitry may be eliminated. Instead,
the test equipment operator will observe the leakage measurement
and then turn the voltage down and stop the test if breakdown
happens. Also, a manual "burn" mode is possible
by simply allowing the current limited arc to continue
while checking for the location of the fault.
voltage level of about 8 to 10-kV is useful for testing
many amateur radio components. The circuit schematic shows
two alternative methods to get a variable high voltage
source. One method uses a common fuel oil furnace ignition
transformer. This transformer has a 120 Volt AC primary,
and a 10-kV secondary, with the case being the transformer
secondary center tap. This means that the transformer
case is electrified with 5-kV AC, and must be insulated
from the mounting base. Also, take care to not come into
contact with this case during operation of this instrument.
A simple Plexiglas shield will keep you at a safe distance.
The high voltage tester I have uses one of these transformers.
It was originally a flea market find at a bargain price.
Sure, it was rusty, and needed some paint, but the internal
voltage windings were intact. The fuel oil ignition type
transformer will generate 10-kV DC at the tester output
electrode. A common neon sign transformer will also work.
small adjustable AC primary transformer called a variac
is used to vary the input voltage to the high voltage
transformer. I used a small panel mounted type of variac
on my tester. Since these are expensive if purchased new,
you may not want to dedicate an individual variac to this
test instrument. In this case, consider using an external
variac you may already have in your workshop as a variable
other method of generating the variable high voltage source
uses a microwave oven transformer. This transformer was
obtained from a bad microwave oven. The output of this
transformer is about 2100-Volts AC, with a 120-Volt AC
primary. Nearly any microwave oven transformer will do,
even the ones from the relatively small ovens are satisfactory.
Old ovens may have a useful diode too. A voltage quadrupler
shown in the circuit diagram boosts the voltage up to
about the 8-kV range.
light bulb in the primary of the high voltage transformer
is an old but good method of limiting the current to the
transformer primary in the event of a major component
failure. For example, if the primary of the high voltage
transformer completely shorts, all that happens is that
the lamp will burn at full brilliance. The idea behind
this is not new. It has been used for years, so it is
well proven. It also gives a visual indication of the
relative amount of current flow into the transformer.
Under normal use, the bulb may glow some at the higher
end of the voltage output range.
that when using the microwave oven transformer circuit,
the light bulb may glow much more brightly as the primary
voltage approaches 120-Volts AC. This varies with each
transformer, and is caused by a relatively high saturating
current. This is typical of the microwave oven transformer.2
If you are using this type of transformer, and your measured
high voltage output is reduced to less than 8 kV by the
limiting action of the light bulb, just increase the light
bulb wattage to about 150 to 200 Watts. Otherwise, a light
bulb rating of about 125-Watts in the fuel oil ignition
transformer circuit works well. Smaller wattage light
bulbs will reduce the maximum high voltage output of this
tester. Do not eliminate the light bulb from the circuit.
It doesn't cost much and is an important circuit protector.
the panel meters are in contact with high voltage, make
sure to use an insulating front panel material. I chose
common Plexiglas since it was available at a local hardware
store, and they would also cut it to size for me. I used
a 6-inch by 12-inch piece for the front panel. Plexiglas
is also available in various colors, but I chose the basic
clear variety since I can see through it to locate any
evidence of corona points. The base plate for this tester
is also recommended to be an insulator. A good choice
here is wood. My tester has a 12-inch by 16-inch painted
plywood base 1/2-inch thick. The paint would conduct slightly
and give leakage current indications until it became very
on the Plexiglas front panel include the small variac,
two 50-microAmp panel meters, and two phone plug jacks
for the electrodes. Two right angle metal braces mounted
on the base plate support the Plexiglas front panel. The
high voltage transformer, light bulb current limiter,
and a small circuit board occupy the main plywood base
support. The circuit board is a small piece of plain perforated
fiberglass material, and is supported by four 1/2-inch
tall ceramic standoff insulators. The various components
on the circuit board are simply wired in breadboard point
to point style.
insulating clear Plexiglas cover that surrounds the tester
is used to protect the operator from accidental contact
with internal circuitry. The dielectric strength of Plexiglas
is 30 kV/mm. A 1/8-inch thickness will then have a dielectric
strength of about 95-kV. With the voltages used in this
circuit, this gives an insulating safety factor of about
ten. The external Plexiglas shroud is assembled with 1/2-inch
aluminum angle obtained from a hardware store. Since the
front panel meters may not provide adequate insulation,
a single clear Plexiglas sheet is mounted in front of
both meters. This protects against meter failure and possible
accidental voltage contact. The meters used in this tester
required a 3.5-inch by 5-inch protective front sheet held
to the front panel by two 1/2-inch standoffs.
attention to the voltage rating of the resistors used
in this circuit. Most common resistors have a maximum
voltage rating of about 300 Volts. You may achieve the
necessary voltage rating by simply using a number of resistors
in series to arrive at a safe voltage rating for a specific
resistance value. For example, two 300-Volt resistors
in series results in a 600-Volt rating.
high value high voltage resistors are required for this
circuit; 50 MegOhm and 200 MegOhm. Sometimes these can
be found at surplus outlets or on Ebay.
high voltages are involved, it's necessary to build this
tester with plenty of open space between components and
wiring. Try for at least a half-inch spacing. The use
of test prod or high voltage wire rated to at least 10
kV is recommended. Also, the solder joints need to be
as smooth as you can get without sharp points. Breakdown
voltage is also a function of geometry. These points will
encourage corona and will cause slight leakage current
indications with no component hooked up for testing.1
With no component hooked up, just turn the voltage up
until you start to get a leakage current indication. Then
turn the room lights off. In the darkened room, CAREFULLY
look and listen for the small telltale blue corona points
in your wiring. Isolate the trouble spots and then correct
the wiring connections as necessary.
may be impossible to remove all stray leakage current
indications. To compensate for this, just turn the voltage
up to a desired test voltage value, and then note the
leakage current indication on the panel meter. This number
will become your background leakage current. This number
will be subtracted from a leakage current reading taken
with an actual component under test. For example, if you
have a background of 10 micro Amps, and a component measures
35 micro Amps, your real leakage is 25 micro Amps. Stray
leakage current readings become more of a problem as the
test voltage is increased.
of all: Switch to Safety!
not touch ANYTHING except the voltage control while you
you start testing, remind yourself what you're doing.
THINK and LOOK before you touch any part of this tester.
All components should be tested on an insulated table.
Hook up the component, plug the tester in, increase the
voltage to make your test, drop the voltage to zero, and
then unplug the tester. Allow plenty of time for the filter
capacitors to discharge before removing the component
under test. While the current of this tester is limited
to about 200-microAmps, a 10-kV jolt is decidedly unpleasant.
It helps to use relatively small filter capacitors in
this test instrument. The .001 uF capacitors will discharge
quickly in a few seconds when the voltage is turned off.
If you use relatively large capacitors, like .1 uF or
so, the discharge time is much longer. This means that
even though the instrument is turned off, touching the
output electrode may still shock you. Use the output voltage
meter as a guide for how long the filter capacitors have
a charge remaining. Remember, the output meter can also
fail, so don't depend on it entirely! If in doubt, use
a shorting connection across the output electrodes to
safely discharge any remaining voltage stored in the filter
to test PIV values for high voltage diodes apply reverse
bias voltage (hook positive terminal to diode cathode,
negative terminal to diode anode) until a small leakage
current of one to two micro Amps is detected, then stop
the test. The point at which current just starts to flow
is the PIV value of the diode. Continuing beyond this
point may damage the component. It is also possible to
test relatively low voltage diodes in the range of 0 to
800 Volts or so. Since the front panel voltage meter has
limited resolution, you may carefully use your digital
multimeter as a voltage indicator. Many low cost meters
have a maximum voltage rating of 1 kV or less. It is easy
to exceed this value with the tester front panel control.
The voice of experience says that if you apply excessive
voltage to a multimeter and then hear a small "snap"
sound from inside the meter case, your meter will probably
require repair. Be careful.
breakdown voltage on transistor specification sheets is
usually stated several ways.3 Some of these
BVcbo = Breakdown Voltage Collector to Base, with emitter
BVceo = Breakdown Voltage Collector to Emitter, with base
BVces = Breakdown Voltage Collector to Emitter, with base
shorted to emitter.
Adjust the high voltage until leakage current just starts,
and then stop the test. These test methods will also apply
to PNP transistors by changing polarities.
on NPN transistor
Hook positive terminal to Collector, negative terminal
to Base, leave emitter terminal open.
on NPN transistor
Hook positive terminal to Collector, negative terminal
to Emitter, leave base terminal open.
on NPN transistor
Hook positive terminal to Collector, negative terminal
to Emitter and Base.
like the test method of diodes, you can also test lower
voltage transistors with a small digital meter if you
are careful with the applied voltage levels.
this type of capacitor is easy to test. Just hook up the
voltage terminals from the tester to the capacitor under
test. Slowly bring the voltage up until your test voltage
occurs or until you hear a slight snap or crack, or see
a small blue arc appear. Note the voltage, and immediately
drop the voltage back to lower levels. Other components
with air dielectric may be tested in this same manner.
If you can't see or hear an arc, just watch the output
leakage current meter on the high voltage tester. A rapid
rise in leakage current is a sure sign of voltage breakdown
on the component under test.
these types of capacitors test in a similar manner to
the air dielectric types. However, when these components
arc across inside, you will hear a small metallic "clink"
sound. Just adjust the voltage until you are at your desired
voltage range or until you hear a "clink" (whichever
happens first), then back the test voltage off.
vacuum capacitors with the concentric plates fully meshed.
I have purchased ham fest vacuum variables that tested
fine with the plates partially out, but that failed with
the plates more completely meshed. If you are looking
for high voltage components, consider taking this tester
along with you to the flea market, but leave it in your
vehicle. If you find some interesting capacitors, retrieve
your tester and perform the tests. Using this tester in
this way will pay for itself very quickly by eliminating
the purchase of defective components. Vacuum capacitors
tend to be expensive. Why pay for a dud? This tester will
also identify and sort out bad parts that you are going
test between normally open terminals until your test voltage
is reached, the audible "clink" is heard, or
a rise in leakage current is indicated.
voltage bypass and coupling capacitors:
these test by advancing voltage to the point where current
starts to flow, note the test voltage, and then reduce
Vacuum tubes are checked out of circuit. Hook up the tester
electrodes to the two terminals you want to test on the
tube. On triode vacuum tubes, I typically test the anode
to grid breakdown voltage. I usually apply twice the tube's
rated DC plate voltage and observe the leakage current
reading. Comparing values from known good tubes is very
helpful. On tetrodes or other tubes with more elements,
just make several tests using the various tube terminals.
Power Amplifier Vacuum Tube Typical Leakage Current Measurements
GS35B, GS31B Anode to grid leakage current at about 4
kV = 6 micro Amps
tube types (4CX250B, 4CX800A) show similar numbers.
higher leakage current indicates a shorted tube. One high
voltage short was only noticed when the tube was warm.
This tube checked well when cold, but failed short after
applying filament power for several minutes. When hooking
up tubes outside of the normal amplifier, don't forget
to provide adequate tube cooling air when running filament
AC components with this DC tester:
test an AC rated component with this tester, just use
this formula: AC voltage times 1.414 to get the effective
DC voltage rating. For example, a 2 kV AC capacitor would
require 2.8 kV DC to test.
1. "Dielectric Breakdown and Arcing", ARRL Handbook,
1995, pg 10.14
Randy Henderson, WI5W,"Build a High-Voltage Power
Supply at Low Cost" QEX, Jan/Feb 1998 Pg. 47-51.
Texas Instruments, Inc. "The Power Semiconductor
Data Book for Design Engineers", First Edition. pg
Richard Measures, AG6K's technical
web site for additional design data on HV testers.