Technical Topics



  • Necessary for testing of any high voltage component

  • Tests vacuum tubes, high voltage capacitors, vacuum relays, etc.

  • Testing components BEFORE circuit operation prevents equipment damage

  • To use, just hook up component, adjust the high voltage with panel variac, then read leakage current on panel meter.

  • Easy to make at home. Requires a block of wood for the base and Plexiglass for the panel and enclosure.


Many 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 tubes.

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 enough.

Download complete one page schematic

High Voltage Tester Basics

Dielectric 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

The 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.

Some 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 over.

Some 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.

Circuit Description

A 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.

A 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 voltage control.

The 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.

The 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.

Note 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.

Breakdown Tester Construction

Since 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 dry.

Components 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.

An 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.

Pay 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.

Two 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.

Since 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.

It 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.

Testing Components

First of all: Switch to Safety!

Do not touch ANYTHING except the voltage control while you are testing.

Before 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 capacitors.

Diodes: 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.

Transistors: breakdown voltage on transistor specification sheets is usually stated several ways.3 Some of these ways are:
BVcbo = Breakdown Voltage Collector to Base, with emitter terminal Open.
BVceo = Breakdown Voltage Collector to Emitter, with base terminal Open.
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.

BVcbo on NPN transistor
Hook positive terminal to Collector, negative terminal to Base, leave emitter terminal open.

BVceo on NPN transistor
Hook positive terminal to Collector, negative terminal to Emitter, leave base terminal open.

BVces on NPN transistor
Hook positive terminal to Collector, negative terminal to Emitter and Base.

Just 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.

Air dielectric capacitors: 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.

Vacuum capacitors: 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.

Test 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 to sell.

Vacuum relays: test between normally open terminals until your test voltage is reached, the audible "clink" is heard, or a rise in leakage current is indicated.

High 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 voltage.

Vacuum tubes: 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.

RF Power Amplifier Vacuum Tube Typical Leakage Current Measurements

Russian GS35B, GS31B Anode to grid leakage current at about 4 kV = 6 micro Amps
Other tube types (4CX250B, 4CX800A) show similar numbers.

Significantly 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 voltage only.

Testing AC components with this DC tester:

To 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

2. Randy Henderson, WI5W,"Build a High-Voltage Power Supply at Low Cost" QEX, Jan/Feb 1998 Pg. 47-51.

3. Texas Instruments, Inc. "The Power Semiconductor Data Book for Design Engineers", First Edition. pg 1-30