Isotest 6 - Additional applications

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Gerhard Heigl
Gerhard Heigl
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31.Jan.07 16:56

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I must admit that I developed the Isotest mainly for insulation (leakance) checks of capacitors. But it can do much more beyond that. Here are a few examples.

For tests at mains operated equipment always disconnect the power cord from the line.

Pull the plug. Switching off is not sufficient!

Note: An ON-OFF switch may rarely fail in the "OFF" position - and could cause serious personal injury or major damage to equipment.

Coupling capacitors need an insulation resistance far beyond 200 MΩ. That is out of the Isotest range. But there is a knack to find out if a sample is appropriate as a coupling capacitor. The sample (Cp) is charged with the rated voltage(A). Is the indication (infinite resistance), disconnect Cp carefully while the test voltage remains on (B). Wait at least 10 seconds *), and then reconnect Cp again to the Isotest, which is still on. The instrument pointer must not show a deflection or movement. If it does, then Cp is not suitable as a coupling capacitor. Do not vary the test voltage.

 For small capacitance values (< 0.01µF). The supplementary capacitor (Ch = approx. 0.1µF) is connected in parallel. Ch itself should display an extremely high insulation value infinite () and sufficient voltage rating.

*) NOTE: the longer you wait, the test becomes more critical.


Aged selenium rectifiers often develop increased forward resistance. The voltage drop should be checked under real load conditions (in operation in the application, if possible).

A rather coarse check of rectifiers is also possible, regardless of their material: Germanium, Silicon, Copper oxide, or Selenium.

Silicon diodes and bridge rectifiers should indicate in reverse polarity.

Good Selenium rectifiers usually show 2 MΩ to 50 MΩ in reverse, depending on their voltage rating. Forward resistance should be as low as possible for all rectifier types. Disconnect diodes and rectifiers completely from their circuitry. Do not exceed the rated voltage!


Transformers of any kind and size may be tested for insulation faults of winding to core, ore winding to winding. An indication of infinite would be ideal. The measured real values have to comply with existing regulations and specifications. This in particular for mains operated equipment and installations.

If a leak current limiting switch (so called FI-switch) in an installation is triggered: Home appliances like refrigerator, electric kitchen range, microwave oven, dish washer, washing machine, tumbler etc. may be checked for hidden leakance / insulation faults. Disconnect the unit completely from mains and apply the maximum test voltage (500 V).


To check 3-phase motors and induction (squirrel cage) type motors every jumper /bridge in the terminal box must be removed. This measure will be helpful if a fault needs to be localised. Ideally the Isotest should indicate infinite ∞. Measured values must comply with national/international regulations, in particular for mains operated equipment and installations.

Testing cables or lines of greater length will result in a small pointer deflection, caused by the mutual capacitance between conductors (wires), respectively against ground.

Standard ohmmeters or digital multimeters are not suitable due to their low test voltage.


For glow discharge devices (e.g. neon lamps) the approximate values of ignition voltage and arc drop can be determined (conduction voltage). Slowly increase the test voltage until ignition occurs, which will cause an indication on the instrument. The light will be very dim due to the 10 MΩ source resistance of the Isotest. If the test voltage is  now slowly reduced, the indication will jump back below the arc drop (conducting) voltage: the glow will go out.

The 2nd view shows a simple audio oscillator. It makes use of the hysteresis between ignition and conducting voltage. The frequency of the sawtooth pulse can be varied by C1 and by the test voltage setting. C2 acts as a blocking capacitor for the test voltage to keep it away from the output.

You may also test tubes for internal leakance. Check all pins /contacts mutually against each other. Except filament to filament and internal connections (see applicable base diagram) there must not be any indication below infinite ∞.

Sockets sometimes develop leakage paths, from contamination and flashover. For these tests all leads have to be unsoldered. Do not forget to check against chassis (ground), too.

Even identification of unknown tube connections is possible using the Isotest. Here the filament needs to be heated. See EF27.

This article was edited 31.Jan.07 19:01 by Gerhard Heigl .

Ake Nyholm
Ake Nyholm
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31.Jan.07 19:17

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Dear radio friends

Thank you for your support of translating the ISOTEST article into English.