Zener diode tester



Last updated: June 6, 2012



For a recent project, I needed several zener diodes and I discovered that I did not have any device to test them.

A recent issue of SPRAT (nr.146, spring 2011) contained an article by Richard Wilkinson, G0VXG. †

The tester which he proposes is based on a simple oscillator which acts as a DC-DC converter.

L1 and L2 are bifilarly wound on a ferrite toroid. Richard used 20 turns on a FT50-43 core.

I used a dual choke salvaged from a discarded switching power supply.

For T1, Richard used a 2N3904 which isnít quite safe as it cannot handle more than 45 Volts.

For this reason, I have employed a MPSA42 high-voltage transistor.

Capacitor C1 should have a working voltage of 100 V.


The circuit works in the following way:

If no zener (D2) is connected across C1 and S1 is switched on, the output rises rapidly to 45V DC.

This voltage can be measured with a DVM.

When an unknown zener is connected† across C1, output will be equal to the zener voltage.

If a normal diode (or a zener diode with zener voltage > 45 V) is hooked up, the DVM will measure 45 V.

If a diode is connected with the wrong polarity, the meter will indicate 0.6 or 0.7 V

(even lower if the diode is a germanium or Schottky type).


Resistor R2 limits the current through the zener diode to a safe value.

Since the current through the zener is relatively small, 400 mW diodes are properly measured,

but 1W or 5W diodes will give readings which are (about 5%) too low. No big deal.

The circuit draws about 20mA from a single 1.5V battery.

I have used it for about half a year now and it works quite well.







A plastic box which had contained beef cubes (Aldi supermarket) was a proper housing for this circuit.






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