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Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 671

Posted: 08:21am 04 Jan 2025

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Reading those tiny numbers on a zener diode to find its 'on' voltage from a table is becoming too hard at my age now.
So I decided to build a tester; but it had to be simple, cheap (using parts I had in hand) and not battery operated.

There are lots and lots of designs on the net, from basic to rather complicated and many could not cater for high voltage zeners.

This is what I made:

How it looks:

And what's inside:

A few comments if anybody wants to copy tis idea:

Use the smallest transformers only, preferably with a high 230V resistance (mine have 1000 Ω).
It's possible to get 'zapped' when the test clips are touched with the pot knob rotated to a high value and no zener is inserted. Take care!
With a zener clipped in the max voltage at the clips IS the zener voltage

.

I did not fit the optional momentary push button, having one would require one hand to push it and the other to advance the potentiometer - leaving no hand to touch the test clips

.

Always rotate the knob to zero before inserting a zener diode.
To test, advance the knob until the multimeter voltage no longer increases, this is the zener voltage.
A more accurate test is to have a micro Ampmeter (0-100uA) in series with the zener.
A small current will show when the zener starts to conduct.

The 10Ω resistor in series with the 500Ω pot starts the test voltage at 2V. Removing this, the test voltage will start at zero but my auto ranging multimeter did not like a 0V input (kept switching to the Ω range until a small voltage was applied to the test terminals)
The 800V, 3A diodes are an overkill - its what I had -, 400v, 1A types are OK.
The capacitor value is not critical, just enough to smooth any ripple but it MUST be a high DC voltage type.
The 1 MΩ resistor discharges the cap, this takes a few seconds, so keep that in mind. The multimeter will tell when its discharged and ready for the next test.

The 10 KΩ resistor limits the test current to a few micro Amps when testing.

Revlac

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Joined: 31/12/2016
Location: Australia
Posts: 1114

Posted: 09:44am 16 Jan 2025

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Good Idea

, I'm going to have to build one of these later I have a pile of zener diodes that fell out of the packet on the table and all those with the paper ends had the value written on them and yes they all fell off didn't they

......was looking for a 75v zener but ended up ordering them instead.
I did try using a transistor and power supply some time back to check a few zener's but this looks much better, thanks for posting.

Cheers Aaron
Off The Grid

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