I have inherited a big batch of Zener diodes whose value is unreadable on most of them. In addition, I realized that the majority have quite high values.
To test their value, I had the idea to use a Nixie tube NE555 based power supply that can be found all over the Internet.
In order to realize the Zener tester, one need only to add a resistor and ZUT (Zener Under Test) in series on the "high voltage" produced and add a voltmeter in parallel on the ZUT to read its value.
I drew the schematics with Eagle and produced a small printed circuit board.
The adjustable resistor is used to set the minimum voltage value produced and the potentiometer adjusts the voltage from this minimum to the maximum possible.
This maximum depends on the MOSFET used.
The operating voltage of the output capacitor C4 must be greater than the value of the voltage produced, it must be 400V or 600V.