This is a typical case of doing something complicated while you can do it easy too.
I needed to sort my box of diodes and zeners and so I made this. At first I did not want an analog meter so I used this schematic and the 10 turn pot with scale and a 3 way switch to read upto 35V. The duoled and comparator was to show if the reference voltage was at the same level of the voltage of the current source (1-25 mA) over the DUT. That worked great but then I decided to add a meter too. The meter is coupled to the Vref. It is more easy to couple it to the DUT but the problem is the open voltage is 35V so the meter then goes skyhigh. The advantage now is I can easy match zeners or LEDs because the Vref has more resolution and I cab use a voltmeter to measure direct over the DUT is needed.
If you only want a simple way to test Zeners you can skip the LM393 comparators and two LM317’s. You only need the LM317 currentsource 😉
The pregeulator transistor is because I only had a 50VDV transformer