With electronic scheme below can be made a zener diode tester.
Using this tester and a multimeter can be measured and determined with a high precision threshold voltage of a zener diode.
If contact S1 is closed, the resistance R1, T1 and Zener diode current flow. Base transistor T1 is connected to power supply trough the R4, so the transistor leads.
Zener current is equal to the ratio of base-emitter voltage of Q2 and the resistance value R1.
Resistances R2 and R3 or a combination of R1, R2, R3 can be connected in place of R1 with S2-S3 so that through the zener diode constant current flow. With a supply voltage of 25 volts at the actuation keys S1-S3, the current through zener diode take values of about 2.2,6 and 22mA.Zener voltage can be read with a DC voltmeter connected in parallel with zener diode.