Close switch to operate and read value off voltmeter or oscilloscope. The transistors form a constant current circuit which ensures the device under test (DUT) is not harmed. With the supply voltage at 30V this circuit can be used to check zener diodes up to 20V.
Note that EC zeners have a knee current of 13.6mA. I have chosen quite a low current here so that devices are not damaged but irl you may want to change it to match, or be closer to, the published data for your particular zener.
This circuit is intended as a quick go/no go test.
You can also test ordinary diodes by inserting the DUT the other way around.
