Here is a very simple circuit that can be used for testing Zener diodes. The IC1 NE555 is wired as an astable multivibrator and the output of the IC is stepped up to a high voltage AC using the transformer T1.The unloaded AC voltage will be around 120V. The AC voltage is rectified by the diode D1 and filtered by the capacitor C3.This voltage is applied across the Zener diode through the current limiting resistors R2 or R3 which can be selected by switch S2 for testing the diode at 1mA or 2mA respectively. The digital multimeter in DC voltage mode is connected across the Zener diode under test. The Zener diode is a fine one; the meter will show the correct Zener voltage specified by the manufacturer.
Circuit diagram with Parts list.
- The circuit can be assembled on a Vero board.
- Use a 9V PP3 battery for powering the circuit.
- The capacitor C3 must be rated at least 150V.
- Use a holder for mounting IC1.
- T1 can be a 110V primary , 9V secondary step down transformer connected in reverse.