Testing zener diodes require variable dc power supply.

In the circuit above, the variable power supply is used to regulate input voltage to a suitable value for zener diode being tested. Resistors R 1 limits the current through the diode. With zener diode connect as shown in the picture above there are no current will flow until the voltage on zener diodes with voltage. If the diode is connected in the opposite direction, current will flow at low voltages, typically less than 1 volt. The flow of electric current at low voltage in both directions indicate that the zener diode is damaged.