Zener diodes work a little different than rectifier diodes in that they will conduct with reversed bias voltage applied at a set voltage level.
A Zener diode
These diodes act as an open circuit from the output-in, until a strict specified reverse bias voltage is reached.
At this point, once the threshold is reached, the diode conducts allowing current to pass through.
Zener diodes are commonly used to regulate voltage in circuit and external power supplies.
The testing of a Zener diode requires a variable dc power supply. In this circuit, the variable power supply is used to adjust the input voltage to a suitable value for the Zener diode being tested.
Zener diode testing circuit
Resistor R1 limits the current through the diode. With the Zener diode connected as shown in the above figure, no current will flow until the voltage across the diode is equal to the Zener voltage, sometimes referred to as the avalanche voltage. When this happens, the current will flow through the diode at the supplied voltage.
If the diode is connected in the opposite direction, current will flow at a low voltage, usually less than 1 volt. Current flow at a low voltage in both directions indicates that the Zener diode is defective.
Test Zener Diode
Step 1- Set the multimeter to test a diode.
Set the multimeter to the diode setting. On digital meters, adjust the dial to point at the diode symbol.
Step 2- Test forward-biased voltage.
Measure the forward-biased voltage on the Zener diode. Place the positive red probe on the anode side of the diode, which is unmarked. Place the black probe on the cathode side of the diode, which is marked by a stripe.
Step 3- Review the test results.
An good diode will read 0.5 to 0.7 V when it is forward-biased.
Step 4- Test the reverse-biased voltage.
Measure the reverse-biased voltage on the Zener diode. Switch the multimeter probes from where they previously were in step 2. Place the positive lead on the marked or cathode side, and the negative lead on the unmarked anode side.
Step 5- Review the test results.
You should read a measurement indicating infinite resistance or no current flow. The meter should display O.L.
Step 6- Connect in a 9V battery and resistor for further testing.
Find a 9 volt battery and a resistor for placement into the circuit. Attach the positive side of a 9-V battery to one side of the resistor and connect the other end of the resistor to the cathode side of the Zener diode, so that it will be reverse-biased.
Step 7- Finish wiring in the 9V battery.
Wire the remaining diode terminal to the negative side of the 9 volt battery.
Step 8- Test the completed circuit for voltage.
Place the multimeter on a DC voltage setting. Measure the voltage across the diode by placing a multimeter lead on each terminal. It should read approximately 5.6 volts, though the value may be as low as 5.32 or as high as 5.88 volts. Note that the voltage between the battery and ground remains at 9 V.