1. Compare your DMM diode check measurement of VZ (Reverse bias) from step one to the data sheet.
a. Why do they differ?
The testing of normal diode is easy but for zener diode testing require some additional circuitry. Because the Zener diode only conducts in reverse-biased condition, and only if the applied reverse voltage is greater than the Zener breakdown voltage.
Gradually increase the variable power supply and observe the voltage on the DMM. This reading on the DMM must be such that as we increase the variable supply, DMM output should increase until the breakdown voltage of the diode. And beyond this point DMM should show a constant value of voltage irrespective of any increase of the variable supply. If it so, then Zener diode is healthy otherwise defective.
As zener doide current will not increase abruply but exponentially. So the breakdown voltage can be here or there depending upon the series resistance in additional circuit.
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