Electronic diodes, and other semi-conductor devices with p-n junctions (e.g. the base-emitter of a transistor) can be tested using a digital type instrument using the following procedure:
PROVE the correct instrument operation.
SWITCH the instrument to diode test. If the diode is still in circuit, turn off the power to the circuit,
Discharge all capacitors and remove fuses.
In this test the instrument drives a small d. c. current (a few mA)
through the diode/p-n junction while it also acts as a voltmeter to measure the volt-drop across it.
CONNECT the two probes across the diode.
READ the forward volt-drop across the diode.
This should be between 500 mV and 900 mV (0.5-0.8 V) for a healthy silicon diode or p-n junction.
REVERSE the probe connections and the display should indicate over-range.
If the display indicates over-range in both directions the diode is open- circuit faulted.
If the display indicates less than 1 V in both directions, the diode may be short-circuit faulted.
The associated diode circuitry may be giving false readings so the diode must be disconnected from the circuit then re-tested.