Troubleshooting - Testing Diodes

Testing diodes is a really simlpe process if you have a good Digital Multi Meter which has the diode test function on it. Here is the procedure for testing diodes, forward-bias & reverse-bias.

- Begin by placing your DMM in the diode test mode

** When looking at a diode for the first time you will see a black line closest to one end of the component, this is to signify that the leg closest to the line is marked as the CATHODE and the leg farthest from the black line is the ANODE, this is important to know when performing the following tests.

> FORWARD-BIAS TESTING A DIODE <

- Place the NEGATIVE lead from the DMM on the CATHODE of the diode

- Place the POSITIVE lead from the DMM on the ANODE of the diode

*** Results *** You Should Now Be Measuring About 0.7 Volts (700mV)

> REVERSE-BIAS TESTING A DIODE <

- Place the NEGATIVE lead from the DMM on the ANODE of the diode

- Place the POSITIVE lead from the DMM on the CATHODE of the diode

*** Results *** You Should Now Be Measuring About 2.60 Volts

> Possible Problems When Measuring <

PROBLEM: When forward-bias testing the diode I measured 2.60 volts, what is wrong? (NOTE: NEGATIVE lead is on the CATHODE and the POSITIVE lead is on the ANODE)

ANSWER: The Diode Is Open Circuit, Replace Component.

PROBLEM: When reverse-bias testing the diode I measured 2.60 volts, what is wrong? (NOTE: NEGATIVE lead is on the ANODE and the POSITIVE lead is on the CATHODE)

ANSWER: The Diode Is Open Circuit, Replace Component.

PROBLEM: When forward & reverse-bias testing the diode I measured 0.00 volts in either direction, what is wrong?

ANSWER: The Diode Is Short Circuit. In Some cases if the diode is building up resistance you will measure below 2.60 volts.


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