Diode Fault Indications

Short Circuit
Diodes can be easily damaged by high voltages, especially diodes working in high voltage or high power converter applications such as power supplies, inverter. Thus, it will usually short (resistance become drop to 0Ω) when it measured in either direction. If it happend in a power supply, a large currents can flow and obvious damage occurs such as "cooked" diodes and / or blown fuses. Short circuit diodes that are not obviously damaged show 0Ω or very low resistance in both forward and reverse directions.

Open Circuit
Occasionally, diodes (especially small signal diodes) may become open circuit, and read very high resistance or infinity in both forward and reverse directions. See [prev]

Leaky
Some you will find a signal diode may become "leaky". It's happen, while its forward resistance may be normal, its reverse resistance may be lower than the expected infinity. According to [1] often this fault could only be measured with the diode removed from the circuit it is working in because of the parallel resistances of other components connected across the diode.

Zener Diodes [1]
Zener diodes exhibit similar faults to other diodes, but in addition may become "noisy". They do not make a noise but the normally very stable voltage across them suffers from very rapid fluctuations similar to the spiky waveform you would see on an oscilloscope when looking at an audio signal that was just a constant hiss, (also called noise since it is sometimes present as "background noise"). As Zener diodes are often used to stabilize power supply lines, this "noise" can give rise to strange faults, depending on what is being supplied by the power supply in question. The moral is - If a circuit is behaving strangely, and noise on the power supply is suspected, check any Zener diode stabilizing that line by substituting it with a known good diode. All Zener diodes have a defined voltage, and if the voltage measured across them under working conditions, is not what is printed in the circuit manual (or on the diode if you can see the markings), then the diode is faulty, (probably open circuit) and must be changed.

Reference:

[1] http://www.learnabout-electronics.org/diodes_05.php

 

How to test a diode

Diodes can be tested using a multi meter. It is normally the resistance of the diode in both forward and reverse directions that is tested. There are however a number of points to remember when testing diodes.

The diagram below (Figure 1) shows how to connect a digital meter to test the diode. There are a number of things to remember:

• Make sure you are using the diode range.

• Using a digital meter, connect the black lead to the cathode and red to the anode (forward bias - around 1kΩ).

• Reverse the meter connections (reverse bias - infinity reading).

MUST BE REMEMBER!

If you are using an analogue meter the above polarities are reversed.

SOME METERS give a reading of the junction potential (in volts) CHECK YOUR METER INSTRUCTIONS.

Figure 1. How to test a diode

Bad Diode:
In the case of a bad diode, you will have one of two situations. First, if the meter
indicates ZERO (as it will if the probes are touched together) both ways, the diode has
shorted. Or, second, the meter indicates INFINITY (as it will if the meter probes are not touched together) both ways, meaning the diode is open.

Reference:

[1] http://www.learnabout-electronics.org/diodes_05.php 
[2] http://www.lesterelectrical.com/techservice/servicedocs/diodetesting.PDF