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Testing diode in a circuit?

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Can I test a diode in a circuit without desoldering it first?

I know this is bad practice, but can I at least identify bad diodes in some cases? I assume that there would be some cases where the diode doesn't test bad, but could still be faulty. What I would like to know is, is there any way to test a diode in a circuit that reveals that the diode for sure is bad, in some cases?

E.g. I imagine if the diode has been completely burned over, so there is no connection at all. That should be easy to test with a multimeter? Of course, a visual inspection should reveal that too, but just to illustrate the idea.

So in brief, in what cases (if any) can we test if a diode is bad without desoldering it?
 

MikeMl

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Most Helpful Member
Depends entirely on the circuit that it is part of...

For example, a diode is used as a snubber across a relay coil. If you try to measure it using the Ohmmeter in a standard Digital Multimeter, you will read the relay coil resistance regardless of which way you put the Ohmmeter probes.

If you have a schematic, you might be able to divine what the Ohmmeter should read with either orientation of the probes. Without a schematic, you are shooting in the dark.
 

Diver300

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As Mike said, the circuit across the diode could have too low a resistance. Then the rest of the circuit conducts then you use an Ohmmeter.

If you have a diode test function on the multimeter, that can help, as the current is often a bit larger than measuring Ohms, and the reading is in Volts. A diode on its own will show open circuit one way and around 0.6 V the other. In circuit, if you see more than about 0.8 V in both directions, that would indicate and open circuit diode. I did that with a diode (well the base-emitter junction of a transistor) yesterday. It showed 1.3 V, which meant the transistor couldn't have been working.

If you see less than 0.6 V in both directions, you have to look at the external circuit to know if you have to unsolder the diode to test it.
 

RODALCO

Well-Known Member
In my experience yes, in most cases.
Usually you get about 0.5 to 0.6 Volts across the diode or transistor junction.
Reading Zero, the device is often shorted out and faulty, reading > 0.9 Volts, unsolder and retest.
The meter needs to be in diode test function.
 
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