Testing diode in a circuit?

[halleffector]

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]

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]

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]

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.