How to tell if a diode is fried?

[Bret608]

Hi everyone,

How can I use my DMM to tell if a diode is still funtional? Normally, to test the forward voltage drop, I would touch the red DMM lead on the positive side of the diode, and the black on the negative, correct? What should I look for, both if it's working or not? Also, will this work if the diode is actually soldered to a PCB?

Thanks!

Bret

[jkokura]

Usually the Diode checking on a DMM is the same as a continutity tester. If it beeps, you've got a faulty diode.

To measure forward voltage, I forget how it's done.

Jacob

[midwayfair]

To measure forward voltage, I forget how it's done.

"Continuity" setting, black lead on the cathode side (that's the stripe on almost all diodes, EXCEPT in some Russian diodes that put the stripe on the wrong side). Flipping the leads should give you "1" or a similar setting, indicating infinite, i.e., no conductance at all.

Diodes conduct in one direction only, with a drop in voltage.

If it does not measure a forward voltage drop within specs (check the datasheet), the diode is bad and should not be used.
If it does not measure a forward voltage drop or continuity (beep) at all, the diode is actually broken. D-E-D, dead. It's not even useful as a jumper.
If it conducts (beeps) in either direction or both directions, the semiconductive material is not fuctioning properly and the diode is dead.

For an experiment, pick up a couple known working diodes and play around with them.

[Bret608]

Whew, I think I am good then! I did get a change in voltage with the black DMM lead on the stripe side of the diode in question. I get a '1' when I flip it around. I appreciate the help. I was not looking forward to desoldering a diode from this millenium bypass DPDT board! The reason I needed to ask at all is that I managed to zap the LED attached to this board board back when I was trouble shooting my Pork Barrel (checked continuity with power supply attached, let full voltage jump to the LED!  ). I just wanted to make sure I didn't fry anything else before I use this footswitch in my Wolfshirt on Friday!