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hotscot 26th March 2011 08:02 PM

LM3875 power supply question.
I'm only getting @ 17 V DC when I'm expecting @ 37 V.

Does this suggest one of my diodes has failed?

I have a multimeter but find testing diodes tricky.


AndrewT 26th March 2011 08:09 PM

Plug your transformer in via a bulb tester and tell us what the bulb filament has done/is doing.

pacificblue 26th March 2011 08:14 PM

Depending on your multimeter there are two ways of testing. Both require you to test the diode twice, once in forward and once in reverse direction. Some meters come with a diode test facility that shows the forward voltage in one direction and should show overload in the other. If your meter does not have that, use the impedance measurement. You should get overload in one direction and a low value in the other.
Diodes that show continuity (or forward voltage) in both directions or show overload in both directions are broken.

hotscot 26th March 2011 08:27 PM

Thanks for the input.
Found a dodgy diode with multimeter.
Now replacing.


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