LM3875 power supply question.

hotscot · 26th March 2011, 08:02 PM

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.

Thanks
Alan

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

Found a dodgy diode with multimeter.
Now replacing.

Cheers

26th March 2011, 08:02 PM	#1
hotscot diyAudio Member Join Date: Aug 2004 Location: California	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. Thanks Alan

26th March 2011, 08:09 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Plug your transformer in via a bulb tester and tell us what the bulb filament has done/is doing. __________________ regards Andrew T.

26th March 2011, 08:14 PM	#3
pacificblue diyAudio Member Join Date: May 2008	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. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

26th March 2011, 08:27 PM	#4
hotscot diyAudio Member Join Date: Aug 2004 Location: California	Thanks for the input. Found a dodgy diode with multimeter. Now replacing. Cheers

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