[sdiy] Simple power supply question

[DCMagnuson at aol.com]

In a message dated 4/7/01 11:32:08 AM Eastern Daylight Time, 
vitreousflux at wwnet.net writes:

> I have a simple power supply that uses the EFM SBM schematic (12v ac in,
>  7812 and 7912 regulators, diode pairs and 2 1000u electrolytic caps).
>  
>  Both +12v and -12v outputs are normal with no load.
>  
>  Lately, when a load is placed on it, the +12V drops - the more you load it,
>  the more it drops.
>  
>  However, the -12V stays normal regardless of those loads.
>  
>  
>  Is this caused by a bad 7812 regulator?

I've learned from several other list members that the 78xx and 79xx series 
regulators like to see an input voltage that's about 3V higher than the 
output.  So you should be feeding a 7812 with around 15V DC on the input 
side.... your probably straining the regulator, and it's having a hard time 
producing the 12V output.  

Measure the voltage at the input, and you'll probably see that you have 
around 12VDC (rectified 12VAC, minus a few diode drops).  A 15VAC wall wart 
will probably work better...

The positive rail is probably using a little more current than the negative 
rail, and that's why the problem only appears on one side, and not the other.

Dave Magnuson