Il Mon, 17 Jun 2013 21:16:24 -0700, jurb6006 ha scritto:

Ummmm, to use a 7812 to regulate negative would be illogical.

Which pin is common ground ? On the 78XX it is in the middle. If it is
off to the side that is actually a 7912.

I don't care what it says on it. I'd bet it is a 7912. There is no
reason for the engineer to **** around and go through what it would take
to use a 7812 there. If it is actually a negative regulator with the
ground pin in the middle then it is custom, or a different series. It is
actually possible....but.

They love to use custom parts to create a captive market, but I can
think of a whole lot of other places in a PS like that to accomplish
that, the standup board with the black conformal coating would do it.

Look at the board, I think you are going to find the ground off to one
side, which means you need a 7912 no matter what the numbers say.

Take it from me, I sometimes don't even look at part numbers anymore, I
just look at where something is in the circuit and usually know what is
needed there. Sometimes I do have to look it up, but not all that often
at least when it comes to something like this.

No, it's a LM340T12 (7812). Datasheet says it's a positive regulator.
The scheme used is this.
+14.5 V between In and Gnd of 7812. Out of the 7812 is connected to
output GND, Gnd of the 7812 to the -12V line. And the -12V line is done...
Gnd of the 7812 is connected to Gnd of the 7905, Out of the 7812 to In
of the 7905.
So the regulated 12V of the 7812 feeds the 7905, Out of the 7905 is connected
to the -5V line.
That's a bit odd...

I tested the spare MC7812C off circuit, maybe 14.5V were not enough.
I used 18.5V as input voltage to the MC7812C, resulting in 8.xx volts
as output. So the spare MC7812C is dead. It became so hot that I saw a bit
of smoke.
I need to buy another 7812, my spares are ended...



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