Re: -5V on IIgs

[Don Bruder <dakidd@sonic.net>]

In article <1173050769.686676.189350@64g2000cwx.googlegroups.com>,
 "bieling@terra.es" <bieling@terra.es> wrote:

> On Mar 4, 11:27 pm, Don Bruder <dak...@sonic.net> wrote:
> > The 7905 (a negative 5 volt regulator - normally supplied with negative
> > 7-20 volts on the input pin, and putting out a regulated negative 5
> > volts on the output pin) gets wired into the positive 12 volt supply
> > such that it thinks it's seeing -12 volts, by simply wiring the input
> > pin of the regulator to ground, and the ground pin of the regulator to
> > +12 volts. It's wired "backwards" as far as usual practice is concerned,
> > but to the regulator, it's running on a -12 volt input source. The
> > output pin spits out a regulated -5 volts. Which is precisely what he
> > needs - negative 5 volts.
> 
> Think of it this way: the 7905's output pin is going to be at 5v less
> (-5v) than it's ground pin...
> but,
> It's ground pin is connected to +12v, not to ground...
> therefore,
> It "spits out" 5v less than +12v = +7v, referenced to ground.
> 
> In other words, -5v referenced to +12v
> is not the same as -5v referenced to ground.
> 
> Jorge.

D'OH! Forgot about that factor. 

Yeah, it works, as long as the +12 lead is the "ground" for everything. 
But since "ground" on the other side of the load from the -5 supply is 
going to be (effectively) -12 volts, what's passing through the load is 
actually +7 volts.

OK, for you fans at home keeping track, score that as one "earned 
double-dumbass" on me.

<slinks off to the showers>

-- 
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