Re: -5V on IIgs

[Don Bruder <dakidd@sonic.net>]

In article <1173043981.038947.50150@t69g2000cwt.googlegroups.com>,
 "bieling@terra.es" <bieling@terra.es> wrote:

> On Mar 4, 8:23 pm, Don Bruder <dak...@sonic.net> wrote:
> >
> > The "quick and dirty" method:
> > Get a 7905 regulator (-5 volts regulated output - 78xx and 79xx series
> > regulators are good to just under an amp)
> >
> > Wire the input of the regulator to ground. Wire the ground of the
> > regulator to the +12V line. (hook it up to the supply "backwards") Now
> > use the output to feed the -5V line.
> >
> > Presto!
> >
> > You can do the same, using a 7805 regulator wired normally - ground to
> > ground, input to +12V, output to +5V line - for your +5 volt supply.
> >
> 
> Excuse me Don, you want him to fry his gs or what ?
> What you say would give +7v, not -5v !
> 
> Jorge.

You've managed to misunderstand, methinks... Or missed something while 
reading... Or SOMETHING! :) 

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.

The 7805 (which is a POSITIVE 5 volt regulator) scenario is for feeding 
the POSITIVE 5 volt line from a positive 12 volt source, and has nothing 
to do with the NEGATIVE 5 volt line. It gets wired to the 12 volts 
supply "normally", rather than reversed, like the 7905, and its output 
pin gives a regulated positive 5 volts.

-- 
Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist,
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