[sci.electronics] NEGATIVE VOLTAGE?

crisp@uncecs.edu (Russ Crisp) (03/08/91)

Hello.

I'm in the midst of a construction project that, when complete, will
require a -12 volt power supply, a +5 volt, and a +12 volt supply.
I can handle the +5, and the +12, but the -12 is vexing me.  Can
I use another independent 12 volt supply, and reverse the leads, or
would this be a fire hazzard?  Whats the scoop on negative voltage
power supplies anyway?

Thanks for the advice.

Russ Crisp
Western Carolina University

henry@zoo.toronto.edu (Henry Spencer) (03/09/91)

In article <1991Mar8.140031.20095@uncecs.edu> crisp@uncecs.edu (Russ Crisp) writes:
>I can handle the +5, and the +12, but the -12 is vexing me.  Can
>I use another independent 12 volt supply, and reverse the leads...

If the independent 12V supply has no ground in common with the rest
of the system, this should be workable.  However, unless you're working
with something trivial like plug-in bricks, you need to be very careful
about this.

I'm not sure why -12 is such a problem.  The circuits for negative supplies
are no more complex than those for positive supplies, and IC regulators are
just as available.
-- 
"But this *is* the simplified version   | Henry Spencer @ U of Toronto Zoology
for the general public."     -S. Harris |  henry@zoo.toronto.edu  utzoo!henry

kingdom@hpfcdc.HP.COM (Mike Kingdom) (03/12/91)

> I'm in the midst of a construction project that, when complete, will
> require a -12 volt power supply, a +5 volt, and a +12 volt supply.
> I can handle the +5, and the +12, but the -12 is vexing me.  Can
> I use another independent 12 volt supply, and reverse the leads, or
> would this be a fire hazzard?  Whats the scoop on negative voltage
> power supplies anyway?

You'll probably be best off to buy one from any of the millions (well
that's an exaggeration :-) of surplus dealers around. These are very
common types of supplies. It's tough to build one for less than you
can buy one. 

-- Mike

strong@tc.fluke.COM (Norm Strong) (03/13/91)

In article <1991Mar8.140031.20095@uncecs.edu> crisp@uncecs.edu (Russ Crisp) writes:
}
}Hello.
}
}I'm in the midst of a construction project that, when complete, will
}require a -12 volt power supply, a +5 volt, and a +12 volt supply.
}I can handle the +5, and the +12, but the -12 is vexing me.  Can
}I use another independent 12 volt supply, and reverse the leads, or
}would this be a fire hazzard?  Whats the scoop on negative voltage
}power supplies anyway?

If you don't need more than a few milliamps, the  best solution is a Maxim
IC called the MAX636B.  It converts 5volts directly into -12volts at up to
50ma.  

-- 

Norm Strong  (strong@tc.fluke.com)
2528 31st S.   Seattle WA 98144   USA

jgk@osc.COM (Joe Keane) (03/13/91)

In article <1991Mar8.182508.15573@zoo.toronto.edu> henry@zoo.toronto.edu
(Henry Spencer) writes:
>I'm not sure why -12 is such a problem.  The circuits for negative supplies
>are no more complex than those for positive supplies, and IC regulators are
>just as available.

Unfortunately, this last statement is not true.  For some reason, positive
regulators are more common.  You can probably find what you want in a negative
regulator, but you won't have as much of a choice.

esupg@warwick.ac.uk (Andrew Bargery) (03/13/91)

>In article <1991Mar8.140031.20095@uncecs.edu> crisp@uncecs.edu (Russ Crisp) writes:
>}
>}I'm in the midst of a construction project that, when complete, will
>}require a -12 volt power supply, a +5 volt, and a +12 volt supply.
>}I can handle the +5, and the +12, but the -12 is vexing me.  Can
>}I use another independent 12 volt supply, and reverse the leads,

YES!!

if you have got another 12v power supply handy then this is no problem. The
normal method is to reference 0v to ground. If you connect the +ve terminal to
ground, then the negative terminal will be at -12v wrt gnd (0v). You don't
even have to connect the 0v to ground; all the voltages in the circuit are
relative to each other, so you could use -12,0,5,12 or 0,12,17,24 or
988,1000,1005,1012 volts - the circuit will still work. However, if you are
going to connect scope probes, etc, then 0v on the board must be at the
same potential as in the scope, IE. GND

I have used this method to produce -5.2,-2,0 , -3.2,0,2 and (currently)
-16,-5,0,5,16 volt power rails. However, I don't think you can use this method
if you are using switch-mode power supplies.

------------------------------ esupg@uk.ac.warwick.cu -----------------------
  DISCLAIMER:       Andrew     University of Warwick, Coventry, UK.
                     Bargery   154 Brunswick St, Leamington, CV31 2ER, UK.
I plead the 5th...             vox : +44 926 881264

esupg@warwick.ac.uk (Andrew Bargery) (03/13/91)

sorry to follow my own article up,

[blurf about connecting PSUs in series]

The PSUs that you use must be of the 'Floating Ground Type'. That covers
the majority of lab. PSU you are likely to come accross. They can (usually) be
identified by having separate +, - and Gnd terminals. If there is not a
separate Gnd terminal, it is common to find the - terminal internally
connected to Gnd. In this case, you obviously can't reference the +ve terminal
to Gnd also!

> I don't think you can use this method if
> you are using switch-mode power supplies.

The type I was thinking of have the -ve terminal (0v) connected
(internally) to Gnd. I guess you could also have floating ground SMPS units.

------------------------------ esupg@uk.ac.warwick.cu -----------------------
  DISCLAIMER:       Andrew     University of Warwick, Coventry, UK.
                     Bargery   154 Brunswick St, Leamington, CV31 2ER, UK.
I plead the 5th...             vox : +44 926 881264

jfw@ksr.com (John F. Woods) (03/13/91)

jgk@osc.COM (Joe Keane) writes:
>In article <1991Mar8.182508.15573@zoo.toronto.edu> henry@zoo.toronto.edu
>(Henry Spencer) writes:
>>I'm not sure why -12 is such a problem.  The circuits for negative supplies
>>are no more complex than those for positive supplies, and IC regulators are
>>just as available.
>Unfortunately, this last statement is not true.  For some reason, positive
>regulators are more common.  You can probably find what you want in a negative
>regulator, but you won't have as much of a choice.

Digikey has 79L12's, 79M12's, and 7912's.  In fact, their 79xx series has
almost as much variety as their 78xx regulators (I don't see a 79L09 or
79L10 to match the 78L09 and 78L10, but there are 79M{09,10} and 79{09,10}
models).  You won't find negative regulators surplus all that often, that
is true, but hey, Digikey has no minimum order and pretty reasonable prices,
too.

kingdom@hpfcdc.HP.COM (Mike Kingdom) (03/15/91)

> In article <1991Mar8.182508.15573@zoo.toronto.edu> henry@zoo.toronto.edu
> (Henry Spencer) writes:
> >I'm not sure why -12 is such a problem.  The circuits for negative supplies
> >are no more complex than those for positive supplies, and IC regulators are
> >just as available.
> 
> Unfortunately, this last statement is not true.  For some reason, positive
> regulators are more common.  You can probably find what you want in a negative
> regulator, but you won't have as much of a choice.

The normal thing to do for disk drive supplies and the like which use +/- 12V
is to build the +12V section like usual with a 7812 regulator and build an
analagous circuit using a 7912 regulator for the -12V section. This is a
non-switching type of supply.

-- Mike