How much current at -12volts do you need? Is it only for RS232? If you
are going to use a linear regulator to convert -24 to -12 volts you will
dissipate (I*12) watts. You can use a 7512 regulator backwards to
regulate (or use the 7912 negative voltage version).
Your 2 100K voltage divider was probably too wimpy (120 uAmps). Try lower
value resistors if you are going the divider route. If the -12 is only
used for RS232, the negative voltage is probably not too critical. It
will probably work with -10v.
You could also try a "buffered" voltage divider approach:
+24v
| --------
R | |
R = 1Meg | |\ |
| |-| \__|________ +12 out
-----|------------| /
| | +|/ micro power op-amp
C R = 1Meg
C R C 1 or 10uF
| |
|----|
--- GND
-
Maxim (www.maxim-ic.com) makes a bunch of different swithing voltage
converters which need no external inductors (just a few capacitors). I've
used them before. They have different versision which can derive a
regulated negative output voltage with a positive or negative voltage
input. Digi-key carries them.
#---------------------------------------------------------------------------
# Geoff Parker 
On Sun, 25 Oct 1998, Tim Gray wrote:
> Ok, I have a switching dc-dc supply that generates my +5v +12v but it
> generates a -24v instead of a -12 that is needed. How can I get this down
> to 12 volts?? I've tried a 12 volt zener diode and that sinks all the
> current my supply will handle on the - voltate bus. I've tried 2 100K
> resistors in a voltage divider but that sinks the same amount of current!
> The entire power supply is all SMD and I cannot locate the negative supply
> components. Any help will be great as I cant stand wasting 1/2 the power
> going through dual converters (battery to 5volt then 5 volt to +-12 volt.)
> is it possible to use a 7512 regulator "backwards" to regulate a negative
> supply or is that going to sink my 0.04amps of negative supply.
>
> Why cant serial port manufacturers make all ports with max232 chips so the
> dang +-12 volts isnt required anymore!
>
>
> -----
> I know, I use Windows, but it's only for games!
> Linux is the only OS for real work!
> -----
>
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