Didn't see the original thread but,

If you have + voltage source the good old 555 can give you a negative
voltage at more ma than the 232.  

I Googled this one:    http://www.csgnetwork.com/ne555c1.html       it'll
give you a start. And 555's are cheap.  I have made negative power supplies
and been able to get clean (-) voltage out of it with filtering.

Rich

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Peter Restall
Sent: Monday, October 12, 2009 1:10 PM
To: piclist@mit.edu
Subject: Re: [EE] Use of MAX232 as power supply for operational amplifier in
5V powered system?


On Sun, 11 Oct 2009 15:29:09 +0200, wzab wrote:

> However antoher idea is to use something cheap and standard like MAX232.
> Has anybody tried to use this chip just as a voltage converter?
> Unfortunately I can't find the output characteristics for V+ and V- pins.
> Maybe I'll need also additional voltage regulator to improve the quality
of
> this power supply voltage (or maybe simple RC filter will be
sufficient...).
>
> Has anybody tried to do it?

I remember briefly playing about with this idea, but as you've found out
the output capabilities aren't too well specified.  Since it's a charge pump
there's also a fair bit of high-frequency noise in the output (probably not
too good if you're trying to do a low-noise op-amp application) and the
voltage seemed to droop quite quickly once any appreciable current was drawn
(and it was always less than 2x the supply for no loading anyway, usually
7-8V for a 5V input).  You might get away with this approach for a one-off
or
for high-impedance loads (MOSFET gate drivers ?) without driving any RS232
lines as well, but I dare say the difference between units is marked if
you're planning on multiple devices.

If you're willing to spend a few sheckles on a MAX232, why not just spend
that much on a dedicated IC with better (and more defined) output
characteristics ?  Or if you've got a spare pin from your PIC (presuming
you're using one) then how about spitting out some PWM and using a few
jellybeans to make a buck-boost - I've recently done just this to get a -5V
supply for my op-amps ?

Regards,

Pete Restall
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist