piclist 2000\06\05\042901a >
Thread: PC Power Supply
face BY : Mark Willis email (remove spam text)

Mitchell D. Miller wrote:
> Mark,
> >
> > Hint:  This isn't a "low-noise" power supply.  You may want to use R-L-C
> > filtering if you need cleaner power - don't know your needs.  For PIC
> > projects I just down-regulate from 9V, works.  (7809 inside that panel.)
> >
> Is the noise enough to cause oscillatior problems on a simple SX project?
> I've been trying to use the SX with the Murara resonator that Parallax
> sells, but can't seem to make it oscillate.  Project works fine when
> Parallax's SX Key is providing the clock.
> Why do you choose to regulate down from 9v (which I presume comes from the
> +12), instead of directly using the +5 the p.s. provides?  Noise?
> -- Mitch

I wouldn't expect noisy 5V to cause a resonator to fail to work - more
like to cause brownout problems and crash the SX Key (or repeated
resets?)  Scope Vcc and see what you see - per usual it's late before I
get back to e-mail <G>  Noise tends to CAUSE oscillations (at least when
inside the feedback loop) so I'd think something else is wrong?
(Haven't gotten SX's to the top of my dolist yet - Some day.)

Several reasons why I don't use the 5V directly - I figured the fastest
way to current limit this monster was to put a (2A fused) 7809 behind
the cover plate, that should protect things.  Then let out 9V at 1.5A
max, I used a regular PC style power connector (recycled a HDD connector
IIRC) to feed power to that PC board the 7809 lives on;  I didn't have
any PolySwitches and (at the time) my workbench suffered occasional
Severe Attack Of The Electronics-Eating Cat Incursions, thus fuse
popping was a distinct probability;  No problems yet.  (Have a small
resistor in series there - 0.1 Ohm, 1/2W IIRC - after the fuse & before
the cap across the In side of the 7809, not sure it was *needed* but I
figured it was a good de-coupling idea.  Don't remember if I 'scoped
across it or not!)  I hand-picked a 7809 with lowish Dropout Voltage
(under 3V, obviously!).  Debated using a 7808 with a diode in the base
lead but HAD a 7809 that worked <G>  (It might be some other reg., I
should go look!  Too darn busy here.  It's been a high-interruption
day!  ... life!  <G>)

A lower dropout regulator would be a better design;  I'd suggest at
least some good current limiting inside the PC's case - I like the idea
of using a blank panel as an interface, since it's PORTABLE, easy to
transfer *when* your PC power supply dies, & right there up front where
you need it to be;  Use what works for you.

Been too long since I scoped a PC power supply;  Take a look, it's noisy
in there.  My current O-Scope is a piece of crummy junk, saving to get a
better one.  Now I find it probably should be battery-powered, so much
for the 'scope I had been thinking of!


<393B6489.1B003E56@foxinternet.net> 7bit

See also: www.piclist.com/techref/power/actodc.htm?key=power
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