> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Dwayne Reid
> Sent: 04 February 2012 00:59
> To: pic microcontroller discussion list
> Subject: [PIC] Running a PIC from a pulsating DC supply
>=20
> Good day to all.
>=20
> I've got an interesting question: what kind of performance will I get
> if I run a PIC from an un-filtered, full-wave-rectified DC supply at
> 60Hz (120Hz)?
>=20
> I have an existing product that regulates temperature using a really
> simple 2-transistor diff amp (long-tailed pair) and
> thermistor.  Power supply is a simple full-wave bridge rectifier /
> resistor & zener regulator.  Because its tiny, there is no power
> supply filtering.  Performance below PSU regulation is predictable
> and the units seem to be happy being powered off and on 120 times per
> second.
>=20
> Now I've been asked to add two more thermistor inputs (all with
> different set-points) and I'm running out of space on the board (its
> pretty tiny).  There just isn't enough room to fit in 4 more sot-23
> transistors and associated resistors.
>=20
> My first thought was to use one of the nice, new Silego chips that I
> started playing with but they contain only 2 comparitors.  So: I'd
> have to use a pair of them.
>=20
> My next thought was to use a PIC.  One of the new 10F or 12F parts
> should work just fine, except for that pesky pulsating DC supply.
>=20
> This would not be feasible with the old PICs - they had that
> 1024-cycle oscillator delay before leaving reset.  But the new PICs
> running from internal RC oscillator should start up pretty quickly,
> I'd think.
>=20
> I'm currently looking at the 12f1822 and 12f1840 because I've got
> some here but I may look at the equivalent 10f parts if I don't need
> all the i/o pins.
>=20
> Opinions appreciated.

If you can retrofit a PIC based design, why can't you add a small filter ca=
pacitor for the PIC (via another diode to prevent loading from the rest of =
the circuit)?  Providing the PIC is not driving any significant I/O current=
s then 10uF or less may be sufficient.

Cheers

Mike

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