On 04-Feb-12 01:59, Dwayne Reid wrote:
> Good day to all.
>
> 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)?
>
> 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 sec=
ond.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> Opinions appreciated.

These are great chips and I don't see any problems with your idea. They=20
have BOR so you will just have to reset at 60Hz. 8ms is a very long time=20
if you are running at 16MHz.

You can even keep running from your decoupling capacitor if you measure=20
your own Vdd and slow down the clock when reset is near.

Djula

>
> Many thanks!
>
> dwayne
>
--=20
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