Bob <op1cwk@flashmail.com> wrote...

>I don't know what your talking about (Willis), about using a light=20
>bulb to sense load output (sounds pretty "Rube Goldberg'ish" to me).

"Rube Goldberg'ish"???  Jeepers creepers, Bob, that circuit has only
THREE flippin' parts in it, and they're all passives!!!  What's to not
understand?

Alright, let's make it simpler: I'm going to knock it down to TWO
parts for you.  Hang on to your hat, and read slowly and carefully.

As in my previous suggestion, connect a small light bulb across the
transformer winding that feeds your heater; select a lamp with a
voltage rating such that when your heater is at full power, the lamp
glows reasonably brightly (but, as Olin pointed out, not TOO bright or
the light bulb won't last very long).

Now, take a  small silicon photovoltaic cell (e.g., a "solar cell")
and connect the (-) wire to circuit ground at the PIC chip.  Connect
the (+) wire to one of the PIC's A/D converter inputs.  [Use of a
silicon photocell also addresses Olin's recommendation to use a sensor
with more infrared response- i.e., a better match to the lamp's
spectral output- and also gives you a component that'll be easier to
obtain than a CdS cell; those things aren't as widely available as
they once were.]

Position the photocell so that the light from your light bulb shines
on it, and you're good to go (except for programming, of course).

Any voltage fluctuation that makes your heater hotter will also make
the light bulb brighter.  This will make the voltage coming out of the
solar cell larger, and you can measure that voltage with the PIC's A/D
converter.  The rest of the job is in the PID control which will
adjust the triac's firing angle (as Hallikainen indicated in his post,
mere proportional control won't suffice to give you both precision and
stability; you'll need P, I and D to get good results).

You should keep in mind one shortcoming of this "World's Simplest
Implementation of the World's Most Complicated Voltage Regulator":
that solar cell won't be putting out much voltage, perhaps a half-volt
or so.

I'd suggest amplifying its output (gain of somewhere around five) so
the PIC's A/D converter has more signal to work with, but that'd bring
you up to six components (opamp, supply bypass cap and two resistors
in addition to the lamp and photocell) and we don't want THAT.  Plus,
you indicated earlier that opamps are still somewhat of a mystery to
you, so let's nix that idea.  Use as low a VREF for the A/D converter
as you can get away with, so you get the maximum resolution in
measuring the photocell's output.

So there it is, Bob.  Two components.  I can't make it any simpler.
If anyone else can, a doff of the hat to 'im.

BTW, who the heck is "Willis"?

Dave

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