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Joe Koberg wrote:
>      uint16 current_setting, accumulator;
>
>      void change_heater_setting( float desired_fraction ) {
>          current_setting =3D desired_fraction * 65535;
>          }
>
>      void zero_crossing_isr( void ) {
>          if (accumulator < current_setting)
>              heater_output =3D ON;
>          else
>              heater_output =3D OFF;
>          accumulator -=3D current_setting;
>          }

That's the basic idea, although use of floating point and 16 bit integers i=
s
totally silly if this was done on a PIC.  Just because you can do something
easily in a higher level language doesn't make it a good idea.  The problem
with 16 bits is that the repetition period can be very long, much longer
than the thermal time constants.

In my implementation I used 8 bit integers, 0 is full off and 255 full on.
Even that is more resolution than needed considering it's inside the
feedback loop.  I attached the salient part of my code since the comments
would probably wrap and make a mess if pasted here.  This runs once for eac=
h
of the 32 heaters on every line cycle zero crossing.  The PIC is a 18F4550.

> It was my understanding that it's "bad" to draw DC from the line, so I
> was wary of drawing half-cycles.

It's probably not great if lots of devices did it all synchronized.  In my
case the unit is driving 32 heaters, each independently controlled.  The
chance of them all turning on every other half cycle and being in sync for
more than a few cycles is pretty much zero.

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