> >Even better, why not screw the phase angle and switch in (or not)
complete
> >power line cycles?  At 60Hz that gives you 13 different power choices and
> >still drive the heater with at least 5Hz to reduce stress.
>
> This would be yet another good application of a Bresenham or phase
> accumulator algorithm.

Yes, that kind of algorithm would allow much finer control over the average
heat output.  However, it would also lower the fundamental frequency of the
input.  It's hard to say how much this may matter.  You want to keep the
temperature of the heater elements reasonably constant to reduce mechanical
stress on them as they expand and contract.  I used 5Hz just as an example
that was admittedly pulled out of the air.  I don't know what a reasonable
number is, but the bigger the heating elements, the lower the frequency.
5Hz is probably more appropriate for thin elements not much thicker than
large lightbulb filaments.

Most heaters with thermostats do a good enough job regulating room
temperature with two power settings (off, full on).  I bet the greehouse
heater of the original poster would work fine like that with the right
control algorithm.  Even if not, 4 settings would almost certainly be fine,
with anything over 8 effectively "continuous" for the purpose.  Given that,
I'd prefer to optimize for low noise and low heater stress.


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