On 2011-05-10 16:18, Olin Lathrop wrote:
>
> In a recent project I used a Bresenham style algorithm to dither the outp=
ut
> to some heaters.  Each heater is fully on or off each half power line cyc=
le.
> Since there are many many 1/2 power line cycles within the first order ti=
me
> constant of the heaters, there is not problem introducing low frequencies=
..

I think "re-invented" a similar algorithm for a toaster controller a few=20
weeks ago and surprised myself at just how simple and effective it can be.


     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;
         }


The ISR runs on every negative-going zero crossing (the AC line is=20
connected to an external interrupt pin via multi-megohm resistor). So it=20
only switches whole cycles. This basically produces pulse density=20
modulation with 16-bit resolution.  At the lowest setting it would=20
output a one-cycle pulse every 1092 seconds (a very low frequency indeed.)

It was my understanding that it's "bad" to draw DC from the line, so I=20
was wary of drawing half-cycles. (Imagine if your output setting was 50%=20
and you were consuming every other half cycle).

Joe Koberg


--=20
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