OT-Noise immune zero crossing (phase angle firing)

[Brian S]

Sorry for not logging in on my last post (Guest, as foothill).

Neil, there's a fairly direct way I've used to sense a reasonably stable zero-crossing reference:

I used a "back-to-back" emitter type Opto, reading a line sample which had been clamped and filtered to reduce noise. Both common mode and normal mode filtering was needed.

I started a timer when output was off (ie, when line voltage approaching zero). Stopped the timer when output went on. This gave a window centered on actual zero-crossing (accurate to the threshold voltage characteristics of the opto's). Dividing this time by 2 gave a time-zero reference. I kept a running average of both cycle-cycle period as well as window length; this seemed to screen out much of the noise.

I drove the opto with the highest "safe" emitter voltage, to get fastest turn-on.

Any scheme you use will probably require filtering to remove both common- and normal-mode high dv/dt components from the sampled line...but you're probably doing that already. At 4KW, you may elect to filter only the line-sampling/opto circuit, rather than exhaustively filtering the transformer mains (though that may be required anyhow by the site manager).

I've seen a similar system used on a 20KW resistive propane heater control under industrial conditions with good results.