Purpose of ...

[bryan]

Can anybody tell me what the purpose of R3 and C1 are in this schematic.

 

[Phasor]

R3/C1 appear to be power limiters. So, when the triac is on, the power head gets full power, and when the triac is off, the power head gets some power, but limited.

On second thoughts, given the value of the capacitor (0.1u) - this is about 25kohm at 60Hz (quite high), so it is probably a noise filter.

 

[mechie]

Mains Filter

I reckon it's a low-pass filter to kill fast transients when the triac opens the circuit (agreeing with Phasor)
Such 'snubber' circuits are fairly common across sensitive circuit mains supplies. The triac will only cope with
maybe 400v peak (on 120v mains) and noisy mains supplies can fry them without this protection.

Or ... :?

Do I remember something about inductive loads causing problems with triac triggering - the phase-shifted
current preventing the triac from ceasing conduction; once you fire the triac it will 'flywheel' and you can't
shut it off.
If this is the case then the cap gives an alternative current path while the triac shuts down.

I favour the first explanation though :!: (so that last bit must be a load of @$%^)

 

[bryan]

Thanks for teh reply. I built the circuit without the R/C combo and it worked fine, but I will add them to protect the Triac.