Re: snubber resistor power rating?

From: Arie de Muynck (send.spam.to_at_spammer.org)
Date: 11/28/04 

Date: Sun, 28 Nov 2004 10:18:39 +0100

Ken Smith" ....
> Is this what we are dealing with here:
> ...
> If so, how does the [triac] get turned off with a current flowing in the
load?

No. It is:

   ----------------------------------------
   ^ | |
   ! ) /
   ! ) L (load) \ R
   ! ) /
  Mains | \
   ! ----- |
   ! A V Triac |
   ! ----- ___
   ! Trigger ckt----/ | ___ C
   V | |
   ----------------------------------------

The triac turns off at the zerocrossing of the current through it. Since the
mains voltage will be about maximum then, the snubber limits the slewrate,
preventing the turnon by excessive dV/dt.
The resistor provides damping of the turnoff efect. It also limits the
current when the triac fires at turnon:
    I(pk) = Vmains(pk) / R
and this discussion is about how a 2W 47 Ohm resistor likes that hefty
spike...

Regards,
Arie de Muynck