Re: snubber resistor power rating?
From: John Popelish (jpopelish_at_rica.net)
Date: 11/29/04
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Date: Sun, 28 Nov 2004 19:34:17 -0500
Terry Given wrote:
>
> Ken Smith wrote:
> > In article <41a9a09e$0$566$e4fe514c@news.xs4all.nl>,
> > Arie de Muynck <send.spam.to@spammer.org> wrote:
> >
> >>AAArrghhh...
> >>
> >>OK, here's the right drawing:
> >>
> >>"Arie de Muynck" ...
> >>
> >>>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 (mainly inductive load)
> >> ---------UUUU---------------------------
> >> ^ | |
> >> ! | /
> >> ! | \ 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
> >
> >
> > Ok got it.
> >
> > So, the spike like current in the resistor is a current that starts equal
> > to mains/R and then decreases very rapidly.
> >
> > Does the triac get turned on only at zero crossings in this application or
> > is it phase controlled? If it is turned on at zero crossings, there is a
> > reduced requirement on the resistor. In the phase controlled case, the
> > resistor can end up with 4 spikes of almost a big per cycle.
> >
> > 47 Ohms is a lot of resistance to solve this way but at lower voltages, I
> > have made resistors to protect crowbar SCRs out just a length of hook up
> > wire folded back on its self. The accuracy of the value isn't good but
> > copper wire can take a huge spike with no trouble because the resistance
> > is spread over a large volume and it is very thermally conductive.
> >
> >
> > I have had a lot of trouble finding any resistor that has a good pulse
> > handling ability in surface mount. The ones I did find were very
> > expensive and not very available. They were from one of the Tyco
> > companies.
> >
>
> IRC (the company, I do Recall Correctly) make some great smt resistors.
> And have peak pulse power curves.
Caddock does too:
http://www.caddock.com/Online_catalog/smt/smt.html
-- John Popelish
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