Re: snubber resistor power rating?

From: Ken Smith (kensmith_at_green.rahul.net)
Date: 11/28/04 

Date: Sun, 28 Nov 2004 17:59:59 +0000 (UTC)

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. 

-- 
--
kensmith@rahul.net   forging knowledge

Relevant Pages

  • Re: snubber resistor power rating?
    ... Arie de Muynck wrote: ... >> The triac turns off at the zerocrossing of the current through it. ... >> Arie de Muynck ... the spike like current in the resistor is a current that starts equal ...
    (sci.electronics.basics)
  • Re: snubber resistor power rating?
    ... Arie de Muynck wrote: ... how would typical resistors react to such a pulse? ... >>as carbon comp, carbon film, metal film, MOX flame proof, wirewound, etc? ... > using a film resistor. ...
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  • Re: snubber resistor power rating?
    ... Arie de Muynck wrote: ... how would typical resistors react to such a pulse? ... >>as carbon comp, carbon film, metal film, MOX flame proof, wirewound, etc? ... > using a film resistor. ...
    (sci.electronics.design)