Re: snubber resistor power rating?
From: Terry Given (my_name_at_ieee.org)
Date: 11/28/04
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Date: Mon, 29 Nov 2004 10:35:37 +1300
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.
Cheers
Terry
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