Dwayne Reid 17:38 2005-12-09:
>At 03:47 AM 12/9/2005, Morgan Olsson wrote:
>>I have never seen a triac as a slave to a relay before, but in this app=
lication it seem the best thing.  So... what can go wrong...?
>>
>>As a remedy for relay contacts burning i have put a TRIAC as slave to i=
t.
>>
>>Schematic:
>>
>>    Existing relay contact that
>>    can=B4t handle the motor itself
>>       ,---o/o-----.
>>       |       |<|-' gate
>>   O---+-------|>|------------.
>>230VAC        Triac    Load=3DVery inductive motor
>>   O--------------------------=B4
>
>1) resistor in series with gate is required to prevent triac failure.  I=
magine that the relay closes near one of the AC peaks.  The peak current =
into the gate can be so high as to destroy the triac before it has a chan=
ce to turn on.  Small resistor (270R) should eliminate the problem.

That was my initial thinking.
But, the motor is inductive, so there would not be very much current peak.

Another worry would be that too much current go through gate also when tr=
iac is on, as g is connected to a2.  But my measuremets on TIC206D say an=
ly about 10% pass through g, so 50mA at 0,5A =3D no problem

(byt still the gut feeling is to use a R anyway... but i can find no prov=
en reason to... but for to be very sure.)

>2) I agree with the other respondents who suggested putting a 10R / 0.1u=
F snubber across the relay contacts.  At least try it to see if that redu=
ces the contact sparking & erosion.  Should be a much lower cost than the=
 triac if it works.

I think it is about the same cost, and i think the triac will do better.
(se me previous post about RC)

/Morgan
--
Morgan Olsson, Kivik, Sweden


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