On 4 Mar 2015 at 1:15, stephen.forrest@agilent.com wrote:

> Hi Brent -
>=20
> You are right, it has been impossible to replicate on the bench,
> even with the actual loads. We do have an MOV earlier in the
> circuit, but this whole triac drive is disconnected by a relay
> (2-pole) to isolate it. The MOV is on the other side of the relay. I
> have tried repeated relay openings (1000's under different load
> conditions) but still no failure :o(=20
>=20
> I have been trying to find a mechanism for this type of fault in
> triacs. Gate current pulse, voltage spike, repetitive something... ?
> While I can find reference to the failure mode, and of course all of
> the precautions about dv/dt etc., I have yet to find a description
> of what causes this type of failure.=20

Hi Stephen,

My gut feeling is over voltage... because memory suggests that's what I was=
 told by=20
the supplier/manufactuer of the opto-triac I had issues with some 20 years =
ago. Too=20
far back to find in my emails though.

To try and replicate the failure perhaps you can increase the supply voltag=
e using a=20
VARIAC and see what type of failures arise. Or increase load current or inc=
rease=20
gate current until failure occurs.

Getting further away from your circuit and harder to trace... have you cons=
idered=20
what other AC equipment is operating on the same supply as the devices that=
 have=20
failed, perhaps there is something common in the environment? Look for sour=
ces of=20
electrical noise, say high freq components that could cause resonances with=
 the L=20
in your load and C in your snubber, or just a switched inductive load with =
no=20
snubbing.

Dumb question perhaps - are the two mains terminals on your TRIAC transpose=
d?=20
This might mean you're not operating in the quadrants you think you are whi=
ch=20
probably shouldn't cause a failure, but perhaps there could be other effect=
s I can't=20
think of right now.



--=20
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