On 27 Feb 2015 at 1:43, stephen.forrest@agilent.com wrote:

> Hi Brent - The triacs are permanently damaged. One half cycle is
> permanently on. If the output is observed with a scope (and yes, I
> have HV differential probes) it looks like a diode. The other half
> cycle works fine. The gate is driven by an opto-triac so I believe
> the gate phasing is 1 & 3.=20

Ok, found your original post & circuit.

To be honest I have a very limited amount of experience with TRIACs, but th=
is may=20
be of some value...

I once built a circuit that used a TRIAC opto-coupler to directly power a 2=
30VAC=20
solenoid (load of 6VA =3D only 26mA =3D within the specs of the opto-couple=
r, and so=20
perhaps risky but no external TRIAC was used). Just on/off, no phase contro=
l.=20
There were some failures in the field, embarrasing. The part that failed wa=
s the=20
TRIAC opto-coupler, sometimes the symptom was half waving as you describe.

The problem was thought to be line voltage transients exceeding the voltage=
 rating=20
of the opto-coupler. The fix was to add a VDR across the optocoupler (there=
 was=20
already a series fusible resistor, which would act to limit current during =
an over=20
voltage excursion), and we upgraded the optocoupler to a 600V part (was 400=
V).=20
This seemed to cure it acceptably.

Back to your circuit. No over-voltage protection components that we can see=
,=20
although they could be elsewhere. I'm suspicious of feeding the gate drive =
from the=20
C in the RC snubber - question in my mind is what determines the gate drive=
=20
current, if it might be possible for example that a high voltage/high frequ=
ency=20
transient drastically lowers the reactance of C210 and massively increases =
gate=20
current. Even so you'd think the absolute max 4A peak rating of the gate of=
 the=20
TRIAC and the 2 x 180R resistors would let it survive this.

I'm guessing the fault is not easily reproduced on the bench. When the TRIA=
C dies=20
does the optocoupler also fail? I hope it's not something absurd like the s=
eries=20
resistors breaking down but not leaving any visible evidence?



--=20
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