On 26 Feb 2015 at 21:24, stephen.forrest@agilent.com wrote:

> Thanks to those who replied. The circuit work s fine but just
> occasionally (too often for my boss) we get a failure. Maybe a
> transient but the question is how. Oh well, keep digging...=20
>=20
> > I am having some trouble with a triac phase control circuit
> (attached) where I am getting a half wave conduction fault with the
> triacs.=20

Is the fault self-resetting? ie. is the TRIAC simply turning on for a half =
cycle when it=20
shouldn't, or is the device permanently damaged such that it always conduct=
s half=20
wave? Sorry didn't see you original circuit, not sure if anyone mentioned y=
et that=20
TRIACs will switch on with high dV/dt across them. And you know that whenev=
er=20
they turn on they stay on for a half cycle (actually when current reaches z=
ero, which=20
equals zero crossing voltage point for a resistive load). The spec sheet sh=
ould have=20
values for max dV/dt. That's one reason why RC snubbers are used accross th=
em.

The gate sensitivity is different for all four quadrants of operation (+/- =
gate current,=20
+/- AC phase), so it would seem reasonable to assume a "transient" problem =
would=20
mostly show up in one particular half of the wave.



--=20
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