Most common problems in triac firing is the missing of one half cycle and
not the permanent conduction of one cycle. This because the two thyristors
inside the triac are driven one in quadrant I and the other in quadrant
III. You need more gate current for one quadrant than for the other. See
here: http://en.wikipedia.org/wiki/TRIAC

Almost for sure, your half triac remains ON when it should be OFF because
either you are not switching pure AC (you may have a DC component on the
load) or you have high dv/dt one one half AC cycle. Also check the
latching/holding current of the device and the driving scheme. As I told
you already, you can't have a snubber which is also polarizing the gate.
The snubber is that RC circuit across the A1-A2.

Vasile Surducan
http://www.itim-cj.ro/~vasile/

On Fri, Feb 27, 2015 at 5:26 AM, Brent Brown <brent.brown@clear.net.nz>
wrote:

> 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.
>
> Ok, found your original post & circuit.
>
> To be honest I have a very limited amount of experience with TRIACs, but
> this may
> be of some value...
>
> I once built a circuit that used a TRIAC opto-coupler to directly power a
> 230VAC
> solenoid (load of 6VA =3D only 26mA =3D within the specs of the opto-coup=
ler,
> and so
> perhaps risky but no external TRIAC was used). Just on/off, no phase
> control.
> There were some failures in the field, embarrasing. The part that failed
> was the
> 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
> of the opto-coupler. The fix was to add a VDR across the optocoupler
> (there was
> already a series fusible resistor, which would act to limit current durin=
g
> an over
> voltage excursion), and we upgraded the optocoupler to a 600V part (was
> 400V).
> This seemed to cure it acceptably.
>
> Back to your circuit. No over-voltage protection components that we can
> see,
> although they could be elsewhere. I'm suspicious of feeding the gate driv=
e
> from the
> C in the RC snubber - question in my mind is what determines the gate dri=
ve
> current, if it might be possible for example that a high voltage/high
> frequency
> transient drastically lowers the reactance of C210 and massively increase=
s
> gate
> current. Even so you'd think the absolute max 4A peak rating of the gate
> of the
> 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
> TRIAC dies
> does the optocoupler also fail? I hope it's not something absurd like the
> series
> resistors breaking down but not leaving any visible evidence?
>
>
>
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