The triac should already have inside a low resistor value (120 ohm or so)
between it's gate and A1 (driving anode). Check this using a DVM as ohmeter
after unplug the circuit from power supply. If not, mount it outside.
The triac can be fired by dv/dt, but if you have resistive load, this is
unlikely.
If still have the problem, try to take out C210 and build the entire
snubber separately across A1-A2, the schematic you've posted is mixing the
snubber with the opto-triac supply.

best wishes,
Vasile Surducan

On Tue, Feb 24, 2015 at 6:43 PM, Isaac Marino Bavaresco <
isaacbavaresco@yahoo.com.br> wrote:

> It is a long time since I last used a TRIAC driver, but the circuits I
> used (with the MOC3063) had a resistor between the gate and MT1, to
> ensure any leakage current could not build enough voltage to trigger a
> sensitive-gate TRIAC.
>
> Isaac
>
>
>
> On 24/02/2015 02:52, stephen.forrest@agilent.com wrote:
> > Hi Everyone -
> > Well, after probably 15 years lurking, a couple of posted responses and
> a few private ones, I have my very first question of the list!
> >
> > I am having some trouble with a triac phase control circuit (attached)
> where I am getting a half wave conduction fault with the triacs. (This is=
 a
> failure mode where the triac conducts for half a cycle even with no gate
> signal.) I have found numerous references to this failure mode around the
> web but never any clue as to its cause. The load is either a resistive
> heater pad or a blower. Neither are of high power - about 100W - and I ha=
ve
> found the fault with both types of loads. I have seen no sign of
> over-heating, nor of any other stress. The triacs trigger properly on the
> other half cycle.
> >
> > Any ideas as to the cause of this type of fault? I am at a loss...
> >
> > Cheers
> > Stephen
> >
> >
> >
> >
>
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