Thanks for your thoughts Vasile -

We are switching mains voltage so I think there will be no DC and with an o=
pto-triac driving the trigger the triggers pulses will be in the correct qu=
adrants. Our triacs are definitely damaged and behaving as a diode for one =
half cycle. The other half cycle triggers normally.

Regards
Stephen

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 embedded systems
Sent: Friday, 27 February 2015 6:27 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Triac "half wave" failure mode

Most common problems in triac firing is the missing of one half cycle and n=
ot the permanent conduction of one cycle. This because the two thyristors i=
nside 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 e=
ither 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 alread=
y, 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/

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