[chbot] Triac snubber

[Mikael Stewart]

Hi Jasper,

Don't forget the opto-triac may be more dI/dt sensitive than the one 
switching the load. Also, driving an SMPS this way might be difficult 
because of the peaky current draw. The conducted mains noise input may 
also be an issue if you're driving something noisy off or near same outlet.

A conservative approach would be to

1. RC snubber both anodes of the power triac (A2 to A1).
2. Place a series resistor from A2 of the power triac (load end) to 
opto-triac, then capacitor to A1 of power triac.
3. Capacitor across gate and A1 of power triac. A resistor in parallel 
with this if no internal one (hurries gate voltage below threshold 
before zero crossing inversion).
4. Use X rated caps. Size resistor dissipation for faulting triac 
(always on) and flash through voltage.

As a starting point, lets say 100n for all 3 caps. 120 ohms for resistor 
in (1), 360 ohm for resistor in (2), 360 ohm for resistor in (3) 
assuming power triac has weak resistor. You can use 3x 120 ohm for the 
360's.

Have you considered a series diode full bridge with transistor across 
the output? Sometimes saves space over a triac + bulky x caps.


Mikael





On 21/01/2013 8:12 a.m., Jasper Mackenzie wrote:
> Good day,
>  Having a great deal of difficulty switching a small smps with a 
> triac. I assume it has to do with holding voltage AND high dI/dt. I 
> need to use the smps for its size in this app, and will need to switch 
> bigger ones in the not too distant future.
>  The triac is driven from an opto, and the cirucit is simply the pump 
> driver from a gentle annie chopped out with a hacksaw. resistor values 
> have been changed etc. I have tried a cap across MT1/2...
>  Its good for a while, then it starts moemntary self triggering for a 
> day or so, then full on forever...
>
> Cheers.
>
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