[chbot] Triac snubber

[Mark Atherton]

This is quite an interesting discussion.

I think the starting point is to consider the load that the TRIAC is 
driving - a SMPSU which is usually a full wave bridge rectifier 
driving a 47uF 400V cap - or thereabouts.

With the secondary of the SMPSU driving a moderate load, the primary 
47uF cap will never approach being fully discharged during any point 
of the 10ms (20ms) power cycle.

It is therefore reasonable that there will only be current peaks into 
the 47uF cap when the mains approaches a crest. During the other 
time, the input bridge is reverse biased...

So how about reverse leakage current of the input bridge holding the 
TRIAC on - you mentioned that this takes about a day to become a 
nuisance, I am guessing that this is a self heating  effect (?).

So how about a small resistive load across the input to the SMPSU - 
maybe 2W at 230VAC - maybe 22k / 5W wire-wound just to see if this 
affects the behaviour.

Anyway, just my 2c.

-mark



At 08:27 p.m. 21/01/2013, you wrote:
>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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>
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