Triac snubber

What I'm trying to do is use a triac to switch an assortment of loads on/off
using a low voltage control via an opto-diac. The problem is that once the
unit has been running and is then switched off - the triac keeps firing but
not at full duty-cycle! The triac switch works fine with just a transformer
load (wall wart adapter) but when combined with loads that include a
switching PSU, I think the SMPSU pulses cause high Dv/Dt transients across
the triac.

What I think is needed is a snubber network to limit the Dv/Dt, but I do not
know how to calculate the values of the capacitor & resistor. Having tried a
couple of searches for triac application notes, I'm coming up a little thin
on articles about how to handle the math for this - so I'm obviously not
wording the search string well enough!

 

I'd try a 0.1uF and 100R for a start, you can get them in one package
with the cap being an X class type. If all else fails use a relay, you
can really small ones these days with low operating powers.
Most triac application notes would have been written in the seventies
and the companies will have changed their names several times by now.
There's a lot of stuff on the STmicroelectronics site.
I used to have a good book on the subject from IR.
Dv/dt is a problem in some applications. An odd half-cycle in a heating
load is not much of a problem but can be a real pain in a big (kW)
transformer.

 

Thanks for reply,

After a couple more searches I got loads of hits and downloaded hundreds of
triac related pdfs, now I have to search through them to find the info I
want. In the interim I tried a 0.22uF cap in series with a pair of parallel
470 Ohm resistors (235 Ohm), this made matters worse!