Triac snubber

Apr 05, 2006 2 Replies
I

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!


Vote
Twitter Facebook LinkedIn
N

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.

In message , I.F. writes

Neil J. Harris
Vote
Twitter Facebook LinkedIn
I

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!
Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required