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Provide SC protection for Triac O/P

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pavanmg

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I have designed a triac circuit to driive AC 220v solenoids using moc3020 ic, but am stuck about how should I provide Short Circuit protection to the triacs since the slightest spike on O/P makes the triac blow off.
Anybody suggest something ??
 
For a start use over specification triacs!, that will help, and you should have capacitor/resistor networks across the triacs as well (I'm trying to remember what it's called? - something like dV/dA? - with the greek letter delta).

The triac datasheet should explain it!.
 
Nigel has the right idea. You need to install a "snubber" across the triac. Directly across the Triac is best. Most commonly this is done as a simple series circuit of resistor capacitor. For 60 Hz, the usual values are 100 Ohm, and 0.1uF capacitor, this likely will be good for 50Hz also.

The term Nigel refers to I think is dV/dT, change in voltage over change in time. When the load is switched off, ( especially in an inductive load,) there can be a dramatic rise in voltage across the Triac. Triacs have a dV/dT rating that cannot be exceeded or they fail, or refire, mostly with the wrong results.

This applies to relays also, to prevent arcing across the opening contacts before the zero crossing.
 
thanx a lot guys but I have selected the triacs with a lot of over rating - both current and voltage. Yes about the snubber RC for dv/dt improvement I will give it a try btu I doubt it will help in case of Short Circuits on O/P (even a momentary one).
Any more Ideas ?
 
Since you are driving an inductive load, over current and Short Circuit protection are going to be a little tricky.

You could try a Triac module that has this built-in. Not always the cheapest solution, and one that may come back to haunt you if the design is long term and you need to maintain it 5 years from now, if the device is obsoleted. Fast blow fuses inline can help, but are a nuisance if they can't handle the inrush, or the Triac dies before the fuse.

For absolute SC protection you are going to need intelligent current sensing. You will have to monitor load current, decide what is beyond inrush, or is a sustained short, and disable the power source. This can get messy and complicated, and probably beyond what you need to do.

I would choose a Triac that is capable of handling the available supply, with generous headroom, then protect it with a snubber. Honestly, I have seen more Triacs die an unatural death from exceeding their dV/dT rating, than from a SC's that were not designed out in the first place.

I would also change to a zero-crossing opto-coupler, unless for some reason you absolutely have to have random phase triggering. You said you are controlling solenoids.. can they wait until a zero-cross to be operated? At 50Hz, given a solenoids electrical and mechanical behaviour, I think zerocross switching should be fine.
 
As for short circuits on the outputs?, it's ALWAYS been a big problem with TRIAC's, however, for your application, it sounds like it shouldn't be a very likely event? - why should your solenoid valves have a short circuit across them?.

In disco light units the TRIAC's often fail when a bulb dies, bulbs commonly go S/C as they fail - this very often blows the TRIAC's as it happens. At one time you could buy special fuses designed to protect the TRIAC's - but they only had limited success in doing so, and the fuses cost more than the TRIAC's.

Certainly it would be a GOOD idea to use a zero-crossing opto-coupler to feed the TRIAC's, triggering at minimum current has got to be good for the life of the TRIAC's, particularly as you have an inductive load!.

BTW, you say your TRIAC's are over rated, what are their actual ratings?, and what are the solenoids requirements?.
 
Yes providing SC protection is a very tricky job for triacs as I noticed. By the way the triac ratings I used is 800V 15A whereas the solenoids I am switching are rated for 220V 2A max.
I have provided sufficient snubber RC hoping my triacs wont blow cos of high dv/dt. But the solenoids I am switching work in a moist atmosphere that increases the chances of shorting very much.
thanx a lot guys
 
It sounds as thought the solenoids you are using are not suited for their working environment.

I would suggest you check the IP ( Ingress Protection ) rating of the ones you are using. Probably a IP56 rating is the minimum you require.

If you are woring with existing equipment you cannot change, then remidies can be taken to protect what you have. Connections can be insulated with dual-wall ( adhesive lined ) heatshrink, and the coil body could be sprayed with a silicone based conformal coating.

Here is an explaination of IP ratings:
**broken link removed**
 
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