back emf AC
Tobias
Posts: 95
···· I will be controlling a AC contacter with a relay that is controlled by a transistor which is controlled with the BS2, the relay is rated for 120vac at three amp I think I should be OK with the ratings, My concern is, the back emf? how does a guy go about that?
Thanks Toby
Thanks Toby
Comments
Are you using a solid state relay, or an electro-mechanical relay? If the former, you're probably good to go. If you are using the latter, you need to place a reverse biased diode across the coil leads of the electro-mechanical relay.
Regards,
Bruce Bates
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
Um, I can't think of an instance where I'd want to PWM a mechanical relay's coil...
Tobias,
I sense that, given the presence of a contactor, you're going to be driving some pretty heavy loads. It's quite likely, if the contactor is anywhere near the Stamp, you could get enough electromagnetic interference from the arcing or current transients to upset the Stamp's operation. There are a number of threads in the forum discussing issues like this, so you may want to do a search and read them. Forewarned is forearmed!
My personal preference would be to use an optically-isolated SSR to drive the contactor coil and to locate the Stamp some distance from it. I might even use a line driver chip on the Stamp's output to boost the available drive voltage/current to the SSR's input to help counteract any noise pickup. Another option would be to put the SSR near the stamp and locate the contactor more remotely. A good SSR will have a built-in snubber and zero-cross circuit to minimize transients — things a mechanical relay would be lacking.
-Phil