Switching inductive loads

And greetings to you, fellow Earthling. Since Klaatu is busy (man, we could use him now), I guess I'll have to do.

I'm assuming you're talking about switching inductive AC loads with TRIACs. This can be a bit of a problem because, at the moment of zero voltage crossing, current is still flowing which prevents the TRIAC from turning off.

There are two approaches to your problem. One thing which is commonly done is to place an R-C snubber across the TRIAC to get the power factor down a little and help reduce the current going through the TRIAC at zero criossing. The RC snubber is also good for limiting dV/dT, which can be responsible for false turn-ons. This solution has limited usefulness, mostly because you have to be very aware of the load inductance, and you're wasting power while the load is off.

The other approach, which is much more commonly used, and which will truly minimize the problem, is to use inverse-parallel-wired, or back-to-back SCRs, and trigger each of them on the appropriate half-cycle. Since an SCR blocks current when reverse-biased, it will turn off at zero crossing, and will stay off (assuming the voltage across the SCR doesn't exceed Vr(max). The difficulty with this is that it requires a more complex triggering system, because you have to keep track of which SCR is next to be triggered, and the gates are at different line potentials.

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One easy solution I've used successfully a number of times over the years is Teccor Electronics alternistors. These are TRIACs which have been specifically designed to switch inductive loads. They act the same as back-to-back SCRs, but only have one gate. They require a little more gate current to turn on, and only operate in quadrants I, II and III. But, you can use them with any standard TRIAC circuit where you've got a positive gate pulse with a positive voltage on the load (Q1), and a negative pulse when there's a negative voltage on the load (Q3). They're made to control voltages up to 800V pk., and currents up to 40 amps. Look up Alternistors in the Digi-Key catalog, and get a one-chip solution. Here's the data sheet:

For the standard newbie circuit of an RC charging circuit with a DIAC driving the gate of the TRIAC, all you have to do is remove the TRIAC, insert the alternistor, and you're good to go. (view in fixed font or M$ Notepad):

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