Author Topic: Capacitor selection in triac chopper w/ RFI filter & snubber  (Read 1641 times)

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Offline iroc86Topic starter

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Capacitor selection in triac chopper w/ RFI filter & snubber
« on: November 22, 2019, 02:35:27 pm »
I'm designing a simple triac chopper circuit for use as an AC motor speed controller. The input is 120 VAC and the load will draw 1 A or less. I want to include an LC filter on the input side for RFI suppression and an RC snubber for the inductive load. I'd like to get some advice on selecting the proper type (not value) of capacitors to use in each portion of the circuit.

I've attached a generic schematic below. It's essentially the same as a common lamp dimmer circuit. L1/C1 is the input suppressor, C2/C3 are the triac trigger caps, and R3/C4 is the snubber.

I know that C1 should be X2-rated because it's across the line, but I'm less sure about C2, C3, and C4. These caps should probably be film/foil to handle the surges from operating the triac, but is an X/Y class rating necessary? I recognize that the safety classes only describe the failure mode, not the use case, but they're still technically an across-the-line application even though the caps are in series with resistors and other components. Does it really matter whether they fail short (X) or open (Y)? I've also read that safety capacitors are intended for line suppression and aren't really designed for snubber circuits and whatnot.

I took apart a commercial lamp dimmer and found a circuit similar to the one I'm building. The equivalent of C1 is a boxed film cap (no safety rating from what I could tell) and C2/C3 are dipped film/foil, but who knows the composition. Just looking for some guidance on best practices... I'd appreciate any input. :)


 

Offline T3sl4co1l

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Re: Capacitor selection in triac chopper w/ RFI filter & snubber
« Reply #1 on: November 22, 2019, 03:14:10 pm »
Only across the line in the sense that, when the TRIAC is completely off, most of the line voltage drops across them; but they don't need to be X2 rated, because the transient voltage is limited by the TRIAC.  (Instead of avalanche breakdown, thyristors latch on when subjected to overvoltage.)

So any PP or PE film cap rated >=250VAC and xVDC (where x is the TRIAC DC/peak rating) should be fine.

The trigger caps can be lower voltage, but being smaller, probably doesn't matter much in terms of size or cost.  DIACs are usually what, 20-30V?

Tim
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Offline iroc86Topic starter

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Re: Capacitor selection in triac chopper w/ RFI filter & snubber
« Reply #2 on: November 23, 2019, 07:59:36 pm »
Thanks for the help, Tim! I hadn't considered the relationship between triac breakdown voltage and the capacitor rating, but it makes sense.

I ended up going with general-purpose PP caps rated at 630 VDC / 400 VAC for the triac trigger and a "pulse" cap for the snubber, just for good measure, since its dV/dt rating was an order or magnitude greater than the general-purpose part (1000 vs. 116 V/µsec). I still picked up an X2-rated safety cap for the RFI suppressor, but I think the other caps are a better fit for their respective applications.
 


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