As a follow-up to this prior question I'm looking for a zero-crossing mains (120V) power switching solution to mitigate the bounce-related EMI I'm getting using the "stock" traditional ON/OFF toggle switch connected directly to the step-down transformer.

In the abstract, what I'm looking for is this:

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As I search on the web, I haven't come across anything so far that doesn't require a separately powered "control" circuit, with its own 24V (or whatever) DC power supply.

What I envision is something like one of those dimmer switches that fits into a wall junction box and takes whatever power it needs directly from the mains (without needing a transformer of its own).

Is there such a thing? I vaguely sense I just haven't found the right search term yet. I've been using "zero crossing triac circuit".

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    \$\begingroup\$ A simple TRIAC circuit can work, the issue is that the control part of the circuit is basically a capacitor dropper with some resistance and perhaps clamp and/or a diac trigger and so getting the control signal in, working on it, probing, etc safely are all concerns. It is common to put an optocoupler in there (in fact a triac-output one can substantially simplify driving your power triac) but then you need a power supply to drive the isolated side of the opto. It is not clear that soft starting will solve your observed problem anyway, but another soft-start mechanism is an NTC thermistor. \$\endgroup\$ – Chris Stratton Apr 3 '16 at 2:58
  • \$\begingroup\$ Thanks for this comment Chris, it pointed me in the direction of transformer inrush current, which I suspect plays as big a part in the spikes as the switch bounce :) Over the weekend I came up to speed on thyristors and got a triac switch breadboarded. It eliminates the bounce, but there's still significant spiking going on, especially at turn off (might need a snubber). I do think I'll try an NTC thermistor in there for inrush limiting. I wasn't liking that it probably wouldn't help on warm cycling, but I expect it's better than not addressing inrush at all :) \$\endgroup\$ – scanny Apr 4 '16 at 22:40

You can design (or buy) an SSR with a 120VAC control input and zero-voltage switching output. Anything more than a few amperes will need consideration as to how to get the heat out. Probably run you $20-$30 for a safety-agency approved unit from a good maker capable of a few amperes (like the dimmer you mention).

Designing it could be accomplished with a zero-voltage switching triac output optocoupler with a mostly capacitive dropper and bridge rectifier at the input. The optocoupler would drive the main triac. Nothing would be isolated from the line in such a design, so proper care would have to be taken or it could easily be unsafe.

Such an installation (including the use of a UL/CSA approved SSR) may well not conform to code requirements and you'd be well advised to find out what your local regulations are- both for safety and to ensure your insurance coverage is not compromised.

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  • \$\begingroup\$ Ah, now Google is working again, thanks Spehro! Finding room for a small brick for $25 is definitely further than I'd want to go on this project, but it's good to have confidence I'm not missing a simpler solution. As I review the new resources search is now turning up, I'm thinking a zero-crossing circuit might not be best anyway. I'm starting to think that a zero-current turn-off is more important, given the transformer load, and that maybe a simpler no-bounce triac switch approach might be better. I'll continue my research :) The load would be about 250mA btw, it's a 10W unit. \$\endgroup\$ – scanny Apr 2 '16 at 21:41
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    \$\begingroup\$ All triacs are zero-current turn-off (once triggered they remain on until the current through the triac falls to zero) so that bit is OK. Zero voltage turn-on gives you the least switching noise so that's OK too. \$\endgroup\$ – Transistor Apr 2 '16 at 22:31

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