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Watch your creepage distance


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So on Saturday we had a bit of unexpected rain, and my home-made irrigation pump triac switch wasn't installed (yet) out of harms way. So even though it was mostly water tight, it drew enough moisture that two mains-carrying points arced over. Check out the result.


This was mostly caused because there just wasn't enough distance between the two points. Thankfully it wasn't directly across the mains, it had the 850W pump motor to sink the energy into :-)


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Well... chances are the Triac made it. It depends how hot it got. The opto-coupler behind it (not visible on the photo) also very likely survived. I've decided however that for applications where more than a few watt is being switched, at low duty cycles, I'm going to use a relay in future. In fact, already replaced it with a relay.


Found out that you cannot feed a relay with AC. Don't know why I thought that would work (perhaps because it works for irrigation valves), in hindsight that's obvious. Half-wave rectifier (aka 1 x 1N4007 salvaged from an old energy-saver lamp) and a 10uF cap (suitably chosen to deal with peak voltage) and we're back in business.


And all that so the garden doesn't die a slow death while we're away :-)

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Found out that you cannot feed a relay with AC.


You get relays with AC coils and relays with DC coils. DC coils usually have higher resistances while AC coils usually have lower resistances (because on AC the impedance comes into the equation). You should be able to energize a DC relay coil with an AC voltage considerable higher than the DC rating of the relay coil.

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I have a few AC relays, working year in and year out. 


Or am I missing something?

This particular one went "buzzzzzz" on the AC signal. Makes sense to me, some are obviously not designed for AC. I added a diode and a cap... it's been working fine for a few days now.


(Old relay from a Coroma Garage Door Opener, 24VDC coil, was used to run the 230VAC motor that opened the door).

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