AC Switching Question

jcrollman · Mar 9, 2010

I am trying to switch a solenoid on and off in the realm of 500mA of 24VAC.

Can you use a phototransistor reliably to gate a triac? All circuit examples that I have found show the triac being gated by a phototriac.

Relatedly, would it make sense to just ditch the triac all together and simply switch through a girthy SSR?

I was thinking this one might handle my needs:

Digi-Key - 255-1533-5-ND (Manufacturer - AQH2223A)

Thanks! - Jeff

Hero999 · Mar 9, 2010

What's wrong with using a photo TRIAC?

You can get an opto-TRIAC that can drive the solenoid directly.
http://www.datasheetcatalog.org/datasheet/motorola/MOCZ500.pdf

If you can't find anywhere that sells it then use the standard opto TRIAC and TRIAC circuit.
**broken link removed**

RCinFLA · Mar 9, 2010

The AQV20 series should be fine to drive an A.C. relay coil if the de-energizing back EMF from the relay coil does not exceed 600 v rating. It is diode in series with a conducting MOSFET. Might want to add two back to back zener surge suppressors across relay coil to be sure.

For driving triac use a photo coupled diac.

crutschow · Mar 9, 2010

That SSR should work fine for you application. It's input is isolated from the power line and just requires a small DC current to turn on (make sure you use a current limiting input resistor or you will blow the input control diode).

crutschow · Mar 9, 2010

RCinFLA said:
The AQV20 series should be fine to drive an A.C. relay coil if the de-energizing back EMF from the relay coil does not exceed 600 v rating. It is diode in series with a conducting MOSFET. Might want to add two back to back zener surge suppressors across relay coil to be sure.

You should not need any suppressors since a TRIAC automatically turns off when the current through the inductor goes to zero. There thus will be no back EMF.

jcrollman · Mar 10, 2010

Thanks for the input.

In regards to the MOCZ500, isn't there some no-no about using zero crossing circuitry with inductive loads like a solenoid? It had to do with the current and voltage being out of phase, but I couldn't really get my mind around why.

Also, I measured the largest amp draw coil I am to switch, and it was drawing 465mA. If I were to use the MOCZ500, which has a current limit of 500mA, am I not giving the SSR enough room to operate reliably?

jcrollman · Mar 10, 2010

After looking around a bit, it seems that the MOCZ500 rights were sold from motorola to fairchild, but fairchild doesn't even list it as a valid part on their website. It may be falling into the nether regions of part availability. Anybody know of a similar part?

Hero999 · Mar 10, 2010

I can't seem to find it either.

Anyway there's nothing wrong with the SSR you posted.

crutschow · Mar 10, 2010

jcrollman said:
In regards to the MOCZ500, isn't there some no-no about using zero crossing circuitry with inductive loads like a solenoid? It had to do with the current and voltage being out of phase, but I couldn't really get my mind around why.

A zero-crossing circuit only affects the timing of the device turn-on (which, of course, occurs at the voltage zero crossing). It has no effect on the turn-off which occurs naturally at the zero current crossing for all TRIACs and SCRs. It either case I see no problem with driving an inductive load with this. Turning on an inductor at zero crossing certainly has no adverse effect, and turning off the solenoid at zero current generates no transient voltages.

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AC Switching Question

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