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I have been using an HCPL3700 to detect 24VAC, but they are large and relatively expensive. I have tested the design shown in the figure (AC input to the diode, and measuring output collector-emitter voltage to determine if AC is on or off) and it works fine on the bench. My concern is that the diode in the optocoupler is not rated for use as essentially a rectifier. I use 12k resistors all around (capacitor is 10uF), so power dissipation maximum ratings aren't a problem. Can general purpose optocouplers (LTV-816 in this case) handle this duty?

24VAC detection circuit

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  • \$\begingroup\$ Take a look at this: digikey.com/product-detail/en/… \$\endgroup\$ – SolveEtCoagula07 Apr 12 '18 at 17:48
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    \$\begingroup\$ If you're worried about the reverse voltage in the optocoupler add a diode at the input to take the other half cycle. \$\endgroup\$ – JIm Dearden Apr 12 '18 at 17:57
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Put a bridge in front (you can get a 0.8A bridge SMT mount for a very reasonable price) and you'll be able to detect the input faster than if you just block the current with a reverse diode. But do one or the other.

It's not a good idea to subject the optoisolator IR LED to reverse voltage in excess of the absolute maximum ratings.

There are also purpose-built AC-input optoisolators with back-to-back LEDs that are available in quad version and I would probably pick that option myself.

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  • \$\begingroup\$ Thanks! You read my mind - we were wanting a quad (LTV-846, but one diode), so it will be nice to have essentially everybody's solution in one package. \$\endgroup\$ – T2JSplode Apr 16 '18 at 15:37
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You must not apply a reverse voltage of more than 6 V to the LED. (In practice, the actual breakdown voltage will be higher than that, but 24 VAC certainly is high enough to be too risky.)

You could

  • use a bridge rectifier to route both half waves of the input through the LED, or
  • connect an antiparallel diode to the optocoupler's LED pins to limit the reverse voltage to the forward voltage of the diode, or
  • put a properly rated diode in series with the optocoupler's LED to prevent the LED from being destroyed by the breakdown current.
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  • \$\begingroup\$ Thanks for the help. I was considering rectification, but your second bullet point (antiparallel diode) will work well for the application. \$\endgroup\$ – T2JSplode Apr 16 '18 at 15:35

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