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# Detecting AC Line Voltage

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To make a long story short and to keep it simple, I need to detect when line voltage is present and then supply a logic or open collector output as the input to a downstream circuit.

My thoughts were to use a PS2565 back-to-back diode input optoisolator with Vf=1 and If=80mA. Since AC line voltage is 170 peak (120Vrms), I need to drop 169VAC without generating a TON of heat (ie., not using a resistor). My thoughts, use a 250V capacitor. I did this in a design once about ten years ago but don't remember all the design techniques that go along with it. If I want to drop 169V at 50mA I need a 3380 ohm impedance. So far so good.

If I remember correctly the impedance of capacitor is 1/(2pi*f*C), or capactance = 1/(2pi*f* impedance). With impedance=3380ohms and f=60Hz, I get a 1.25uF capacitor. How am I doing to far?

So will this work or am I going to electrocute myself or burn the house down?

That will work but it is best to put a small series resistor and a cap across the led diodes to prevent the case of momentary connect and quick disconnect and reconnect such the charge on the voltage dropping cap is still charged and the reconnect AC happens to add to voltage already on cap. This puts a surge on led that can damage it. The cap across led needs to be at least ten times the series cap to absorb the surge.

I came up with 785nf.

My thoughts were to use a PS2565 back-to-back diode input optoisolator with Vf=1 and If=80mA....... If I want to drop 169V at 50mA I need a 3380 ohm impedance. So far so good.
Why run it at such a high current? An If of 5ma should be more than enough. If you need more current capability at the transistor side, add a 2nd transistor for a current boost. Also note that the transistor will not be ON in a steady state. It'll be be on maybe 99% of the time and off 1% of the time, 60 times a second.

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So let me get this straight, all you need to do is detect the presence of line voltage and send a signal when it is detected?

Why not go the non-contact route? Have you ever seen the voltage pens that Fluke and Greenlee sell? They detect the electric field when 50/60 Hz AC voltage is present.

I imagine that the IC and sensor used in these products is available in non-proprietary packaging, and a simple circuit could be built for your application to supply a logic state when AC is present WITHOUT direct contact or any risk to life or property

Go get an old wall-wart that puts out a few volts DC.

You don't need that much current, I have seen circuits that work very well that use a .33uf series cap. RCinFLA is right you need a surge suppressor cap after the series cap and a few current limiting resistors too. The circuit I have also uses a back to back type of optoisolator.

So will this work or am I going to electrocute myself or burn the house down?
Probably.

Why don't you use a 120VAC relay. It''s safer, isolated and simple.

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