# How Zero Crossing optoisolator to work with 220VAC?

I want to use a H11AA1 to detect Zero Crossing, but I am not sure how circuit should look like. How much voltage should LED receive inside H11AA1?

Here is datasheet: http://www.vishay.com/docs/83608/h11aa1.pdf

By my calculation, please correct me if I am wrong. Forward continuous current from datasheet 60mA.

Resistor should be 220/0.06= 3.6k
With wattage 220*0.06=13.2W


Here is some circuit which I found but not seem legit...

• LEDs don't "receive" voltage; they drop voltage based on how much current is passing through them. – Ignacio Vazquez-Abrams Jan 22 '16 at 2:25
• How much voltage can go trought leds? – Vladimir Djukic Jan 22 '16 at 2:35
• Voltage doesn't go through LEDs, it drops across LEDs. – Ignacio Vazquez-Abrams Jan 22 '16 at 2:43
• Is that led same as this:greenprophet.com/wp-content/uploads/2012/09/… ? I know when I apply to much voltage to it, it will burn. – Vladimir Djukic Jan 22 '16 at 3:15
• It will work, but it only detects 'zero' very approximately (maybe Vout high for |Vin| < 25~35V) . If you're happy with such large values for 'zero' (and thus wide pulses), it may work for you. – Spehro Pefhany Jan 22 '16 at 4:32

The maximum current through the LEDs has to be limited to 60mA. That would not be the normal operating current for that optoisolator.

The 230VAC RMS with 66K ohm series resistors allows a current of 230/66K = 3.4mA RMS through the LEDs. The peak current would be 4.8mA when the AC voltage reaches its very peak value.

These conditions should allow that optocoupler to work just fine.

Why do you have a doubt about the circuit now?

• What wattage of resistor should be? – Vladimir Djukic Jan 22 '16 at 2:49
• Each resistor will have approximately 115 volts RMS across it. At 33K the actual RMS power in each resistor would be (115 * 115)/33K = ~ 0.4W. So it seems safe to recommend 1W resistors. If you could have good cooling air flow you may be able to get 1/2W resistors to be OK. – Michael Karas Jan 22 '16 at 2:57
• Thanks very much, I understand now better. I will ask u some more question if I get some more problems when I try this... – Vladimir Djukic Jan 22 '16 at 3:13
• I have one more question... Is possible to calculate voltage after current go trought resistors? – Vladimir Djukic Jan 22 '16 at 12:36
• Are you wondering about the voltage drop across the LEDs themselves? If so that is specified as the LED Forward Voltage in the data sheet and the data sheet Figure 1. – Michael Karas Jan 22 '16 at 12:51