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...

enter image description here

  • \$\begingroup\$ LEDs don't "receive" voltage; they drop voltage based on how much current is passing through them. \$\endgroup\$ Jan 22, 2016 at 2:25
  • \$\begingroup\$ How much voltage can go trought leds? \$\endgroup\$ Jan 22, 2016 at 2:35
  • \$\begingroup\$ Voltage doesn't go through LEDs, it drops across LEDs. \$\endgroup\$ Jan 22, 2016 at 2:43
  • \$\begingroup\$ 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. \$\endgroup\$ Jan 22, 2016 at 3:15
  • \$\begingroup\$ 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. \$\endgroup\$ Jan 22, 2016 at 4:32

1 Answer 1


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?

  • \$\begingroup\$ What wattage of resistor should be? \$\endgroup\$ Jan 22, 2016 at 2:49
  • \$\begingroup\$ 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. \$\endgroup\$ Jan 22, 2016 at 2:57
  • \$\begingroup\$ Thanks very much, I understand now better. I will ask u some more question if I get some more problems when I try this... \$\endgroup\$ Jan 22, 2016 at 3:13
  • \$\begingroup\$ I have one more question... Is possible to calculate voltage after current go trought resistors? \$\endgroup\$ Jan 22, 2016 at 12:36
  • \$\begingroup\$ 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. \$\endgroup\$ Jan 22, 2016 at 12:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.