I am making an arduino controlled fan speed controller that would digitally control the speed of my ceiling fan working on 230 VAC and 50 Hz. I am using the following circuit which is designed for 110VAC and 60Hz enter image description here

I took this circuit from this website:- http://playground.arduino.cc/Main/ACPhaseControl

Working:- When the AC voltage goes from zero to peak, the optocoupler is active and activates the output side. The arduino pin connected to the output of the optocoupler senses a HIGH voltage. When the pin reads a low it means we are approaching at a zero crossing of the AC mains voltage and depending upon the timing, the arduino will activate the TRIAC driver that is optocoupler MOC3052 that will further activate the TRIAC providing the power to the ceiling fan.

My doubt is that, in the datasheet of H11AA1 optocoupler ic the absolute max forward voltage of the optocoupler is not mentioned. How is this ic working directly with AC. I understand that the current is low since there are two 15k Ohms resistors, but the voltage is still around 55VAC at the pins 1 and 2 of H11AA1. How is this IC not destroyed?? Also will the circuit work for 230VAC 50Hz??

Datasheet of H11AA1: - http://www.vishay.com/doc?83608


1 Answer 1


What makes you think that the voltage at pis 1 and 2 is 55VAC? There are back-to-back diodes across pins 1 & 2, and as long as you don't exceed the current rating of the diodes you will never see more than the diode's forward voltage drop between pins 1 & 2.

The max forward diode voltage is specified as 1.5V at 10mA. In your circuit the peak current will be the peak of the AC line, maybe 155V across 30K or around 5mA. So pins 1 & 2 will have less than 1.5V across them.

If you limit the current to a safe value you could use the circuit at 230VAC as well.

There are safety issues with use and PCB layout of this kind of device that you have to understand, but the device itself should be fine for these applications.

  • \$\begingroup\$ thanks John D, I wasn't thinking about the voltage drop because of the diodes. You just made my day !! \$\endgroup\$ Feb 4, 2017 at 21:18
  • 1
    \$\begingroup\$ No problem, @RishiSharma, but please be careful, seek advice and learn about creepage and clearance rules for high voltage isolation. Working with 230VAC is not recommended unless you really know what you are doing. \$\endgroup\$
    – John D
    Feb 4, 2017 at 21:24

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