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I have made a simlpe zero crossing circuit to control the speef of a single phase AC fan through conduction angle control with a Triac. The circuit looks like this with the use of 4n25, an optocoupler:

enter image description here

L and N represent live and neutral out of the mains supply.

So ideally i thought while the mains sine wave is above or below zero, you would get 0V beneath the 10k pullup resistor since there is condcution on the infrared diode and thus, conduction through the transistor. and when the sine wave passes through 0, it would make a pulse with 5V at its peak. But when i put this to test, I see a sqaure wave of 50% duty cycle and 50Hz frequency, meaning the conduction through the transistor basically changes state when the mains sine wave goes from negative values to positive values. I am struggling to see the reason why and could really use some help.

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  • \$\begingroup\$ There are issues like the minimum current required by the optocoupler, which you don't seem to have incorporated in your design (200k total resistance). A better way to detect zero crossing is to use a small voltage transformer. \$\endgroup\$ – Abdullah Baig Jun 18 at 8:01
  • \$\begingroup\$ I can't still understand why your transistor doesn't turn on in one of the negative and positive cycles. It can be a wrong connection or some other problem. But I will suggest an alternate way which is commonly used and I have also used it myself. \$\endgroup\$ – Abdullah Baig Jun 18 at 8:03
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You have ground connections on both sides of the opto-isolator and this is significantly problematic for isolation reasons AND will cause half wave rectification of the signal and lead to your output waveform being how you describe it. Just do this instead: -

enter image description here

Note also that controlling of an AC fan by using a triac may not result in any speed reduction and could lead to problems with the fan - it depends on the motor used inside the fan - standard AC motors are intended to be used for fixed speed applications. The devil is in the detail.

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  • \$\begingroup\$ Fans are regularly controlled by traic controllers. In our homes, we have a speed controller for each fan in each room. \$\endgroup\$ – Abdullah Baig Jun 18 at 7:57
  • \$\begingroup\$ The devil is in the detail. Many can't. Maybe make an answer yourself and explain what AC motors can be used for speed control and what can't? \$\endgroup\$ – Andy aka Jun 18 at 7:58
  • \$\begingroup\$ connecting to same ground, as the OP has done, may create a risk of electric shock, but should not interfere with the full wave rectification. \$\endgroup\$ – Abdullah Baig Jun 18 at 7:59
  • \$\begingroup\$ @AbdullahBaig Neutral is normally connected to ground. Think about what will happen. \$\endgroup\$ – Andy aka Jun 18 at 8:00
  • \$\begingroup\$ If the N is connected to the ground labeled in OP's diagram, then the full wave rectifier will give one cycle out output and shortcircuit the input (through 100k) in the other cycle. That may be the reason why OP is getting half wave signal, not full wave. But for this short to successfully happen, he must connect the ground connection in the circuit board to the actual Earth ground, which is rarely done. \$\endgroup\$ – Abdullah Baig Jun 18 at 8:22
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I think you objective with this question is to find a way to detect the zero crossing of the mains voltage.

  • If you are trying to design a completely analog circuit, you can use something like this. These circuits are used everywhere and you can find the components easily. Although you may need to adjust the values of resistors and capacitors if you are working with 110 V mains.
  • If you are using this circuit to enable a microcontroller like Arduino to control the speed of the motor, I would suggest you use a small voltage transformer (or potential transformer as it is more commonly known). You can get the smaller ones at the size of about 1x1x1 inch. The transformer will transform the mains voltage to about 12Vac, then use a resistive potential divider to reduce it to about 1.2 Vac, so it swings from +1.7 to -1.7 V. Then tie one end of it to 2.5V DC so the other end goes from +3.7 to 0.3 V. This will be perfect for a microcontroller to process. You will know a zero crossing when the voltage passes below 2.5 V. All of this circuit also comes in a module known as ZMPT101B. You can find one easily on Amazon, Ebay etc or your local hobby electronics store.
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