Zero crossing circuit

Question

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:

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.

Answer 1

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: -

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.

Answer 2

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.