AC Phase control

Posted on Feb 5, 2014

One method of controlling power to ac circuits uses a triac to turn the power on and off at precisely timed intervals that are synchronized with the ac signal. This method is called AC phase control. It is the method used in many light dimmer and heater power control circuits. Using an Arduino microcontroller with some simple circuitry, we can monitor the ac wave to determine the

AC Phase control
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proper time to turn the power on and off with the triac. The circuit I used to do this is shown below. The circuit consists of an opto-isolated zero-crossing detector and a opto-isolated trigger circuit for the triac. The opto-isolators are necessary to keep the low voltage signal circuits away from the power circuits and provide an appropriate level of safety. As will all circuits involving mains voltage, make sure you know what you are doing. The zero-crossing detection circuit provides a 5v pulse every time the ac signal crosses zero volts. We detect this with the Arduino and leverage interrupts to time the trigger circuit precisely in synchronization with these zero-crossing events. The method for power control is shown in the diagram below. Once a zero crossing is detected, the triac remains off for a controlled amount of time (t1). The longer this time is, the less power the ac circuit receives. Once the off-time , t1 has elapsed, the microcontroller turns on the triac by applying a voltage to the gate (shown in red). Once turned on, the triac will remain on even after the gate voltage has been removed. It will turn off if the gate voltage is zero the next time the ac wave crosses zero. Because of this, we do not need to take care to turn the triac off when the ac signal crosses zero again. All we need to do is to ensure that the triac gets turnd off inside of the period of ½ wave (t3). The duration of the gate...

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