Switching low voltage AC with a microcontroller

AC_laagspanningThis circuit can be used to switch a low voltage (12V) AC load with an Arduino or other microcontroller
A positive signal on the entrance will bring T1 into conduction, triggering the Triac via emitter resistor R2. D1 and C1 are just there to provide DC-power to the transistor.
R3 will pull the base low. It can be left out, but it avoids accidental switching when the microcontroller port is undefined (like at start up). I would really advise to put it there.
The Triac should be cooled with a metal profile. remember that the Gate is galvanically connected with the metal housing of the Triac.

WARNING: This is for LOW AC voltage only. Switching a high voltage AC requires use of opto couplers to isolate the High Voltage from the microcontroller.

Notes. I have used an SC141B Triac rather than a TIC206 as I had that around and it is 110 V. max. In a 220 V. country I am not likely going to use that soon for another project.
The picture of the mounted PCB does not show the heatsink yet. It is advisable to use a heatsink.
The printdesign can be downloaded here. There is space for various size capacitors. Mine seems very big but it is a very old one I still had lying around. You may want to move the position of D1 to make it a bit easier to mount a heatsink.
The Diode D1 does not really need to be a 1N4007. The DC part of the circuit does not really need much power. a 1n4148 would be OK, but they have the same price and I had a 1n4007 lying around

Print mounted with components
Print mounted with components

6 thoughts on “Switching low voltage AC with a microcontroller”

    1. void setup()
      pinMode(12, OUTPUT);
      void loop()

    1. That depends on the TRIAC used. The TIC 206 can stand 3 Amps. The Watts depend on the voltage that you use. The TRIAC is capable of 600 Volts but I really would not use this circuit for 600 Volts

  1. But the Triac will open at 3V approx. That means, the lamp will not get a full sine wave. From the zero crossing point, there is a delay and the the curve jumps up to 3V. Not such a good solution.

    1. Psaitz, i am not sure i understand your comment. The best solution for what?. It shows a way to switch AC with a micro, but sure there are more ways to skin a cat. One could use a relay, an ssri, 2 anti parallel thyristors, a bjt or fet with a rectifier bridge and even a servo or actuator pressing a mechanical switch. They all have their pro’s and con’s. Whether something is or is not the best solution all depends on the problem in question.

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