This is a circuit that allows you to control an AC load in an analogue fashion from a PWM signal from a microcontroller,

This circuit is perfect for controlling a mains powered heater or a lamp in a closed loop system with a PID controller like a heated chamber for a 3d printer or a hot bed. Conventional solid state relays can't be used for this because they are on/off, can't provide an analog control of the load. This circuit can.

The input for controling the power expects an open drain/open collector signal like the ones from a heater output of a 3d printer board. If you have a TTL signal you should add a transistor to provide the correct input to the circuit.


This circuit uses a triac to control an ac load varying a delay between the zero crossing of the AC mains and the activation of the triac. that delay is commanded from the pwm input.

All the mains powered parts are opto isolated from the low voltage signal so it's very safe.

The PCB also includes a 12v fan output with 2 modes. in manual mode it can be enabled from the fan pin on the signal connector, and on the auto mode the fan tuns on when the mains power is present.


In concept this circuit is similar to this arduino application: https://playground.arduino.cc/Main/ACPhaseControl

but it replaces the microcontroller with a 555 timer so it's much simpler/cheaper.

Jan 09,2019
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