Logic level TRIAC control

[KerimF]

For too many decades, triac was the main element in my produced products mainly for light control.

(1) the triac is turned on by a gate-MT1 current.

(2) The triac input is equivalent to two diodes in parallel.

(3) In power triac, an internal resistance is added between gate and MT1 for better stability. In general, even for low power triac, it is better adding this resistor externally. Its value is not critical (it could be from 100 to 470 Ohm) but this increases the triggering current of the triac.

(4) If a digital multimeter is set for diode checking and the reading between gate and MT1 (same in either direction) is much lower than 0.6V, it would be a sign of an added internal resistance.

(5) For usual AC applications, a triac has a higher gate sensitivity (lower triggering current ) if the trigger current is from MT1 to gate (quadrants II and III).

(6) If the board is out of reach, MT1 could be connected to Vcc of the MCU.

(7) Even if the triac is sensitive (Ig about 10mA), I recommend using a transistor as a buffer.

(8) An NPN transistor could be used. Its emitter is connected to GND and its collector to the gate via a series resistor. The connection of the base to the MCU pin depends on the I/O pin type.

(9) In case of C-51 Atmel MCU as AT89C2051, the pin output is an open collector with a weak pull-up resistance. So the base could be directly connected to a pin. If the pin is low, the NPN transistor turns off (I_base=0). If it is high and the pull-up resistance is relatively big to turn on the transistor, an external resistor (as 4K7 for example) could be added between the pin and Vcc.

 

[betwixt]

I have several of these already but please be more specific about the requirement for logic control. All mine use standard phase shift networks and diacs to fire the triac, if you want to use logic control you also need extra circuits to detect the zero crossing points and a control signal to decide the firing angles.

Brian.

 

[schmitt trigger]

I have several of these already but please be more specific about the requirement for logic control. All mine use standard phase shift networks and diacs to fire the triac, if you want to use logic control you also need extra circuits to detect the zero crossing points and a control signal to decide the firing angles.

Brian.

Agree 100 percent.
A potentiometer, capacitor, diac and triac.is all that is required. Unless there are additional requirements.