help in design of electrovalve with optotriac-triac circuit

[Adam Kott]

Hey everyone, hope I'm posting in the right place (if not, please move it to the right one admin). I'm designing a garden sprinkler circuit with 24VAC electrovalves. I have to power them somehow using optotriac-triac circuit and I can't make out how... I found some circuit design used for powering 230V devices but I believe it's useless for my 24VAC electrovalves... Can anyone help me with it? It's very important sice it's an academic project :/

 

[FvM]

The circuit can perfectly work for 24 VAC. The 360 ohm resistor might be better changed to a lower value, e.g. 50 or 100 ohm.

 

[Adam Kott]

really? great! can you explain shortly why this 360ohm resistor should be changed to lower?

 

[FvM]

The resistor is there to limit peak gate current during the short period before the main triac is triggered. Because the triac has a minimum trigger current, the resistor will delay triggering of the main triac until Vinst/R > Itrigger after each zero crossing, cutting part of the full wave.

 

[Adam Kott]

one more thing that I'd like to ask. After the 330ohm resistance (the one closer to the uC), is any transistor needed? My colleague used simmilar design for 230V heater and before connecting the circuit to the uC he placed a BJT but honestly I don't know why.

 

[FvM]

As shown, the circuit will have hardly ever worked. AT89C51 series has only µA high output current. You'll either change the circuit to active low drive or use a transistor (or both).

I would always prefer active low, otherwise the processor will activate the valve in reset state.

 

[Adam Kott]

hmmm. datasheet says that the pins have not µA but mA output current... anyways, if I wanted to change the circuit to active low one, how should I modify it? I found something on using a switch connected between gnd and output but it concerns external pull-ups and my uC has internal ones.

 

[KerimF]

As FvM said it is better using the MCU pin as active low.
First, you connect the LED Anode of the opto IC (pin 1) to Vcc (5V).
Second, you connect the 330 Ohm to the LED cathode (pin 2).
Third, if the MCU pin is high, the output triac is off. If it is low, the triac is on.

 

[Adam Kott]

ok, I drew a simple scheme (don't know if I understood correctly) so if anyone can check, would be great. I used a BJT to further improve the circuit as FvM suggested and connections as KerimF suggested (I believe so)
and I changed 360ohm resistor to a smaller one as well

 

[KerimF]

By using a PNP transistor as you did, the LED anode (pin 1) should be connected to the 330R resistor and the LED cathode (pin 2) to ground.
Note please that here the pin is also active low (output triac is on when the MCU pin is low).