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Connecting firing circuit to triac through optocoupler

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admnovsk

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Hi

Im designing a TRIAC control circuit. The problem that im having is im not really sure how to connect the output of the comparator to the TRIAC by using an optocoupler. I have 2 choices of optocoupler which is the MOC3021 and also 4N25. I have looked through other thread in this forum but i must say i cant really understand. I cannot upload the circuit that im building because the forum wouldnt allow me to...

Thanks in advance.

---------- Post added at 16:20 ---------- Previous post was at 16:07 ----------

I'll just describe it more or less. The firing circuit has a comparator output. This output is connected to the base of BC546 transistor. Emitter is grounded.6V DC is connected to a 470resistor ---> optocoupler input LED ---> BC546 collector. This is how i connect the input side of the optocoupler to the output of my firing circuit. Is this acceptable?
 

betwixt

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It sounds good to me but if you are using it to control an AC circuit, use the MOC3021, the 4N25 will not work. The reason is that the MOC3021 is an opto-triac, it will conduct in both polarities when LED current flows and this is neccessary for the main triac to conduct throughout the whole AC cycle. The 4N25 has a transistor output which will only conduct over part of the cycle.

Brian.
 

admnovsk

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Thanks betwixt. by the way is it possible to connect the LED directly to the comparator output such as

comparator output ---> resistor ----> LED anode ---> led cathode ---> ground.

If im not mistaken this connection will trigger the LED directly from the comparator, with the resistance limiting the flowing current through the LED. Ill try to upload my circuit diagram in a moment.
 

betwixt

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If the comparator can supply enough current, and the supply voltage is high enough to operate the LED, it will work without the transistor driver.
Calculate the resistor value by taking the difference between the comparators output high voltage and the LED forward voltage, divided by the LED current.
Without seeing your diagram, I would warn that some comparator ICs have open collector outputs so they can't directly supply any current at all. If this is the case, consider swapping the comparator inputs so the 'logic' is reversed then sink current from supply into the comparator output instead.

Brian.
 

admnovsk

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Im actually using a LM324 opamp as my comparator.I attach my circuit diagram below.



The output high of the comparator is 4v.and im using 4n25 in tis simulation because i dont have the model for moc3021

---------- Post added at 20:21 ---------- Previous post was at 20:18 ----------

or is it better of to connect the optocoupler as in the following diagram



btw thanks for the help Brian. really appreciate it
 

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Sorry for the late reply - I've been away all day.
I assume from your high voltage of 4V that your supply voltage is about 5V, you should be able to source about 10mA maximum directly from the LM324 but if you take more, the output voltage will drop rapidly and you may not have enough to operate the LED. It might be safer to use your circuit with the transistor driver, the LM324 will be happier and the LED will be sure of a healthy current supply.

I would suggest you add a resistor between pins 1 and 3 of your triac. If the MOC3021 is turned off, it leaves the triac gate floating and you might get random switching, a resistor to discharge any leakage current or static charge will ensure it only turns on when the MOC3021 tell it to.

Brian.
 

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