using moc3023 triac opto coupler to fire a triac

Thread Starter

denison

Joined Oct 13, 2018
252
This is a further note on the Fairchild schematic for using moc3023. The .05uf capacitor obviously has to be able to withstand 240vac as can be seen from the schematic. It is being charged through a 470 ohm resistor and the load even when the moc 3023 is turned off.
The .05 capacitor is not optional but is vital for the operation of the moc3023. In my circuit the capacitor fires at 29 volt providing 80ma of current to the gate of the main triac.
The 360 ohm resistor shows 2.7w of instant power and the 470 ohm 3.6 watts. However the average power is very low being less than 0.25 watts.
I initially used a 560 ohm 3 watt resistor without the capacitor and it burnt out.
Use a 3 watt resistor for the 360 and 470 ohm resistors to be on the safe side.
I hope this will assist other uses of the moc3023.
 

Thread Starter

denison

Joined Oct 13, 2018
252
hi,
I have posted a clip from the MOC3023 d/s so that others can refer to a circuit diagram, using your text
E
View attachment 174820
Hi Eric Gibbs, I tried to upload the schematic but it was apparently in the wrong format to be accepted on the forum website. Thanks for adding the circuit for people to view. Unfortunately Fairchild did a poor job of explaining the circuit.
I usually switch the neutral in my triac trigger circuits as it is safer than having the hot wire switched.
 

Thread Starter

denison

Joined Oct 13, 2018
252
hi,
I have posted a clip from the MOC3023 d/s so that others can refer to a circuit diagram, using your text
E
View attachment 174820
Hi Eric, I have tried this circuit using a BT138X-600E sensitive gate triac and moc3023. The load is a 25W 240v filament globe. I cannot get it to work. Even tried the snubbers. The globe does not turn on. No damage seen to any of the components.
The only way I could get the triac to fire was to use a capacitor and diac on the output of the moc3023.
I have no problems with the zero crossing moc3043 triac opto coupler and the recommended Fairchild circuit with the BT138X-600E.
Any ideas how I can get the moc3023 to work? I may contact Fairchild regarding this but it is very hard to get these people to reply.
Here is a thought on why it may not work. To turn on a triac requires a rapid rise of voltage. This would happen with the zero crossing opto and also with the diac coupling.
Perhaps I should be looking for another triac which does not require the rapid rise of voltage to turn on.
 

Ian0

Joined Aug 7, 2020
3,768
Does it work without the 0.05uF capacitor? I never use it, and it has always worked, and I have never had any problems with interference. I rarely use snubbers either, because I use the SGS-Thomson "snubberless" (W suffix) triacs.

The MOC3023 is not a zero-crossing part. Is the LED current high enough? Is the trigger pulse long enough? (the MT1-MT2 current must exceed the holding current for it to remain on after the end of the trigger pulse).

To turn on a triac requires a rapid rise of voltage.
It doesn't - it requires a gate current more than the minimum trigger current for the quadrant in which it is being triggered.
A rapid rise of voltage between MT2 and MT1 may trigger the triac, but it is a situation to be avoided.
 
Last edited:

Thread Starter

denison

Joined Oct 13, 2018
252
Does it work without the 0.05uF capacitor? I never use it, and it has always worked, and I have never had any problems with interference. I rarely use snubbers either, because I use the SGS-Thomson "snubberless" (W suffix) triacs.

The MOC3023 is not a zero-crossing part. Is the LED current high enough? Is the trigger pulse long enough? (the MT1-MT2 current must exceed the holding current for it to remain on after the end of the trigger pulse).


It doesn't - it requires a gate current more than the minimum trigger current for the quadrant in which it is being triggered.
A rapid rise of voltage between MT2 and MT1 may trigger the triac, but it is a situation to be avoided.
The moc3023 is not a zero crossing opto. I know that Ian. The trigger pulse is continuous when I clamp it to the 5v line. Not working with or without the snubber capacitors. I have increased the led current to 50ma. max is 60ma. still not working.
Tried the moc3020 opto. still not working. It may be the holding current as I am using a 25watt globe and the current is very small. I will try a heavier load. However I have no trouble turning on the triac with the moc3043 zero crossing opto using the same 25 watt globe and the same triac(holding current the same).
All very peculiar and you say that you have had no trouble with the random phase opto. I guess I can also try a snubberless triac.
 

Ian0

Joined Aug 7, 2020
3,768
With a continuous LED current the triac in the MOC3023 should be on. If that hasn’t switched the main triac on, then the resistors burn out.
With a continuous LED current, the triac should be on even if the load current is lower than the holding current.
Voltage readings across the opto and the resistors should show what is going on.
The only problem a snubberless triac will solve is spurious triggering.
Also, triacs almost never fail open circuit, so your triac most likely isn’t faulty.
 

Thread Starter

denison

Joined Oct 13, 2018
252
With a continuous LED current the triac in the MOC3023 should be on. If that hasn’t switched the main triac on, then the resistors burn out.
With a continuous LED current, the triac should be on even if the load current is lower than the holding current.
Voltage readings across the opto and the resistors should show what is going on.
The only problem a snubberless triac will solve is spurious triggering.
Also, triacs almost never fail open circuit, so your triac most likely isn’t faulty.
The 390 ohm resistor I am using connected to the mains did not burn out even though the triac failed to fire. You are right of course about switching the live wire instead of neutral. In future I will do it that way.
I will check the voltage readings. I will get back to you if I find the problem as it will also help others who I have seen on the internet have had this problem.
 

Ian0

Joined Aug 7, 2020
3,768
Try putting a switch (make sure it is mains-rated) across the output of the opto-triac.
The main triac should then switch on whenever you close the switch.
 
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