How is the MOC 3052 triac driver optocoupler supposed to work?

[Wrichik Basu]

I have the following circuit with the MOC 3052 Triac driver optocoupler:

The diagram of the MOC 3052 has been made with respect to its datasheet. The two legs of the transformer are labelled T1 and T2. The green wire of the optocoupler has been connected to the Gate of the BT 139 Triac. Also, as you can see, the LED is connected to T2 via a resistor. To switch on the LED, its other terminal has to be connected to T1. (This has been tested to work.)

Now, the two main terminals (MT) of the optocoupler are supposed to be connected only when the 5V DC supply is on. Am I right?

If yes, this is what I found instead: when the 5V DC supply is on, the LED glows when touched to either the yellow wire or the green wire (or to the gate of the Triac). Fine, that's expected. Now, I switch off the 5V DC. The LED glows when connected to the yellow terminal (expected), but it glows when connected to the green wire as well, which is unexpected. Why does this happen?

I have rechecked my wiring, and everything seems to be fine.

Also, when the DC is on, the Triac is supposed to be turned on by the optocoupler. I checked and found that the Gate is getting a voltage of around 7.4V AC (expected, as mains is currently > 220V) , but the Triac is still not on as the LED does not glow when connected to the orange wire. Where am I going wrong?

 

[DaveE]

it glows when connected to the green wire as well

Because current can flow through the Triac from Gate to MT1 to complete the circuit.

Gate is getting a voltage of around 7.4V

Somethings not right here, maybe you could describe it more carefully? The Triac Gate should look like a diode, and can't support 7V.

Try rewiring your circuit like Figure 7 in the datasheet, so the Gate trigger current flows to/from MT2 through the resistor & IC to the Gate and then through MT1 to return to the source.

Also, your LED test load may not have enough current to make the Triac latch on so you can't trigger it with a pulse. However, it will turn on when the Gate current is flowing.

This is a good review of how Triacs work:
https://www.electronicshub.org/triac/

 

[alan123hk]

If there is a clearer electronic circuit diagram, it will be easier to understand. In any case, I suspect that the problem is related to latching current and holding current. Please try to carefully check whether the latching current and holding current are sufficient to maintain the normal operation of the circuit.

 

[DaveE]

If there is a clearer electronic circuit diagram, it will be easier to understand. In any case, I suspect that the problem is related to latching current and holding current. Please try to carefully check whether the latching current and holding current are sufficient to maintain the normal operation of the circuit.

Yes, that will be an issue, but not for this experiment. He is continuously triggering in this test.

However, I 100% agree. There is a good reason that EEs always have a schematic.

 

[Wrichik Basu]

Because current can flow through the Triac from Gate to MT1 to complete the circuit.

Okay, that explains it.

Somethings not right here, maybe you could describe it more carefully?

Currently the potential difference (P.D.) between T1 and T2 of the transformer is 7.28V (RMS). I replaced R2 from 560 Ω to 1 kΩ. With the multimeter, the P.D. between the gate and T2 is still around 7.28V.

The BT 139 datasheet says that maximum gate current is 2 A, and peak gate power is 5 W. The power rating of my resistor is 250 mW, so with the peak voltage being 10.3 V, the current came out to be 0.0243 A. Using V=IR, the resistance comes out to be 424 Ω. I am using 1 kΩ currently. Am I going wrong somewhere in my calculations?

Try rewiring your circuit like Figure 7 in the datasheet, so the Gate trigger current flows to/from MT2 through the resistor & IC to the Gate and then through MT1 to return to the source.

That's exactly the problem — I mistakenly mixed up MT1 and MT2. Now the LED, when connected to MT1, glows only when the 5V DC supply is on. Thanks!