# Isolation circuit using optocouplers for three phase voltage circuit

I'm trying to design an isolation circuit, using optocouplers, for a circuit like this one:

This system allows to control the thyristors turn on angle and change the resulting voltage on the resistive load.

For what I understand from the image, the control circuit has three outputs and each one of them is connected to the isolation circuit.

I've worked with a single phase AC voltage circuit previously, this other one:

And I used this circuit to isolate the control circuit from the main circuit:

My question is: Can I use the same isolation circuit with three optocouplers, each one with a different input Ig1, Ig2 & Ig3 and connect all of the optocouplers emitters to a single cable that goes to the common point, where the three thyristors cathodes are connected? Something like this:

Providing you use an isolating DC-to-DC converter with sufficient break-down voltage rating, the three-phase circuit should be OK. I mention this because you don't appear to have covered this in your question and I'd hate to think that you thought it could all be powered from the control signal side of the opto-isolators.

For the opto-isolators, I'd be tempted to use something a bit more powerful and quicker than the 4N25 but, if you are happy with that part and its ability to drive the thyristors you intend to use, then it should be fine.

You also said this: -

...and connect all of the optocouplers emitters to a single cable...

I think you meant to say "collectors" and not "emitters".

Also note that with a 10 kΩ emitter resistors $$\t_{PLH}\$$ is about 50 μs (which is why I would recommend a better coupler circuit like the FOD8343 or similar).

• Does the k in the first diagram mean that the three optocouplers emitters are connected to the same point? Commented Jul 2, 2021 at 16:20
• I don't know. the language used is not English and G and K may stand for the equivalent of anode and cathode but, that's just a guess. Commented Jul 3, 2021 at 9:26