I designed this circuit to control a TCR (Thyristor Controlled Reactor) using two SCRs in antiparallel, which should be triggered through two MOC3021 drivers. To properly supply the MOCs and maintain isolation, I included a 4N25, whose function is to increase the voltage of the 3.3 V pulses and isolate the control side from the power stage. In theory, when I apply the pulses, the MOCs should activate the SCRs and thus modify the waveform of the current on the load. However, it is not working: at the output I only observe the sinusoidal signal unchanged, as if the thyristors never trigger. That is why I need help to identify what is failing in the triggering stage and what adjustments I should make so that the circuit works as intended.

 

ADDED:

to Danko
TS wrote that the load is not a resistor, but an inductor

Yes, reactor in text, but no inductance value and more: on circuit shown resistor...

 

to Danko

TS wrote that the load is not a resistor, but an inductor

 

Driving a thyristor with an opto triac can be problematic. If it is triggered when the thyrsitor is reverse biassed it will exceed the maximum reverse voltage of the gate. It needs a series diode.
Also the circuit says MOC3031 which has a zero-crossing detector, but the title says MOC3021 which isn't. Which is correct?
And why are there two stages of isolation? Isn't the MOC3021 isolated enough?

 

hi kevin,
Are you trying to achieve phase angle controlled firing of the SCR's, using a Zero Crossing opto triac, which are typically are used for sine wave 'burst' control?

E

 

I designed this circuit to control a TCR (Thyristor Controlled Reactor) using two SCRs in antiparallel, which should be triggered through two MOC3021 drivers. To properly supply the MOCs and maintain isolation, I included a 4N25, whose function is to increase the voltage of the 3.3 V pulses and isolate the control side from the power stage. In theory, when I apply the pulses, the MOCs should activate the SCRs and thus modify the waveform of the current on the load. However, it is not working: at the output I only observe the sinusoidal signal unchanged, as if the thyristors never trigger. That is why I need help to identify what is failing in the triggering stage and what adjustments I should make so that the circuit works as intended.

Hi,

You are using a zero cross detector opto which turns on at the zero crossing. If you want phase control you have to use a regular opto coupler of some kind.
Also, you have to show your waveforms or else we cannot accurately analyze it or simulate it the way you want to run it.

 

@kevin_solarte
You can control current through inductance
using one MOC3021 and two SCR:

 

hi Danko,
I believe the TS problem, he is using Zero Crossing opto triac's
E

 

Eric;
That, and the fact that he is cascading a photo triac and a photo transistor for no reason.
In fact he could simplify the circuit even further by employing a single triac instead of a pair of antiparallel SCR.

 

Another observation; he hasn’t built the circuit yet, it is only a simulation. And he hasn’t included any ground symbols anywhere!

To the TS: look at Danko’s circuit, it has ground symbols both on the primary and secondary sides, which are required for a simulation to run.

 

Eric;
That, and the fact that he is cascading a photo triac and a photo transistor for no reason.
In fact he could simplify the circuit even further by employing a single triac instead of a pair of antiparallel SCR.

I doubt that. I've not seen many triacs rated at 1.25kA.
https://en.proton-electrotex.com/T253-1250

 

Eric;
I have re-read the TS original post, where does it mention the 1.25 kA?

And he hasn’t built a physical circuit yet. It is a simulation that doesn’t work.

 

Hi s-t,

Eric;
I have re-read the TS original post, where does it mention the 1.25 kA?

I didn't.
E

 

Eric;
I have re-read the TS original post, where does it mention the 1.25 kA?

It was me not Eric, and you are correct he didn't say 1.25kA. It was @Danko.

 

Ok A case of doubly mistaken identity

 

hi Danko,
I believe the TS problem, he is using Zero Crossing opto triac's
E

Hi Eric,

Yeah that's probably the main problem. I'm waiting to see some more information on this project too.