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I'm trying to trigger a TRIAC BTA16-800 using an MCO3021 optocoupler. I have a two-channel power supply, in one of them I'm pulsing the optocoupler at 120 Hz and the other channel is configured as a sinusoidal source of 24V amplitude and 60 Hz for power on the other side of the optocoupler and the TRIAC. But apparently the TRIAC is not triggering as it should.

When I measure the output of the optocoupler I get the following waveform (yellow is the 24V source and blue is the optocoupler output). If I vary the phase of the pulse that I am applying to the optocoupler, the output is also varied, as expected.

Opto 1 Opto 2

Connecting the circuit as follows, applying a pulse to the input of the optocoupler and a sinusoidal voltage of 24V amplitude to the output of the optocoupler and the TRIAC, I cannot get any voltage across the load. Is there something wrong with the circuit design? Or am I forgetting something? Measuring it wrong?

schematic real

Thanks in advance, I've been trying to get the TRIAC to work with the optocoupler for some time now and I still haven't gotten any results.

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  • \$\begingroup\$ The circuit is ok so you need to do a bit of fault finding - for example, verify that you are actually injecting current into the gate, show that on the scope. If the triac did ever work it would burn out the 1/4W resistor in series with it. \$\endgroup\$ Apr 13, 2022 at 0:52
  • \$\begingroup\$ Can you get the triac to operate without the opto-isolator? \$\endgroup\$
    – HandyHowie
    Apr 13, 2022 at 7:11

2 Answers 2

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In your circuit, the gate will not get voltage, as you the coupler is connected between the gate and A1. Swap A1 and A2 triac conections, that is, connect coupler resistor to the other 12 Vac supply point and check.

enter image description here

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  • \$\begingroup\$ Thank you very much, I moved the load and the power supply and it worked perfectly! \$\endgroup\$
    – Lucas Mazz
    Apr 13, 2022 at 11:37
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The typical application of MOC3021 is just a little "complicated".
There is a capacitor to store a "charge" that will be released by MOC.
Does this function with a lower voltage?

Note also that 15 mA is needed at a minimum (5v-Vled)/100 > 15 mA and < 60 mA max ...

From this

enter image description here

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  • \$\begingroup\$ Thank you very much! Later on I intend to implement the circuit with the capacitor. For now I was just wanting the characteristic waveform of the TRIAC on load, to check if the component was working as it should. \$\endgroup\$
    – Lucas Mazz
    Apr 13, 2022 at 11:39

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