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Good day. I am designing a power control circuit for dimming a 100W bulb using BTA16. In the circuit shown below, when I apply power to the circuit I only receive the positive pulse at the output (of both MOC3020 and the Triac, triggered perfectly at the desired angle) but it completely blocks the negative cycle. (Circuit and output diagrams are attached below)

I have tried few solutions given below but still I am facing the same issue.

I have tried several resistance values, but no gain. Of course, if I increase the value too much, then the TRIAC doesn't even trigger in the positive half. But for the negative half, even low resistance values don't work.

Can anyone please guide me on what I am doing wrong here? Many thanks. enter image description here

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  • \$\begingroup\$ You need to tell us what the MCU is doing. \$\endgroup\$ Dec 24, 2021 at 4:14
  • \$\begingroup\$ MCU is giving pulses to the MOC3020 for each positive and negative cycle. I can clearly see the pulses for both halves (in almost the middle of both cycles), but the MOC only responds to the positive cycle. \$\endgroup\$
    – Noman
    Dec 24, 2021 at 5:29
  • \$\begingroup\$ wnich triac are you using? \$\endgroup\$
    – Jasen
    Dec 24, 2021 at 10:20
  • \$\begingroup\$ I second this - which triac and what is your 100W lamp? Incandescent? \$\endgroup\$
    – Kartman
    Dec 24, 2021 at 10:24
  • \$\begingroup\$ @Kartman Thanks for commenting. I am using BTA16 (also have tried replacing with BTA20) and the load is incandescent light bulb, I also have tried putting 1000W heating element. Both have same waveform across them. \$\endgroup\$
    – Noman
    Dec 24, 2021 at 12:40

3 Answers 3

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If you out the load on the "cold side" you may need snubbers as shown in the bottom circuit. Otherwise, try placing your load on the "hot side" as shown in the top circuit.

from datasheet of your opto-triac

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Try increase the LED current (and maybe pulse length if it is marginal). The MOC3020 is the least sensitive triac driver of that series and can take as much as 30mA to trigger (15mA typical at room temperature). The opto-triac is not necessarily perfectly symmetric.

Most MCUs cannot drive that much current reliably so you may need a BJT or MOSFET to drive the LED.

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TRIAC triggering with resistive load

You don't need a phase shift capacitor at all for a resistive load. Look at any of the datasheets. If you are using an MCU to define the timing, you don't want any phase shift imposed by an external network.

Also note that for resistive starting the resistor values are typically much lower.

enter image description here

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  • \$\begingroup\$ Thanks for replying. I have used the circuit from Fig. 5. I am having the same issue, nothing changed in the output. \$\endgroup\$
    – Noman
    Dec 24, 2021 at 14:41
  • \$\begingroup\$ How are you deriving your Zero crossing Sync pulses? Are you using 50Hz or 100Hz triggers for the MCU? \$\endgroup\$ Dec 24, 2021 at 16:24
  • \$\begingroup\$ I am using 100Hz triggers (triggering both for positive and negative halves of AC sine wave). I have tried reducing MOC Led resistor but same issue (which is obvious as it is working perfectly for positive half) \$\endgroup\$
    – Noman
    Dec 24, 2021 at 17:04
  • \$\begingroup\$ I suggest setting up a test bench with a safety transformer (24-32VAC) and just use the MOC3020 with a resistive load. You can safely probe this type of setup to check all your logic and timing. It may be that your main TRIAC is not working or not as sensitive as you expect. What device is your main TRIAC? \$\endgroup\$ Dec 24, 2021 at 17:15
  • \$\begingroup\$ I am using BTA16 in that particular circuit but I have tried BTA20 as well, same response. For MOC3020 testing, should I connect 100W bulb directly to the MOC pin 4 and check ? (The max output current for MOC3020 is 1A, I'll be safe ?) \$\endgroup\$
    – Noman
    Dec 24, 2021 at 17:20

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