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i am doing a cycloconverter single phase design and i have a problem , i use microcontroller to control the firing angle ,i have used zero crossing voltage (741) to trigger INTERRUPT port of MCU , i have controled two positive waves well but two negative waves dont work in reality . my question is " i want to use directly MOC3021 to control the two negative waves with load resistor 100ohm ?" And i will use MOC3021 inside triac to control negative waves instead of 2 SCR negative direction but it doesnt work.Can the MOC3021 drive directly load 100ohm Vin=6VAC

enter image description here

[![enter image description here][2]][2]

enter image description here VIN=6VAC Rload=100ohn

Thanks guys!

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You need to exceed certain AC voltages on the switching side for this to work properly. Once triggered the triac will have an on-state voltage. For the MOC3021 this is typically 1.4 volts but can be as high as 3 volts. However, the AC waveform needs to exceed this by some margin to get the device to trigger. I think, if it's working on positive cycles, you are close to making it work on negative cycles too. Maybe try lowering the load resistor a bit and see if this begins to happen.

BTW what is your diode forward current? Your question doesn't specify this.

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  • \$\begingroup\$ Hi Andy , thanks for replying me soon , i have updated and edited the schematic for you to easy to image . 2 positive cycles works well by 2 SCR 2P4M in schematic , now i want to use isolated opto triac to get the 2 negative cycles (MOC3021-U6-U11). but i got the problems.I have read the datasheet carefully .THe holding current its got about 100uA. What am I missing something?What about increase the Vin voltage up to 12VAC or decrease the Rload down? \$\endgroup\$
    – Electrica
    Dec 26, 2016 at 12:01
  • \$\begingroup\$ Increasing the ac voltage is probably the best idea. I'm not sure what the peak ac current of the opto trial is so just check on this. They are not intended for driving loads directly but 100 ohms on 12 volts should be ok. What is your ultimate aim? I ask this because there may be other options. \$\endgroup\$
    – Andy aka
    Dec 26, 2016 at 12:15
  • \$\begingroup\$ I just want to do the cyclo converter , but the 2 SCRs on negative cycles didnt work like the positive SCRs, so i want to try this option to easy to control , if you have other options plz show me how to deal with to 2 SCRs in the negative cycles, and how to control the gate of neagtive SCRs to work well . Thanks ! \$\endgroup\$
    – Electrica
    Dec 26, 2016 at 12:28
  • \$\begingroup\$ No, what I am asking is what you are trying to achieve - don't think about method, just state what you are trying to make in layman's language. \$\endgroup\$
    – Andy aka
    Dec 26, 2016 at 12:33

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