I designed a pcb that contains an optocoupler and a triac to work as a dimmer for AC

But when I give a high value to the GPIO corresponding the triac doesnt fire

So I am looking for the mistake I did ...

Is my schematic wrong ??

enter image description here

if its correct , what could be the problem on the pcb leading to this problem ??

How can I check it ??

I want to search for the problem but I dont know where to look

  • \$\begingroup\$ You need to check if by mistake you did not swap triac MT1 with MT2 terminal. \$\endgroup\$ – G36 Jun 4 '17 at 12:33
  • \$\begingroup\$ you mean connected the gate in place of MT1 or MT2 ?? \$\endgroup\$ – ihsanogluu Jun 4 '17 at 12:50
  • \$\begingroup\$ No, not the gate but MT1 and MT2 lens.unifi.it/ew/… \$\endgroup\$ – G36 Jun 4 '17 at 12:54
  • \$\begingroup\$ thank you for your help , Firstly I am pretty sure that I didn't swap any pin with another .. But since this triac conducts current in both directions and I am working with AC main I dont think this will be a problem even if I swapped the two pins .. I am new to triacs and if there is wrong in my understanding to how it works please tell me about it \$\endgroup\$ – ihsanogluu Jun 4 '17 at 13:15

It appears that you do not have sufficient current through the opto LED. 15 to 50 mA is recommended to meet the transfer function.

First check that your uP can drive at least 15 mA from its output. If so, then a series resistor of ~ 220 ohms should be used. If the output cannot drive that much current, add a simple NPN or FET driver to control the higher drive current.

  • \$\begingroup\$ so changing the opto's anode resistor would fix it ?? did I understand you correctly ?? \$\endgroup\$ – ihsanogluu Jun 5 '17 at 0:18
  • \$\begingroup\$ Does your GPIO support 15 mA of drive? \$\endgroup\$ – Glenn W9IQ Jun 5 '17 at 1:44

About the schematic, at the first look, you should (at least) add a snubber circuit in parallel to the TRIAC, to prevent a quickly slew-rate to turn it on (turn the TRIAC or the MOC!). You can find more info in this application note from Fairchild Semiconductors about the MOC30XX family that I share with you: https://www.dropbox.com/s/hwek0h4thuyi5go/MOC30XX_AN-3003.pdf?dl=0

About the firing problem I think Glenn has answered it in part. Can you supply the minimum current needed? I would activate the moc with a signal in low connected to the pin number 2, so you can connect the pin 1 to Vcc (5V) through a pull-up resistor of 220 ohms.

  • \$\begingroup\$ Welcome to EE.SE. The OP said, " ... the triac doesnt fire." A snubber isn't going to help. Tip: copy the OP's question into your answer, quote his actual question and answer it, then delete the rest. I've found it a useful way of ensuring that I address the real issue. \$\endgroup\$ – Transistor Jun 13 '17 at 17:14
  • \$\begingroup\$ It is a different issue but the question in the title said: is my schematic correct? I'm just trying to prevent a future problem. I think Glenn has answered the firing problem! But I'm going to edit my answer thanks for the feedback! \$\endgroup\$ – Paco2L Jun 13 '17 at 17:23
  • \$\begingroup\$ Ha! I missed that aspect and I've done it before too in answering questions. Maybe I should change my habit to copy both title and text. Thanks. \$\endgroup\$ – Transistor Jun 13 '17 at 17:25
  • \$\begingroup\$ Thanks to your comment I think I have improved my answer to the main problem :D \$\endgroup\$ – Paco2L Jun 13 '17 at 17:34

Yes, the optocoupler resistor should drop to approx 220 ohms, since it needs to turn on harder than what the 470 ohm resistor will do. Also, you should have a snubber circuit on the Triac for any inductive load, such as a motor. or compressor. You might not need it with a resistive load, like a heating coil, but It can't hurt.


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