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I'm designing a fluorescent lamp dimmer circuit which will work with Arduino pro mini. The mcu will detect zero cross from mains outlet and will give out offset for TRIAC.

I would just like to get sanity-check on this circuit, why it would or wouldn't work?

enter image description here

  • X_DETECT_IN - Zero cross detection for Arduino
  • OFFSET_PULSE - Offset pulse from Arduino

[Ignore 2 AC generators, didn't find AC+ and AC- in KiCad]

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    \$\begingroup\$ Your opto circuit U2 at the top appears designed to short a battery, except as Olin pointed out, the U2 LED will fry first. U1 LED also lacks current limiting. \$\endgroup\$ – Tut Aug 8 '17 at 11:54
  • \$\begingroup\$ U2 should go up in flames quite spectacularly. \$\endgroup\$ – winny Aug 8 '17 at 13:43
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Yikes! The first thing I see is a LED being driven directly from the output of a full wave bridge that rectifies line voltage. POOF!

You need to limit the current to what the LED in the opto can handle, and to minimize the power dissipation. You also have to think about the maximum reverse voltage of the LED. That's usually around 5 V, but the definitive answer is in the datasheet, of course.

For detecting line power, I usually use two optos back to back. Each LED is driven from opposite polarity, so protect each other from excessive reverse voltage. The two outputs are wired in a push-pull arrangement. That gives you good drive and symmetric response.

Another problem is the gate drive of the triac. It doesn't look like you have anything turning the gate on, only off.

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  • \$\begingroup\$ Thank you for taking a look! > Another problem is the gate drive of the triac. It doesn't look > like you have anything turning the gate on, only off. How does triac turn on and off ? I thought that the triac in this circuit would be on when the photocoupler recieves offset pulse from mcu. The pulse is lesser than Half-period of the AC sine, so wouldn't triac turn off when the sine period is over ? \$\endgroup\$ – flowian Aug 8 '17 at 12:30

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