I am working on project to dim the LED bulb(Dimmable) with Atmega328. Everything works fine when duty cycle is above 50% but as soon as i am decreasing duty cycle below 50% LED bulb start flickering. Please suggest what is problem with circuit.

Conditions:- 1.> I am generating 12V power Supply and then using lm317 to generate 3.3v from 12v.

2> Using Zero Cross detector with atmega328 which is working fine.

3> If i drive gate of both mosfet from a separate 12V supply instead of my actual supply, circuit is working fine.

Please suggest something....


simulate this circuit – Schematic created using CircuitLab

I have modified my circuit and now its working fine. Thanks for your comments and rectifying my schematic.


simulate this circuit

  • 1
    \$\begingroup\$ What makes you believe the LED can handle the duty cycles below 50%? Also I don't like anything about your 0 volt/earth connections in your schematic. \$\endgroup\$ – Andy aka Apr 25 '17 at 10:42
  • \$\begingroup\$ What's the circuitry like inside the LED bulb? \$\endgroup\$ – Dampmaskin Apr 25 '17 at 10:44
  • \$\begingroup\$ Strange that is doing something, your M3 MOSFET should be dead at the very first time. \$\endgroup\$ – Marko Buršič Apr 25 '17 at 10:50
  • \$\begingroup\$ Are you attempting to use leading edge or trailing edge dimming? \$\endgroup\$ – Finbarr Apr 25 '17 at 10:56
  • \$\begingroup\$ Since when dd a LED look like a 100 ohm resistor?! get a proper model for your LED . \$\endgroup\$ – Autistic Apr 25 '17 at 12:39

This looks like a mess waiting to catch fire or electrocute someone.

You don't say where this is, but almost certainly some part of the AC line is tied to ground somewhere. That means you have a hidden conduction path from somewhere in the "AC Supply" block to the source of both FETs.

The only way the concept is valid is if this "AC Supply" block is a completely isolated secondary of a transformer, but that's a long way to go to light a LED.

If this thing is still working at all, then the reason it might be flickering is because of a beat frequency with the AC line. You didn't even mention the PWM frequency, so it seems you didn't think about it and it could therefore be a issue.

  • \$\begingroup\$ Thanx for your comment. Actually i figured out the problem of flickering, its because of source is connected to DC ground which makes the things messy. Can you please suggest how to isolate both Source and DC-Ground. \$\endgroup\$ – vikas goyal Apr 25 '17 at 17:47

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