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Hi, As you see in schematic, the circuit is a normally-closed AC dimmer. Therefore when the connection between microcontroller - which produce pwm - and mosfet get halted, the lamp will be always on without any dimming. I want to make this circuit in a way which eliminating microcontroller would turn the lamp off. Is it possible?

  • \$\begingroup\$ This looks like a rather hefty C1! At which frequency do you do your PWM? \$\endgroup\$ – Marcus Müller Sep 6 '18 at 7:26
  • \$\begingroup\$ It works by arduino uno. maybe 490 hz. \$\endgroup\$ – ETeddy Sep 6 '18 at 7:37

Something like this should work. The transistor Q1 takes the place of the optoisolator in shunting the gate voltage. A second 15V supply from the second zener and 100K resistor is shorted out by the 4N35 to turn the transistor off and the MOSFET on. You will have to reverse the PWM coming from the MCU as well, or add an inverter to do it with hardware or bright will be dim and vice versa.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thank you very much. I will try it as soon as possible and comment the result. Why do you use R3 , 10k? \$\endgroup\$ – ETeddy Sep 6 '18 at 12:08
  • \$\begingroup\$ R3 makes sure the transistor turns off- another 10K resistor from base to emitter could help too.. It might be better to substitute an LED which would give a couple volts drop, and might allow you to avoid D2 (but if the LED opened up then the 4N35 could be damaged). \$\endgroup\$ – Spehro Pefhany Sep 6 '18 at 12:20
  • \$\begingroup\$ Many Thanks, the circuit works properly. What is your idea to reduce blinking of the lamp when it is dimmed? \$\endgroup\$ – ETeddy Sep 6 '18 at 17:32
  • \$\begingroup\$ Blinking? You mean at the PWM frequency? \$\endgroup\$ – Spehro Pefhany Sep 6 '18 at 21:37
  • \$\begingroup\$ At lower than nearly half of the nominal power of the lamp (in my case PWM<125) it start blinking. It seems it is related to characteristics of incandescent lamps. \$\endgroup\$ – ETeddy Sep 7 '18 at 1:03

Convert your inverting optocoupler circuit to a non-inverting one. Done!

You'd achieve that by putting the optocoupler in series with D1 on the high side.

  • \$\begingroup\$ It won't work. In this way the gate of mosfet won't get discharged immediately to make PWM work. \$\endgroup\$ – ETeddy Sep 6 '18 at 12:13
  • \$\begingroup\$ well, pull down resistor? \$\endgroup\$ – Marcus Müller Sep 7 '18 at 10:49
  • \$\begingroup\$ Again doesn't work. I tested with three resistors (560, 680 & 1k) but the lamp will be either on or off. I think the resistor will shape an RC circuit with MOSFET's gate's capacitor and eventually lingers gate's charge and discharge time. \$\endgroup\$ – ETeddy Sep 7 '18 at 12:16

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