Variable resistor separate from control circuit?

Discussion in 'The Projects Forum' started by biggerB, Oct 4, 2015.

  1. biggerB

    Thread Starter New Member

    Oct 4, 2015
    Good morning,

    I am making a simple AC chopper for a dimmer circuit for a table lamp. It consists of a simple Triac which is fired using a RC circuit with a variable resistor. I've attached the schematic.

    My problem is that instead of using a potentiometer like in the circuit, I now want to control this circuit using a microcontroller to vary the resistance. The problem is that there is 220vAC flowing through the variable resistor and I obviously have a control signal of 5vDC coming from the microcontroller.

    I have controlled AC switches before using DC control circuits by using simple relays which have 2 completely isolated coils. What I'm asking is, is there a similar way to control a variable resistor?

    I've been researching photocouplers, looking for a solution. But it seems photocouplers behave similar to relays (in that there are only 2 states, either on or off). Any help would be appreciated.

    Thanks !
  2. Colin55


    Aug 27, 2015
    opto couplers will operate with variable resistance as the output if you illuminate the LED inside the opto-coupler with very little current.
  3. bertus


    Apr 5, 2008

    You can use MOC optocouplers to drive the triac from the micro.
    Use a zero crossing circuit to synchronyze the micro and use a delay for the firing angle.

  4. biggerB

    Thread Starter New Member

    Oct 4, 2015
    Thankyou, this is what I've settled on doing. I've got a pspice model up and working and hopefully it works just as well when its all assembled !

    Yes, this was my initial plan aswell. But it turns out that most optocouplers dissipate very small amounts of current and were just impossible for me to implement without burning them to a crisp. I've decided on using optocoupled triacs instead. Thanks for your help!
  5. Bordodynov

    Active Member

    May 20, 2015
    I am sorry. New Reg.png: REG.png