Driving power BJT with optocoupler

Discussion in 'The Projects Forum' started by sp3tsnazz, Mar 21, 2013.

  1. sp3tsnazz

    Thread Starter New Member

    Mar 12, 2013
    Hi everyone,

    I'm working on voltage equalizer for supercapacitor stack.
    Attached is the schematic for my project.

    Now I have a problem with driving BJT, that will allow the supercap to dissipate excess power through the resistor R2. In order to do this, I try to use an optocoupler, PC817, which received logic level input from microprocessor.

    I also try to set up the darlington pair as shown in the circuit, so there is no need for external power supply. But basically use power from supercap to drive the BJT.

    My question is, will this circuit be operational? (I've not buy these components yet, as I'm not sure it will works)
    What are the things that I've miss, if there is any?

    All suggestion is welcomed.

  2. k7elp60

    Active Member

    Nov 4, 2008
    You need to put a resistor between the collector of the optocoupler and R2 to limit the power dissipation of that transitor, and a resistor from the emitter of the same transistor to the base of the the BD135. Actually the best thing to do is but a resistor from the base of the BD135 to its emitter, a low value so it will turn off when the opto coupler is off. If you can tell us the maxmum voltage on the super capacitor and the value of R2 we can help you with the values of the other resistors.