Question about my control circuit

Discussion in 'The Projects Forum' started by ATM, Oct 14, 2009.

  1. ATM

    Thread Starter Member

    Oct 8, 2009
    Hello, I have a question regarding my control circuit for a solenoid valve. Note, i'm still at the beginning of my design stage and nothing is final yet, but its going to be a 12vdc solenoid valve drawing around 300-400mA. I've included a rough schematic of what I have so far below.

    My question is; The valve has to be controlled by the output of the PIC (the pin connected to the darlington pair), but i'm not getting enough voltage to operate the 12v solenoid. What can I do to obtain more voltage?

  2. bertus


    Apr 5, 2008

    The solenoid can be connected between the +12 volts from the powersupply and the collectors of the switching transistors.
    Also mount a diode accross the solenoid for protecting the circuit against back EMF from the solenoid.
    This diode should be connected with the cahode to the +12 Volts and the anode to the collectors of the transistors.

  3. ATM

    Thread Starter Member

    Oct 8, 2009
    Thanks for the reply, bertus.

    Do you mean like so?

  4. SgtWookie


    Jul 17, 2007
    Yes, that is what Bertus is talking about.

    However, R1 is too low in value. A PIC uC is limited to +/-20mA per I/O pin; and there is a total I/O current limit that is particular to each uC.

    The lowest value you should use for R1 is 220 Ohms. However, since you're using it to control the base of a BC517, you only need about 0.1mA to control 500mA collector current. You could increase R1 to 10k, and still have plenty of current to fully saturate that Darlington.