how to reverse polarity (12v) on a timer

Discussion in 'The Projects Forum' started by wickk, Jun 7, 2013.

  1. wickk

    Thread Starter New Member

    Jun 7, 2013
    Very similar to this discussion, I have a linear actuator that I'd like to automatically reverse polarity to after each run, using a programmable timer.

    Couple differences --

    My entire circuit is 12v. This is for a chicken coop door, so the actuator would be extended for 12 hours & retracted for 12 hours.

    I'm using a solar-recharged 12v battery as a power source, so for instance I probably can't have constant power to a DPDT relay coil wired as an H-bridge, for 12 hours.

    The linear actuator (specs here) draws 4.8 amps max has built-in limit switches. It also has 3 wires for the potentiometer output that I'm not sure how I could use, but I could if it helps.

    The timer I have at the moment to use is a SPST... Amico CN101A. It has a minimum run time of 1 minute, which is longer than the actuator needs but the actuator's internal limit switches make it so that doesn't matter.

    (Judging from the CN101A timer instructions, they also make a CN102A timer which looks to me like a DPDT timer & probably what I need, but I can't find a source for it.)

    Any way to make a circuit alternate polarity after each run, using 1 timer? Or is there a cheap 12v timer with programmable polarity?

    The way I've been able to think of so far to make this work is using two SPST timers.. one would energize the DPDT coil for its run, & the other timer would not (that'd work, right)?

    I'm an electronics beginner, so sorry in advance :)
    Last edited: Jun 7, 2013
  2. crutschow


    Mar 14, 2008
    You could use a MOSFET H-bridge, as shown below, to provide the reversal voltage. That takes very little steady-state power to drive.

    The two bottom N-MOSFET gates are connect to the NO and NC contacts of the SPDT relay. The common relay connection goes to +12V.

    The MOSFETs can be just about any N channel (bottom) and P channel (top) devices rated for at least 25V and 25A.

    Edit: I forgot to show the two 10k ohm resistors needed from each N-MOSFETs gate to ground to establish 0V when the respective switch is open.

    Last edited: Jun 8, 2013
    wickk likes this.