DC Controller protection

Discussion in 'General Electronics Chat' started by Momoe, Apr 7, 2011.

  1. Momoe

    Thread Starter New Member

    Apr 7, 2011
    Hi, I'm looking for help to try and resolve what I hope is a relatively simple problem.
    I've acquired a kids tractor powered by twin wheelchair 24v geared motors rated at 11amps. These are controlled by a PWM control box with an integral DIN socket which uses a potentiometer for speed control.

    This works fine except that the control box is not isolated when the potentiometer is off, if the tractor is on a hill and rolling the motors generate current which blows the control box.

    I was thinking of installing relays on the feeds from the control box to each motor, when power is present from the control box the relays would actuate, when the power is off the relays would then effectively become isolating switches.

    Simple but I was wondering if there was a more elegant solution, I've also wondered about the PWM regulated current actuating the relay.
    Any suggestions appreciated.
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Rolling forwards or backwards?:rolleyes:

    If the motors are unidirectional you could add a power diode rated at maximum motor current in series with the DC feed to the motor(s), to prevent the backflow of current - if that is indeed the problem. You'll have some forward voltage drop in the diode(s)when operating the motor(s) and hence some power loss & requirement for adequate heat sinking of the diode(s).

    Do you know if the controller has a free-wheeling diode across its output?

    The relay idea sounds simple enough to implement. Maybe there's a solid state DC relay option - but they tend to be expensive.
  3. JDT

    Well-Known Member

    Feb 12, 2009
    Sounds like a very poor design of motor controller. If you go for the relay option go for a relay with change-over contacts that when off, disconnect the motor from the controller and short-out the motor terminals. Make sure the contacts are "break before make" so that the controller is not shorted.

    This will apply dynamic braking to the motor and prevent it rolling with no power. You will need a relay with high-current contacts.

    One relay for each motor.