Polarity reversing switch

Discussion in 'The Projects Forum' started by Robin Badwelder, Apr 2, 2014.

  1. Robin Badwelder

    Thread Starter New Member

    Apr 2, 2014

    I'm hoping some kind soul will give me some help.

    I'm building an electric go-kart for my four year old. It's not a high budget project. All the welding is done and Dan can steer himself round the garden if I push him. Now I need to sort out propulsion.

    I've cannibalised a kid's stand up scooter off ebay for motor and battery (among other things). For reasons I won't bore you with I can't use the switch etc from the scooter. I need a proper speed controller.

    That's no problem. For 15 quid I can get a PWM controller intended for an electric scooter. Wiring the controller itself seems straightforward enough.

    The problem is these cheap PWM controllers have no reverse function.

    The motor is just an ordinary DC motor that I can drive backwards by reversing the polarity. But the currents involved are quite hefty. 250 watt motor at 24V means DC currents of the order of 10A, I think. If I were to just wire a switch to reverse polarity and if the driver switches it over when the motor is at full throttle I imagine nasty stuff could happen.

    What I'm thinking is this. I need a reversing arrangement that can only switch polarity when current through the switch is low (or maybe it's easier to do it only when voltage across the motor is low). Maybe the user's switch could just toggle between forward and reverse, subject to switch current/motor voltage being low enough.

    Does that sound daft?

    I keep trying to doodle arrangements of multiple relays but I'm obviously reinventing the wheel.

    What's the right answer please? Any help hugely appreciated.

    As someone who's perpetually trying to make something he barely understands, it never ceases to amaze me how generous people are on these forums with their time and expertise.
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    If you want to keep things to all wires, switches, relays, and other electro-mechanical stuff, your options are limited. If you want to play around with MOSFETs, you're in luck. 24V at 10A is nothing to today's switching power MOSFETs. Four of then in an arrangement called an H-bridge will do exactly what you want.

    Back to electro-mechanical stuff, can junior handle a 3-step reversal process? Switch off the battery (SPST), flip the direction switch (DPDT), switch on the battery. Moving the protection protocol from hardware to wetware eliminates a lot of hardware.

  3. crutschow


    Mar 14, 2008
    You could use a DPDT relay to reverse the relay but use a interlock so that the relay can't change states when the motor has power applied. This may require a simple logic circuit.

    How low a voltage do you think you need to detect for the interlock?

    Do you want a momentary push-button or a toggle-switch to control the direction?
  4. MaxHeadRoom


    Jul 18, 2013
    By rights you need to reverse not only when the current is low, but when the vehicle has stopped, otherwise the motor is still generating in the opposite polarity especially if still connected to the drive.
    You could place a 'zero rpm detection' relay across the motor, this would drop out and enable the opposite direction when either the fwd or rev had been switched.
    Is the motor connected permanently (hopefully) or is there a 'Neutral'?
  5. inwo

    Well-Known Member

    Nov 7, 2013
    I don't know what's available where you are.

    Low cost reversing pwm modules for scooters are common here. As are the hall effect foot controls.

    250 watt is on the very low end of the scooter motor world.

    To be honest, I wouldn't even bother with pwm for a 250 watt motor. It's going to be underpowered for all but the smallest rider. ps. I just notice "4yo" PWM may be wasted on a 4yo. My experience with young riders is "on and off".

    A motor for each wheel could give you half and full power.

    Use hefty reversing solenoid relay. A "center stop" reverse switch and a little current limiting. Along with instructions not to reverse until stopped you should be safe.

    It might be possible to mechanically interlock a switch to the brake.
    Or use F-R latching relays that could only change state while brake is depressed.
  6. MrChips


    Oct 2, 2009
    That's it!

    Have an interlock on a DPDT switch that releases only when the brake pedal is depressed.
  7. Robin Badwelder

    Thread Starter New Member

    Apr 2, 2014
    Lots for me to think about here. I've a feeling inwo is right and I'm making it all more complicated than it needs to be. One does that. Deep thanks to all who responded. Makes me smile to think that while I was in bed in the UK last night there were kind people across North America helping with my back yard project!
  8. MaxHeadRoom


    Jul 18, 2013
    Like the Pinkerton motto, 'We Never Sleep' always someone on call.;)