reverse polarity for DC motor

Discussion in 'General Electronics Chat' started by Phayilboy, May 9, 2011.

  1. Phayilboy

    Thread Starter New Member

    May 7, 2011
    2
    0
    OK as you can see i am new to these forums and i have a problem that has been adressed many times before (yes i did bother to look :D)

    I have a 4.8v mini servo which I blew the controller board after messing about (I think I wired it wrong and blew the cap on the internal control board thing)and have recently removed it and connected the power directly. I want to be able to move the motor in both directions and after some quick research found that reversing the polarity was the way to go about doing this. My understanding of this is that quickly changing the polarity massively strains the motor and therefore wondered if it is at all possible to create a circuit that not only reverses the polarity but creates a short delay long enough for the motor to stop and then turn in the opposite direction, or in some other way stops the motor before reversing the polarity.

    thanks in advance

    PhayilBoy
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Although the motor is not strained massively by polarity reversal one thing that is, is the circuit.

    Think about it: the motor is spinning normally. You decide to reverse it. Very quickly the motor goes from full speed to nothing and back to full speed (but in reverse.) Where does all the energy go? What will happen is the motor will produce a large spike of voltage (100V+) when this happens, usually enough to damage sensitive driving electronics. So you usually "brake" the motor, by connecting its terminals to a small resistor, converting the kinetic to heat, before speeding it up again.

    N.B. Generally, this is not required for small motors. Larger motors, definitely. But for smaller motors, I would just use a H-bridge. Remember to include flyback diodes if your H-bridge doesn't have them.
     
    Last edited: May 9, 2011
    Phayilboy likes this.
  3. Phayilboy

    Thread Starter New Member

    May 7, 2011
    2
    0
    Thanks for the help

    Yes Using an H bridge is what I am currently doing (Built onto a DPDT switch). Would it be possible to build an H bridge that I can control the changeovers with a pot, possibly transistor based or something not using a straight forward switch

    Thanks
    PhayilBoy
     
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