Need Help on DC motor & diode relationship

Discussion in 'The Projects Forum' started by secretstep, Jun 27, 2008.

  1. secretstep

    Thread Starter Member

    Jun 27, 2008
    17
    0
    Hi,
    I wonder if we need a diode if the current through the motor is supported by NPN transistors.
    My design is one input from sensor, and the other input from controller. Two voltages go into comparator opamp, and the resulting voltage goes to output stages. In the output stages, I have two NPN transistors for the motor. Since the comparator can generate positive or negative voltage, the motor will be able to run in forward or reverse direction. Yes.. it's totally analog design.
    In a simulation with PSpice, it seemed to work fine, so I actually built a circuit and tested. I couldn't find a break down in motor or circuit due to the voltage problem...
    I read that DC motor is inductive device so we need protection diodes or capacitors.. but I don't really get why is that. I hope someone there to clarify more about that with some equations, etc...
    Please Help!
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The voltage across an inductor is proportional to the product of the inductance and the rate of change of current through it.

    v=L*(dI/dt)

    When you turn off your transistor to turn off the motor the current through it falls to zero very rapidly, thus a very high voltage is generated across its terminals which may destroy your transistors. So, you have to protect them by using diodes and if you want capacitors for better results.

    But if you want to reverse the voltage on the motor's termnals to reverse its direction of rotation then you have to use two zener diodes connected in series and in opposite directions. Their breakdown voltage should be 10 volts greater than the motors operating voltage but less than the maximum voltage the transistors can withstand.
     
  3. secretstep

    Thread Starter Member

    Jun 27, 2008
    17
    0
    yes, it makes a sense now. Thank you.
     
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