Stoping a DC motor

Discussion in 'General Electronics Chat' started by uriahsky, May 25, 2013.

  1. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    I have a DC motor which flips a coin into a game. It is run by 24VDC. The motor and pcb that was in the game was able to detect when the motor had jammed and it would shut the power off to the motor and send a signal out to the game. To replace this motor which can happen every year or two is almost 300.00. Only a few years ago it was 65.00.

    I have found another motor and purchased a simple H Bridge DC motor controller from eBay for cheap but I need a way to stop the motor when there is a jam. I also purchased a simple current sensor from eBay for a few bucks and when I stop the motor with a wrench the current goes from .5amps to 2amps and the current sensor changes by .187 Volts for every amp.

    To get to the point of stopping the motor I was thinking of adding a comparator to compare the voltage out of the current sensor to a reference voltage and from there to a transistor triggering a relay.

    Will that work or am I leaving out a buffer or something else that might make this work better? Or is there a simpler way to do this?

    I would really like a ready to go adjustable fuse that can handle 24VDC and does not automatically reset it self, but I haven't found anything like that.

    Thanks very much,
    Russ
     
  2. circuitfella11

    Member

    May 10, 2013
    56
    5
    go with the comparator then relay..it costs less and also simpler circuit in my opinion..

    how about circuit breaker instead of adjustable fuse?

    --regards
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    The motor can draw nearly the stall current each time it starts.
    It would be good to add an RC low pass filter between the output of the current sensor and the input to the comparator to prevent false triggering.
     
  4. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    If I could find an adjustable circuit breaker that would also work but one that works around 24VDC and is adjustable and low cost seems kind of rare. They are out there but not cheaply. I also wish I had some test gear that could do the same thing, an adjustable current level that you can be reset. Something I could take with me for service work. If anyone has seen something like that post a link. I would think that would have been made by someone.
    Thanks
    Russ
     
  5. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    I remember reading about this situation. How does the RC filter stop the false triggering. Shouldn't I put in some type of time delay using a cap? Somehow?
    Thanks
    Russ
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Yes, an RC low pass acts as a time delay circuit. It takes a certain period of time for the capacitor to charge after a voltage is applied to the input.
    The RC time constant is R*C(farads).
    This is the time it takes to reach 0.7 times the voltage in. I think after 5 time constants it is typically considered to have reached full charge.

    For example:
    10K*100uf = 10,000*.0001 = 1 sec.
    RC low-pass.PNG
    [​IMG]
     
    Last edited: May 25, 2013
  7. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    42
    2
    OK, I think I got it, Add that RC circuit after comparator and about how long should I wait? Three seconds too long? Perhaps I will have to experiment. Thanks very much,
    Russ
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Add the RC circuit to the comparator INPUT receiving the current signal. And you might google using hysteresis with comparators to further improve stability.

    You could make it adjustable by replacing the R with a pot and a fixed resistor. The fixed resistor to limit the the adjustmen range and max charge current if needed - depends on the current sensing device you are using.
     
    Last edited: May 25, 2013
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