Current limiting resistor for motor circuit.

Discussion in 'General Electronics Chat' started by ctrmint, Dec 29, 2012.

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  1. ctrmint

    Thread Starter New Member

    Dec 29, 2012
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    Hi,
    I'm attempting to put a circuit together to drive a motor from a 12v supply but limit the current to approx 2A with a resistor. However I'm not sure of the resistance of the motor and thus unsure which resistor to use.

    I'm thinking a 6 ohm 50W resistor would be about right.
    Any ideas.

    Sorry I don't have much info on the motor as its a sealed automotive part.

    Thanks for the help
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    An elecric motor that runs at 2A probably needs 20A to start and when it is stalled.
    If you limit its current then it is useless.

    If you need to limit the current (and limit the torque) then use a smaller motor or reduce the supply voltage.
     
  3. ctrmint

    Thread Starter New Member

    Dec 29, 2012
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    i'm told the motor runs at 2A peak, its actually the drive on a specialist hydraulic pump and I very much doubt its pulling 20A.
    It used to control clutch packs in a differential rather than propel.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    If the peak current of the motor is 2A then it runs at maybe 200mA.
    Or maybe it is only used for a moment when it is at its peak current?

    Why do you want to limit its current? An electric motor uses only as much current that it needs.
     
  5. ctrmint

    Thread Starter New Member

    Dec 29, 2012
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    i was trying to prevent it from pulling too much current and damaging it, as sort of safety measure.
    The component is approx 2k GBP, not the sort of thing to break.
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    What's wrong with using a fuse of the recommended value ?
     
  7. #12

    Expert

    Nov 30, 2010
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    I'm with gerty. You are describing exactly what fuses were designed to do.
     
  8. ctrmint

    Thread Starter New Member

    Dec 29, 2012
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    i see your point about the fuse approach it does make sense.
    But is there a way I can limit the current, i'm just wanting to be in full control of how it will run. Also not really keen on replacing fuses every 5 minutes if the current goes slightly over 2A.
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How fast the fuse blows is depended on the type of fuse:


    This is from the wiki page on fuses:
    http://en.wikipedia.org/wiki/Fuse_(electrical)


    Bertus
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Here is a 2 Amp limiter that puts only 0.33Ω in series with the motor when the current is below 2 Amps.

    You can make the series resistance even lower if you replace the PNP with an op amp (not just any op amp, though).
     
    #12 likes this.
  11. Audioguru

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    Dec 20, 2007
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    I don't think a motor should have its current limited. When it starts it uses a LOT of current for a moment. If you limit the current then the motor might just sit there without starting and continuously drawing the limited amount of current until it burns out.

    Maybe you should have a circuit that detects when the motor is stalled (drawing a very high current and getting extremely hot) then the circuit disconnects the power.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I feel the same way. I just think the OP might have to see it to believe it.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Which smokes the most, the current limiter or the motor?
    Both smoke the same amount?

    My electric RC model airplanes have a circuit that disconnects the motor when it gets stalled. One airplane's brushless motor makes music when it is turned on and it SHRIEKS and disconnects when it is stalled.
     
    Last edited: Dec 30, 2012
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yeah, you would need a BF heat sink for the MOSFET if the motor shorted out, or maybe even stalled.:eek:
     
  15. Jaguarjoe

    Active Member

    Apr 7, 2010
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    Measure the resistance of the armature when the motor is stationary. This, along with I=E/R will be its peak current. I never had much luck fusing 12v DC motors, some fan motors start at 130+ amps and settle down at 10 or so. A circuit breaker is a better choice.
     
  16. RodneyB

    Active Member

    Apr 28, 2012
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    The 0.33R resistor what is the wattage
     
  17. MrSoftware

    Active Member

    Oct 29, 2013
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    The brushless motor is most likely a 3-phase AC motor. The controller feeds it high frequency power to make the "music". I believe the logic in the controller probably stops the output when the motor is stalled.

    For the other motors you might be referring to an auto-resetting fuse.
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Legacy thread, the OP has not been back since Dec 2012!:rolleyes:
    Max.
     
  19. be80be

    Senior Member

    Jul 5, 2008
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    He's been crying a long time he burnt up his motor waiting on it to start. Maybe.
     
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