DC Motor Speed Control using LDR

Discussion in 'The Projects Forum' started by phoebe_, Jul 8, 2012.

  1. phoebe_

    phoebe_ Thread Starter New Member

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    I found this simple circuit of controlling the speed of a DC motor using a potentiometer. I'm wondering if this could still work if I replace the potentiometer with an LDR. I have a variable voltage supply up to about 15V.
  2. bertus

    bertus Administrator Staff Member

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    Hello,

    The LDR would probably burn.
    It can most likely not handle the current needed for the motor.
    You could try to make a PWM circuit that reacts on the LDR.

    Bertus
  3. phoebe_

    phoebe_ Thread Starter New Member

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    Oh I see.. But why would the potentiometer work?
  4. bertus

    bertus Administrator Staff Member

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    Hello,

    They are using a rheostat, that is an high power potentiometer.
    It is also strongly dependent on the current drawn by the DC motor.

    Bertus
  5. phoebe_

    phoebe_ Thread Starter New Member

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    How about this circuit? Would a regular potentiometer (10k or 100k) work?
  6. bertus

    bertus Administrator Staff Member

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    Hello,

    A 10K potentiometer might work.
    A 100K potentiometer will not work.
    The transistor needs a certain current as input to control the motor speed.
    The 10K might already be on the border of a correct working of the circuit.

    On the following page you will find some circuits for motor control:
    Motor control

    Bertus
  7. phoebe_

    phoebe_ Thread Starter New Member

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    I only have some simple components such as resistors, capacitors, potentiometer, voltage supply, transistors, etc.. I don't have ICs so I can't use PWM. Could you suggest other ways of doing it aside from the Darlington pair?
  8. bertus

    bertus Administrator Staff Member

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  9. Kermit2

    Kermit2 Well-Known Member

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    Using a straight resistance based control scheme will waste lots and lots of power creating HEAT.

    [​IMG]

    The circuit above uses transistors as a variable resistance to lower voltage. The designer used a 5 inch square chunk of aluminum heatsink material, with fins 1.7 inches tall. The fan was included to keep temperature down. Avg current for the device in use is 10-20 amps and that will result in a 160 degree avg. temp rise. at the transistor mounting points. It was used to provide a slow ramp up of power for bench testing of RC plane motors.(paraphrased from the original descriptions)

    Using a PWM scheme with a 555 IC might prove to be easier to do than what you see above, AND it would save lots of power that can be used by the intended load.
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