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Halloween prop - wiper motor - question on walwart

Discussion in 'Sensors and Actuators' started by jousley, Sep 19, 2014.

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  1. jousley


    Jan 14, 2013
    Hey all - it's a simple question for you all I'm sure. I have a Halloween prop with a car windshield wiper motor attached to a rocking chair. think the rocking chair rocks on it's own... works fine when I attached it to my 12V motorcycle battery. However, the motor does run pretty quick. I plan to connect the wires from the motor to wallwart - taking the 120V from the wall to a lower voltage to run the motor.

    My question is this:

    the wiper motor is designed to run on 12 V. if I get a walwart that will lower the voltage from 120 to like 10V or 8V will this make the wiper motor run slower? which is my goal. will this cause any problems or concerns? I'm sure this is an easy question, but I don't do this kind of stuff too often.

    thanks for all knowledge.

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    A lower voltage *may* cause the motor to run slower. If you can find a PWM controller designed for 12V you should be able to achieve a far more controlled range of speed control.

    What current does the wiper motor draw when run from 12V?
  3. Gryd3


    Jun 25, 2014
    (*steve*) has asked about the current draw of the motor. Some starter motors can draw a large amount of current. If the wall-wart you plan to use cannot provide enough you run the risk of damaging the wall-wart or having it shut itself off.
    Also, you may reduce the voltage to the motor, but you could end up with inconsistent results that could show as a 'weak' motor that strains to move through part of its cycle. If it's too weak, the motor could stall which would draw more current than normal, and again, damage the wall-wart.

    How are you with electronics? If you can answer steve's questions, we may be able to recommend a better solution.
    Also, do you have a budget you are trying to stay on track with?
  4. shrtrnd


    Jan 15, 2010
    How about a potentiometer to vary the input voltage to the motor.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    For a wiper motor? You'd probably be talking a 50W to 100W rheostat. depending on the current the motor draws.

    Oh, jousley, It would be nice to know three things:

    1) The current the wiper motor draws when there is no load
    2) the current the motor draws when it's rocking the chair
    3) the resistance across the motor (with no battery connected!)
  6. KMoffett


    Jan 21, 2009
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Excellent. That suggests that a wallwart is unlikely to be sufficient. Given the stall current of around 14A, I'd use a Computer power supply (which would need t be capable of in excess of 15A on the 12V rail) and a PWM controller capable of 15A or more. You would also need a diode (preferably rated for 10A or so, but you would probably get away with less) to place across the motor to protect the PWM controller.

    You may be able to find PWM controllers for LED lighting that are capable of operation at 12V and up to 15A. That may be the cheapest option. Here is an example of a device that seems generously rated AND is specified for motor control so the additional diode will not be required.
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