What resistor value which could slightly decrease the speed of geared motor?

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What value of resistor to use so that it can slightly decrease the speed of geared motor,not too much,but just slightly decrease.For example so that it would be easier to explain,imagine a geared motor is connected so that it need to pull up a garage door on a small project model,so the speed should be enough that it takes a bit time to open up the door,should not be too fast nor too slow.

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? Best Answer

  • need the current or power rating of the motor. “small” is relative. Some would consider a 1 HP motor small.

    But given that a 9v battery operates it, the current could be 50 mA to 200 mA or so, at a guess.
    At 100 mA, a 10 ohm resistor would cause a 1 volt drop, use a 1/4 watt resistor
    At 100 mA, a 20 ohm resistor would cause a 2 volt drop, use a 1/2 watt resistor
    At 100 mA, a 40 ohm resistor would cause a 4 volt drop, use a 1 watt resistor

    edit: or better yet, use a lower voltage battery. 4 – AAA batteries is 6 volts, try that. or 3.
    – Chosen by Asker

  • Any resistor would slow down.
    Problem is that a resistor is the worst way of controlling a motor, the startup current of a motor is very high (can be 5-10x the running current) and will cause a very high voltage drop on the resistor; As a result the motor may be fed too little power to start turning, resulting in a prolonged high current which can damage both the resistor and the motor.
    Now if it’s a really small motor chances are that it would work, but the value of the resistor is highly dependent on your operating voltage and the rated power of the motor..

  • Try running it from a 6 Volt battery (4 AA’s should do it if you are using a small 9 Volt battery such as used in some smoke detectors).

    Source(s):
    Electrical Engineer

  • Instead of a resistor, try putting a small flashlight bulb in series, like a 6V lamp.

  • speed of small DC motors is linearly proportional to voltage. you can use resistors but if you need adjustment, you better go with wirewound potentiometer wired as rheostat (series with motor). the problem with using resistors is that they both drop voltage and limit current. you can also use one or mode diodes, each diode will drop about 0.7V but current is not limited as with resistors so you can have higher torque if motor load increases. the better way is to use PWM control. there are many simple circuits and kits just for this application. for example

    http://www.paia.com/KRUKits/K67/K67v3.pd…

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