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I have a 12V DC motor that uses 1 ampere from a 12V battery. I want to limit it to 0.5 ampere because it runs too fast on 1 ampere. I know that I can achieve that by adding a resistor but the question is which value of resistor?

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  • \$\begingroup\$ Theoretically 12 ohm, but what you really want is a PWM speed controller. A resistor won’t change the unloaded speed more than a little, just make it bog down under load. \$\endgroup\$
    – winny
    Jul 1 at 13:42

2 Answers 2

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It's generally a bad idea to add a series resistor to a DC motor because it will reduce the torque without reducing the no-load speed at all. It might be acceptable for certain types of loads such as a fan where there is speed-dependent drag but little friction.

It would be better to add a PWM controller to effectively reduce the average voltage to 6V. You can easily design one with a 555 and a MOSFET/diode (which I suggest just because it would be instructive), or you could buy an inexpensive module (eg., there are various permutations including ones that use a microcontroller rather than a 555)

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The total resistance is 24 ohm on 1A. so to make it half a 12 ohm resistor is needed.

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  • \$\begingroup\$ Hi, Thanks for coming back with the answer to your question. (Personally I disagree with this solution, but if that's your answer, then it's your choice.) In order to effectively mark the topic as solved, please consider "accepting" an answer (click the "tick mark" next to that answer, to turn it green). Note that you must wait 48 hours after you asked the question before you can accept your own answer, but you can accept another answer at any time. (If you didn't mean this to be an answer but wanted to know if it was a good idea, it should be an update in the question). \$\endgroup\$
    – SamGibson
    Jul 2 at 18:33

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