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How to calculate resistor values

  1. Mar 3, 2010 #1
    if my 12v motor draws 10amp when running am i right to think if i put 6v on the same motor it will run at half speed? how would i work out the resistor needed to do it?

    i think i need to be using ohm's law somehow but cant remember just how,its been a while since i've needed to do something similar...

    thanks craig

    p.s its for my car with only one speed at the moment
  2. jcsd
  3. Mar 3, 2010 #2
    I'm afraid its not as simple as you may be imagining, a resistor in series or parallel with the motor won't have the effect you desire without the resistor being completely impractical and consuming just as much power as the motor. The most common way to control the speed of a DC motor is through pulse width modulation.
  4. Mar 4, 2010 #3


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    If you don't mind 'wasting' a lot of power - for instance, if you are only running slow for a short while - then a series resistor would be OK. Old Electric railway trains used resistors to eliminate over current on startup, for instance.
    The other point is that you would need something more than 6V for the motor to run at half speed. 6V would represent only 1/4 Power into a resistive load.
    If you want to go for a 'brute force and ignorance' approach (not a bad idea to start with) you could obtain a selection of series high power resistors (tens of Watts rating) of 0.1, 0.3 and 0.5 ohm sort of values. Don't touch them 'cos they'll get hot! Not very pretty but it could do what you want. You could even use a long length of ordinary electrical flex (no coiled up - it may melt the insulation if it is) Use both blue and brown conductors in series and measure the resistance with a multimeter. It's not difficult to get 0.5 ohms that way, using thinnish wire. Cheap cheap, too.
  5. Mar 4, 2010 #4


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    What do you mean that this motor is for your car? What does it run?

    To control the speed of the motor, you should be using pulse-width modulation (PWM) of the 12V drive signal.
  6. Mar 4, 2010 #5
    thanks for the replies, looks like then its more complicated than i thought... it is for the car,the heater motor. i'll hold my hand up and say i'm not that up on electronics so making a 'pwm' is probably beyond me.

    cheers craig
  7. Mar 4, 2010 #6
    It's really not too bad. You can get an integrated PWM controller. All you have to do is give it a supply, attach some resistors and capacitors according to the data sheet, then it will output an on-off signal for a heavy duty transistor that supplies your motor. Turning a supply on and off can be just like splitting the voltage.

    Look at the http://focus.ti.com/lit/ds/symlink/tl5001a.pdf" [Broken]. The data sheet shows a typical configuration that should work for you. Just be sure to replace the transistor with something that can handle 12 amps. Also, they're 1.65USD at Digi-key. It comes in a DIP package (Dual-inline-package) which means it can be mounted on a simple breadboard.
    Last edited by a moderator: May 4, 2017
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