# DC motor current limiting

Discussion in 'General Electronics Chat' started by bug13, Jun 18, 2013.

1. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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Hi guys

I have a 12V 160mA DC brushless motor, like the one use in CPU cooling, how do I calculate the resistor value to limit the current to 100mA @ 12V?

Thanks

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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If you lower the current the speed will alter, choose another fan for replacement, or lower the supply voltage.

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3. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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Maybe I should ask in another way, say I have 13V supply, my brushless fan is 12V 160mA, can I add a series resistor to limit the current to 160mA, and how do I calculate the value?

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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The back EMF of the motor will vary with speed of the motor. As designed, the motor will draw 160mA of current running under its rated load from a 12 volt supply. Raising the supply to 13 volts will increase the speed of the motor and this will essentially just increase the motors back EMF and give an end result in the motor once again drawing approx. 160mA when driving the same load.

Figure a resistance that will drop ONE VOLT(to go from 13 to 12) when it is passing 160mA of current.

1/.16=6.25

You need a 6.25 Ohm resistance by my calculations.
a 1/4 watt resistor will do, but a 1/2 watt one would be better.
Two three ohm, 1/4 watt resistors in series would be my suggestion.

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5. ### THE_RB AAC Fanatic!

Feb 11, 2008
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If the motor draws 160mA at 12v, it will draw approx 100mA when running at 7.5v.

Even though it has electronic commutation the speed/volt and current/volt of the fan will be similar to a DC motor.

So I'd try a first test dropping 4.5v on the resistor at 100mA, which is R=E/I = 45 ohms. Say 47 ohms. Then tune the resistor value as needed if the result was not perfect.

Another option would be to use a LM317 regulator with a constant current resistor of 12 ohms, will give a constant current to the fan of approx 100mA, and drop about 3v in total.

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6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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I mis-understood the poster to be asking for 12V at 160mA. I see NOW that it was actually 100mA.

and that would make the suggestion to select a different motor the best answer here.

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7. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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So I can't simply add a series resistor to limit the current of a 12V 160mA motor to 100mA? I don't mind lower the speed.

Is series resistor with Dc brush-less motor a bad idea? because both Kermit2 and DoggyDave suggested that I should look for another motor.

8. ### THE_RB AAC Fanatic!

Feb 11, 2008
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A lot of those little fans used in electronic goods are speed controlled by varying their voltage.

I think you would be fine reducing the voltage with one resistor until it draws 100mA. But yes, the fan will then run quite a bit slower.

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