Reduce Fan Speed Using Resistor

The more the resistors that takes part the more the voltage drop generated thus the less the speed of the motor.

Reduce fan speed using resistor.

As for the resistor wattage here comes the good news. The resistor will dissipate 0 55 50 25w. But notice that you are wasting 15w of power. If we use a 10kω as our r1 resistor plugging in the values we get r2 v r1 vin v 1v 10kω 5v 1v 2 5kω.

On the third position 3 resistors takes part and on the last position there is no resistor connected. So we can use a 2 5kω resistor as our r2 resistor with the r1 resistor being 10kω. If the motor draws constant current then the resistor will drop a constant voltage and the motor will run at a fixed lower speed. So to achieve a voltage drop of 5v we would need a resistor with value of 42 ohms and capable of dissipating 0 6w of power.

The problem with using a series resistor is that its voltage drop is proportional to current. If you add a resistor of 50 ohms then you will have an effective load of 400 ohms which will reduce the current by 220 400 0 55 and the voltage across the fan will be 190v. So it s easy to see now how any voltage can be obtained with a resistor voltage divider circuit. I m not sure how slow the fan will go though.

You could also use a simple 1n4001 diode in series with the positive wire to drop 7 of a volt for each diode you use doing it this way still lets you draw exactly the same amount of current about 500ma and it purely drops voltage and nothing else whereas a resistor in line will have an effect on current also. The speed of a permanent magnet dc motor is primarily determined by voltage. You can use a selector with more positions for more speed adjustments. Say if you have the same circuit above with 5v but only want 1v.