Electronic – 5V to 3V, calculate resistor value (or use diode?)


I want to connect this dv motor to the 5V output of an Arduino board. Of course I need to get the 5V down to 3V in order for the motor to function correctly. I've read that I could use a diode with the correct voltage drop for this, or just use a resistor.

Going the resistor way, I've got some trouble figuring out the correct resistor value.

From the datasheet I calculated that the resistance of this motor would be 40ohms at 3V (3 / 0.075 = 40). Is this correct?

The ratio between 5V and 3V is 0.6. So I figured I need a (40 * 0.6) = 24ohm resistor to get the correct voltage over the motor.

Are these calculations correct? I'm still an electronics beginner so I'm not quite sure :).
And even if they are correct, would I be better of using a diode (or maybe even a voltage regulator)?

Best Answer

It takes 75 mA at 3V, which is far too much for an Arduino output. You need to drive it from a transistor using the following circuit:

Motor driver

Any similar NPN transistor could be used.

You could use three diodes in series to drop the 5V supply to about 3V, or use a suitable regulator.