# 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)?

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:

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.

• Thanks for the answer! I know 75mA is way to much for an Arduino pin. I planned on using a transistor and flyback diode to overcome this (exactly like in your circuit). I was just trying to figure out how to drop the 5V output pin to 3V. So you are saying I should use 3 diodes in series? What are the advantages of multiple diodes vs a resistor? Commented Mar 18, 2011 at 14:15
• You could use a resistor. I make it 2V/75mA = 27R. Commented Mar 18, 2011 at 14:36

http://ww1.microchip.com/downloads/en/AppNotes/00905a.pdf is a fair description of dc motor control issues. You should look at the Basic Drive Circuits section to see some appropriate circuits. Set VCC across the FET/motor to be 3V and use a FET that can cope with at least 5V gate input and you should be fine.

In particular, you should note the diode placed in parallel with the motor. This is extremely important to prevent the mosfet and your controller from damage. The motor is essentially an inductor and it isn't possible to make an instantaneous change in the current flowing through an inductor. When you turn the motor off, this means that the current flowing through the motor gradually decreases. If there isn't a diode placed as shown, then the voltage at the motor will increase and potentially be high enough to damage other connected devices. With the diode in place, the voltage is limited to its forward voltage and so no damage occurs. This is a well known phenomena called back EMF.

• Thanks, nice overview! But I already knew I needed a flyback/snubber diode to prevent back EMF from doing any damage. I was just wondering what would be the best way to get the 5V output down to 3V :). Commented Mar 18, 2011 at 14:35
• Ok, good. Always best to check to avoid explosions :) Commented Mar 18, 2011 at 16:32