# Problem running a motor

I am new to electronics, and I’m trying to run a simple DC motor (Mabuchi 130 motor) off my Ardunio UNO R3. I have read that it is best to use an external power source (like a 9V battery), a motor shield, or an H-bridge (not sure what that is exactly), but the motor is so small, I think I SHOULD be able to run it directly from the Ardunio.

I think this is probably the spec sheet (none was included in the shipping package).

http://www.hobbyengineering.com/specs/MABUCHI-fa130ra.pdf

There are only a black and a red wire which are attached to the motor (I assume power and ground).
I am trying to run it at 1.5V, which will require 0.2A, according to the spec sheet.

Therefore, the resistor I am using across the circuit should be:

R = V / I
= (pin output voltage – voltage drop across element) / current supplied to element
= (5V – 1.5V) / 0.2 A
= 3.5V / 0.2 A
= 17.5 ohm
However, when I put a 100 ohm (the smallest resistor I have) across the circuit, the motor does not run. Why is this, and what could be the cause?

All I am doing with the code is just setting pin 13 (the output) to HIGH, and then turning it on and off everything 2 seconds. Hey, it works with an LED…

I don't think you can do that sort of maths with a motor. When it is stationary or moving very slowly an electric motor has very little internal resistance and will draw a large current - the stall current is quoted as 2.2 amps.

The 0.2A you mention is the no-load current. You must design to accommodate the stall current.

You haven't said how you are planning to connect your motor to the Arduino. DON'T think of connecting it to a digital PWM pin (even with no load on the motor) as they can only supply a max of 40 mA.

...R

DC brushed motors have significant friction from the commutator (and because they don’t use
ball bearings), so most of the quoted 0.2A no-load current is required to overcome static friction,
its the minimum current needed to allow the motor to move at all.

A motor shield is usually one or two H-bridges - if you don’t know what an H-bridge is you will find out
with a search on these forums and elsewhere!

I have read that it is best to use an external power source (like a 9V battery)

The small PP3-sized 9V batteries are unsuitable for motors, they provide only a tiny current (think 0.05A to
0.1A without the voltage/capacity dropping precipitously).

An external power source is usually needed to prevent supply drop-outs when the motor starts or reverses
(since most motor supplies don’t attempt to provide enough current for stall conditions, whenever the
motor starts from rest it pulls the supply voltage down and resets and microcontrollers sharing the supply).