Driving a 6V DC motor with a Motor Shield

Forum 2005-2010 (read only) Hardware Interfacing
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Hey guys,

Beginner here, so please excuse any stupid questions. :-/ I just finished assembling the motor shield kit from Adafruit. I'm trying to get a 6V DC motor running. I've read the tutorial on using it but there are just a few things I am confused about. It says to use two external power sources (one for the Arduino, and one for the motor) for best performance. I am using a 9V power supply to power the Arduino, but I am not sure what to use to power the DC motor. The Adafruit site says this:

If you would like to have 2 seperate DC power supplies for the Arduino and motors. Plug in the supply for the Arduino into the DC jack, and connect the motor supply to the PWR_EXT block. Make sure the jumper is removed from the motor shield.
If you have a Diecimila Arduino, set the Arduino jumper to EXT. This is a suggested method of powering your motor project!

I am wondering about what type of power supply to use. It also says this:

the EXT_PWR terminals on the shield do not have a protection diode (for a fairly good reason). Be utterly careful not to plug it in backwards or you will destroy the motor shield and/or your Arduino!

What does it mean that it does not have a protection diode? Does this mean I can only use an exact 6v power supply? Again, I am sorry for such basic questions. I am still in the process of learning.

I have also seen a picture where the motor shield has a servo, dc motor, and a stepper all plugged in and the only power supply is connected into the Ardunio DC jack. What power supply would this be possibly with? Again, I am only trying to power a 6v DC motor, so could I potentially just use a 9v power supply plugged into the DC jack to power the Arduino and the DC motor?

Thanks in advance for the help!

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A diode allows current to flow in only one direction. The Arduino does have diodes on the input lines, so it you connect the power supply backwards, no current flows, and the Arduino is unharmed.

The motor shield does not have those diodes, so current will flow backwards, which is generally not a good thing.

An unregulated power supply can be used, but it's loaded voltage should match the motor voltage. That is, the power supply should say that it is a 6 volt power supply. When you measure the unloaded voltage, it might read much higher than 6 volts, but as soon as a load is applied, the voltage will drop.

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What does it mean that it does not have a protection diode? Does this mean I can only use an exact 6v power supply? Again, I am sorry for such basic questions. I am still in the process of learning.

What it means is that if you connect an external supply to the shield (after removing the jumper), and you connect it up "backwards" (that is, the positive of the supply to the negative on the shield, and the negative of the supply to the positive on the shield), you will fry the shield. So don't do that.

I have also seen a picture where the motor shield has a servo, dc motor, and a stepper all plugged in and the only power supply is connected into the Ardunio DC jack. What power supply would this be possibly with?

In this case, everything is likely being supplied power from the on-board voltage regulator of the Arduino, which can output a maximum of 5 volts DC @ 1 amp (in reality, it could never supply 1A without burning up, because there's no real heatsink other than the PCB). Doing such a setup should only be for "demonstration purposes only"; the regulator on-board the Arduino was not meant to be used for that many external parts, where each may draw a significant amount of current. If you have parts (like relays, solenoids, motors, etc) that are external to the Arduino and possibly draw a lot of current (this even includes other external logic circuits!), you should power them separately from the Arduino with another power supply rated for the voltage and current draw of the device in question.

Again, I am only trying to power a 6v DC motor, so could I potentially just use a 9v power supply plugged into the DC jack to power the Arduino and the DC motor?

Probably not, unless the motor you are using can take the extra voltage. You say your motor is a 6V motor - if it truely is, then running it on a 9 volt DC source will likely burn it out, cause the bearings to fail, or the brushes to fail/weld to the commutator - or all of the above. You should try to find the specs for your motor.

I would suggest running on an external power supply; instead of a separate wall-wart supply or such, get a 6V battery pack that holds 4 AA or C cells, remove the jumper from the motor shield (to use external power) then hook the positive/negative wires of the pack to the positive and negative terminals on the motor shield's external power input. Just don't switch the wires around, and it should work fine.

:slight_smile:

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Thank both of you very much for the help. Much appreciated.