2 hobby motors & 12v PS

First, I am basically a novice when it comes to electronics, so pardon any wrong terms.

My Arduino is controlling 2 hobby motors via transistors. (I am only concerned about one direction, so no need for a H-Bridge, plus I don't have one lying around, but I do have a few transistors) If I am running one motor at 'full' speed, as I increase the speed of a second motor the first motor slows down. Here comes my limited knowledge in electronics, this slowing down is a result of a lower available current, correct?

What I want to happen is for them to spin at a constant speed regardless of what the other is doing. So, I think I am asking how to control the current to each motor? If my power supply can supply 1A, how can I limit the current to each motor to say 300mA? (that is, if I am understanding this correctly)

Would using a H-Bridge be easier for this? I could use a resistor (correct?), but would that not waste power? (the end goal will be to make it battery powered)

I am following this tutorial: http://itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads...except I have a different transistor.

this slowing down is a result of a lower available current, correct?

Most likely, it means your power supply current rating will not operate both motors running full speed. Either get a supply with more current capacity or you could try resistors in series with the motors to lower the max current each can draw. They will be slower but might not effect each other, if that makes sense?


I would first find out how much current each motor draws; hook one up in series with a multimeter; put the probes on the multimeter into common (black) and the 10A (or whatever the high-amperage current check plug is) for the red probe. Set the meter to current measurement (this is important, or you'll blow the meter's fuse or worse), and the PSU voltage to the max voltage the motor will be run at.

Hook it up like:

PSU +V ---> meter ---> motor ---> PSU GND

Set the PSU to maximum current output (if it is adjustable), then turn it on. Note the current being drawn by the meter at that voltage. Do the same with the other motor (if they are the same type/model of motor, they should be similar in current draw, but check to make sure). Be sure to also note stall current (try to stop the motor briefly with your fingers - assuming these motors are small enough to do that).

Once you have that information, then you'll know how to size your power supply for running both, and also whether your transistors are properly sized for running both.

Thanks you two.

At the moment, I am not too concerned about speed, so even if they were running slower in order to have them not affect each other's speed, that is not an issue. I'll take a look at using resistors.

Cr0sh, I just bought a new batter for my tester, so I was unable to do any of that testing. Hopefully tonight I will have time to test.