Hi I'm using a adafruit motor shield v2 and I need to power a DC motor 6-12V but I'm not sure if my battery will be okay for the shield. Will it burn the shield? I'm going to be using 6V so it should draw less current?

  • The 6Amps is the maximum current the battery can supply. It will only supply what it needed. You only need to look at the voltage, and 6v is fine for both the shield and the motors.
    – Gerben
    Jun 11 '15 at 20:04
  • On the shield it says 1.2A/motor (3A max) Jun 11 '15 at 20:06
  • "1.2A per motor", so the battery doesn't really matter here, only the motors. What is the current rating of the motors at 6v?
    – Gerben
    Jun 11 '15 at 20:08
  • Here's the link to the motor specs: jaycar.com.au/Electromechanical-Components/Mechatronics/… Jun 11 '15 at 20:10
  • 14A full load (at 12 volt). 0.45A at no load (at 12V). So it depends on what is attached to it. You could try measuring the current by connecting the motor directly to the battery and see how much current it uses at 6v when attaching the maximum load.
    – Gerben
    Jun 11 '15 at 20:16

If you take the "full load" to be the "stall current", then you can work out the resistance of the winding.

R=V/I = 12/14 = 0.857Ω

From that you can then work out the stall current at 6V:

I=VR = 6×0.857 = 5.143A

That, being the stall current, is the maximum the motor will draw - the current drawn when it is trying to move an impossible amount of load and the rotor cannot move. At that current you will melt the motor drivers (or they will go into thermal shutdown).

Of course, with a geared motor, that becomes a (comparatively) massive load and would be quite hard to achieve. You have a 50:1 gear ratio, so the stall load is effectively 50x greater.

So there is going to be a limit to the load that you can drive with the motor before it becomes a problem for the driver. There are ways of calculating it from the given torque, gear ratio, load/noload current ratings, etc, but I don't have all that at my fingertips right now.

So you can either "play it safe" and invest in a driver that can cope with the stall current, or experiment and measure what current is drawn by the motor with different loads and see if it stays below the 1.2A limit of the shield.

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