Motor (6v) won't spin when connected to voltage divider (6v)

Hi there

I have a question about powering my 3-6v DC motor. When it’s plugged into the 5v or 3.3v supply on my Arduino, it works fine. If I connect it directly to a 9v battery, it works fine. However, when I try and make a voltage divider from a 9 or 12 volt supply, the motor will not turn at all.

I’m more or less a complete beginner and feel like I’m probably making a schoolboy error, but I just can’t figure out the problem. I’ve checked the voltage available across the voltage divider and can successfully create 3 or 6 volts from a 9v battery and 6 or 9 volts from a 12v battery, however when I connect the motor it does nothing. I’ve tried using different resistors (10k & 20k; 2k & 1k, 200r & 100r) and still no joy. I’ve tried the same circuit but just using 2 LEDs in series (~6v consumption) and it works fine.

Essentially, 9 volts directly from a battery (or 3.3 or 5v from Arduino) spins the motor, however 9 or 6 volts from a divider does nothing. I’ve spent a long time now just doing sanity checks and getting nowhere, it’s driving me crazy.

I'm not sure what this means or if it helps but I'm getting a reading of either 0.00 or 0.02 volts on the multimeter, when measuring across the motor. If I replace the motor with 2 LEDs in series, when taking a reading from the same point I get 5.90 volts.

Any help would be greatly appreciated.

However, when I try and make a voltage divider

Yup, that's a schoolboy error OK. What's the impedance of your divider? And the impedance of the motor?

Hi, The motor is part of the voltage divider as soon as you connect it. So the resistances are WAY off.

Also, motors have a large startup current (low resistance) which then changes as the motor spins. You need some voltage regulator or PWM speed control; resistive voltage dividers will not work here..

Resistor dividers are only used for sensing voltages, no use at all for power, that requires a voltage regulator. For instance the Arduino has an on-board 5V regulator for when its powered via the power jack or Vin pin.

But you'd never use a voltage regulator for a motor, that's wasteful (and generates loads of unwanted heat), you use PWM to limit the motor drive instead. That's basically a switch-mode power conversion, switch-mode is more efficient, its the only technique used these days for high power.

Small 9V batteries are hopeless for powering motors, they are happy to about 50mA, motors want much more than that.

Thanks for your replies guys, very helpful. Back to the drawing board it is!