Powering a Small DC Motor with USB

Question

I am designing a device that uses a small 12V N20 geared motor. For convenience, I would like to power my device via micro usb. I intend to use a step-up boost converter to get from the supplied 5V to the necessary 12V to power my motor.

I’m wondering what it would take to power my device from a host device like a laptop instead of a USB wall charger. The motor has a rated current of 200 mA, but can draw up to 500 mA with a locked rotor. Assuming a max 2.5W (0.5A@5V), I believe I could get just enough power (0.2A@12V) for the rated torque. This assumes that my device can negotiate for the 500 mA max, which I understand can be very complicated.

1. It seems many power sources don’t bother with the negotiation process and supply up to 500 mA by default. Am I overthinking this?
2. If negotiation is necessary, is there a chip that will do this for me?
3. Does my device need to handle overcurrent protection in the event the motor gets locked up for some reason, or will the host device automatically cut it off?
4. Should any usb-powered device be capable of running off of any usb power source? In other words, is it common for usb-powered devices to be designed to run off of only a usb power brick with adequate current and not a host device like a laptop?

Answer 1

Start current same to stack rotor. Just short time, current reducing then turns up. Nothing wrong with any PS which can provide necessary power. 500 mA@12 gives approximately 1.5A before voltage converter. Computers USB ports have current limits. USB 3.0 may hold 1.5A but, my guess, the load is not only motor.

Answer 2

It should be possible to use a large capacitor charged up to 12V, and then draw the 500 mA needed long enough to start the motor. You could also implement a PWM low voltage start until the motor reaches desired speed.

Answer 3

It will not work. As you increase the voltage you increase the current by the same ratio ignoring efficiencies. So we look at the 200mA * 2.5 and you get 500mA, leaving nothing for anything else. If it can draw up to 500mA that will require 1.25 amps from the USB port, again just for the motor. I used 2.5 (12/5 = 2.4) as a round number to work with and did not go through the efficiency calculations.