Supplying power to a 12V motor with 5V and 2A adapter

Question

A very elementary power supply question but i couldn’t find the answer with Google, so I would appreciate a response.

I’m using a 12V, 340W DC brushed motor for a personal project. While the motor is relatively powerful, I don’t need the full capacity of the motor, but I do not want to buy a new motor as I already have this lying around.

For the purpose of my project I only need it to operate it at 5V (required RPM from the data sheet). I have a 12V, max 3A, cont. 2A, 30W driver lying around too. I was wondering if I used a 5V, 2A power adapter (that I already have), will it only supply 2A even if the motor tries to draw more than that and will this damage the adapter by any chance?

I really don’t want to buy any additional parts and I have all these components lying around. If I supply 5V to the driver, even if the motor tries to draw more than 2A, will the power adapter limit it, or will it supply more and damage the adapter and driver? Is this okay to do or will I damage anything?

I will be using PWM to slow down the motor and average it out to around 3V or less. The motor draws around 2.4A at 3V when I checked with a bench top PSU and DMM. I don’t want to supply more than 2A and I don’t mind it losing a bit of torque. It only spins a threaded rod and the load is pretty much static once it starts. Do I have to take any additional methods to be safe or will the power adapter limit the current supplied?

Answer 1

Motors want lots of current to start, they want lots, given the opportunity they will take lots, and this will cause your 2A powersuply to quit.

I was wondering if I used a 5V, 2A power adapter (that I already have), will it only supply 2A even if the motor tries to draw more than that and will this damage the adapter by any chance?

Typically they go into over-current shoutdown if over-loaded.

But motors don't need lots of current to start they only want it, if you use a DC-DC converter that has a current limit you can keep the power supply happy and get the motor spinning.

Answer 2

Most (if not all) modern 5V phone adapters will limit the current to what they're rated for. If not, the voltage will most likely fall apart and it's probably not going to burn the adapter.

The current your motor will draw depends on the mechanical load it has to overcome, so if you're not asking much of it it will probably not exceed the adapter's ratings.

Furthermore you can always add a fuse just in case, it costs cents.

Answer 3

You've already established the motor draws 2.4 A at 3 V, which is 7.2 watts. If you restrict it to any less than that, you may well find the motor doesn't turn, through insufficient torque.

However, your 5 V power supply will deliver 2 A, which is 10 W.

If you use a buck DC-DC power converter to get from 5 V to 3 V, or your PWM operates so as to use the motor's inductance to be a buck converter overall, then you will just manage it. In the second case, you may need a large input capacitor, to supply the output pulses of current, to prevent them from overloading the 2 A PSU.