Question Using 12v Wall Wart to drive 12v motor using RPi

[martincregg]

I am using a RPi to switch a relay on and off at certain times of the day. I have an electric motor connected to the relay and would like to use an external 12V DC power supply (wall wart) that I have lying around to drive a 12V DC motor. I have measured various 12V DC power supplies and all of them show >12V: The nearest was 15V and one was 21V. Is that normal? More to the point is it an issue for the motor? LMK if I need to provide more info.

 

[kanewolf]

I am using a RPi to switch a relay on and off at certain times of the day. I have an electric motor connected to the relay and would like to use an external 12V DC power supply (wall wart) that I have lying around to drive a 12V DC motor. I have measured various 12V DC power supplies and all of them show >12V: The nearest was 15V and one was 21V. Is that normal? More to the point is it an issue for the motor? LMK if I need to provide more info.

How important is the exact RPM of the motor?
You were measuring the no-load voltage. You would have to put load on the wall wart to see how much the voltage drops.

 

[USAFRet]

Amperage is at least as important as the base voltage.

My car runs on a 12v battery. But I could not jump start it from a tiny wall wart.

 

[Alabalcho]

If you want finer control of motor' RPMs (eg ramp-up, rotate with less RPMs than max, slow-down), a simple relay is not going to cut it.

 

[martincregg]

How important is the exact RPM of the motor?
You were measuring the no-load voltage. You would have to put load on the wall wart to see how much the voltage drops.

The motor is connected to a peristaltic pump which I will be using to dose fertilizer. So, RPM not that important as I plan to try and calibrate dosing amount of time the motor is running.

 

[martincregg]

If you want finer control of motor' RPMs (eg ramp-up, rotate with less RPMs than max, slow-down), a simple relay is not going to cut it.

I know.

 

[OldSurferDude]

Power supplies come in with different internal resistances. The higher resistance the higher open circuit voltage. Also with the higher resistance will be higher Supply noise. In your case it is just a matter of how many times the motor will rotate to get the dose that you desire. So it doesn't really matter which power supply you use, just calibrate it to dosage that you want.. The power supply that has the higher internal resistance will have more error. Can you tolerate that?

Personally, I would use the cheapest power supply just to get that power supply out of my stack of power supplies. That is, if it works.

OSD

 

[faalin]

What is the amp rating on the motor and how many amps can the wall wart handle?

If the wall wart cant handle the amps you can buy one of these, which depending on which one your get can go up to 40 amps. It also has a pot so you can adjust the voltage from 10.2 to 13.8v

 

[martincregg]

You were measuring the no-load voltage. You would have to put load on the wall wart to see how much the voltage drops.

Could you help me understand why a 9v battery measures 9v but a 12v power supply measures something different.

 

[kanewolf]

Could you help me understand why a 9v battery measures 9v but a 12v power supply measures something different.

They don't measure something different. But the way the voltage is created is very different. The circuitry in a wall-wart that creates voltage is different than the chemical reaction in a battery that creates voltage.