motor controller
I am building a robot that sports 2 motors ripped from an old RC car. After many attempts to find the data sheets, nothing of use turned up. So, I ran some test and found this out about the motors.
My question is, How many amps does my motor driver circuit need to be able to handle? Could I get away with a 1A driver, or should my circuit need to be able to handle the full stall amperage?
The circuit looks reasonable, but it looks like the decription is wrong.
Where it says:
The MOSFET used on the ground-side of the motor doesn’t actually need to be there. This is the brake which let you electronically stop the motor from spinning.
It should say the MOSFET used across the motor, this would be used for the braking. This is confirmed in the truth table, where it says when N = 0 and P = 1 "MOTOR FRENADO", which means motor braking. So N is for on/off (N = 1, P = 0 is "MOTOR AVANZA" i.e motor advances) and P is for braking.
The reason you see the sparks is because you are open-circuiting a charged inductor (ie the stator of the motor)
Thou shall not open-circuit a charged inductor, thou shall not short-circuit a charged capacitor.
The stator of the motor has been "charged"... you have at one point applied a voltage to it to permit current to flow. For whatever reason (intent or oversight...) the relay has opened the stator terminals. The inductor MUST keep the current flowing & due to: \$V = L\frac{dI}{dt}\$, a sudden (attempted) change in current can only result in an increase in voltage which leads to sparks, avalanche or insulation breakdown.
To remove such sparks you could consider using a H-Bridge arrangement but these are more involved. As a "poor mans" replacement, some sort of snubber across the motor would be the next best thing. An R-C-Z would be one option
Best Answer
The easy answer is that it should be able to handle up to 2.5A. Since that's the max the motor can draw, the driver's limit won't be exceeded.
Another approach is to use a weaker driver and then actively limit the current somehow. This is not so good for two reasons. First, at a lower current like 1A you will have lower torque. If the motor is presented with some mechanical resistance, it might not be able to overcome it at the limited current. Second, it makes the control electronics more complicated.
A H bridge or low side FET drive isn't much harder to make for 2.5A than 1A at your low voltages. I wouldn't try to play games cutting corners on the driver. That's the most likely component to fail since it would be the most stressed, especially if it can't really handle the full current.