klricks wrote: ↑
Tue Jan 23, 2018 4:35 am
And current is proportional to voltage and inversely proportional to resistance.
So when voltage is applied to the motor the motor takes the current it needs and provides torque which remains constant provided the resistance (and voltage) does not change.
Aye. And there's the rub.*
A motor isn't just a resistance, it's also a generator.
If it's kept stationary, current will be proportional to voltage (neglecting the inductance of the windings) but as soon as it starts turning, it generates a voltage proportional to its speed.
So the voltage required to maintain constant current (and therefore constant torque) will vary depending on it's speed.
I've no idea where (or whether) you can buy the requisite driver board.
However, you have a couple of options.
One is to use a linear constant current source such as a LM134, or maybe a LM317 regulator connected as a linear constant current source.
Another is to put an ACS712 in series with the motor to measure the current and so control the voltage in a feedback loop (but be aware of the problems people seem to have with these! viewtopic.php?f=63&t=203012