I wanted to open and close my curtains with a nema17 stepper I have laying around, so I built two simple H-bridges out of DB139 and DB140 transistors.
But then I measured the resistance of the two coils in the stepper motor; 1.5 Ohms.
Driving this with 5 volts would mean a current of about 3 Amps, which seems retardedly high. But I use the steppers in my reprap aswell, where they are driven with 12v.
What do the stepper controllers from the reprap do so that the steppers work on 12v without burning out, and how would I build a simple driver for this myself?
You drive low impedance bipolar steppers with current-controller chopper drivers,
not H-bridges. These are effectively constant-current sources and buck-regulators
combined into one. People use the A4988 a lot for Nema17 steppers in RepRaps etc,
typically from 24V or so.
You need the high voltage supplies to turn the motor rapidly, as the back-EMFs from
steppers are high at even modest rpm. Big CNC setups have motors with perhaps
5A rating and 0.5 ohm windings, supplied from 80V rail via industrial chopper
driver units. The 80V supply will see about 0.5A with the motors stationary at
5A, upto perhaps 7A when the motors are going full pelt under load (3000 rpm
sort of speed)
If you want a stepper that's easy to drive get a high-impedance unipolar motor, not
a low-impedance bipolar motor. It won't turn as fast though.
I do have some small unipolar steppers, but they are way too slow to move a curtain nicely, so that won't work well..
As last try at making my own driver, would it be an idea to PWM a 12v supply at about 5%, 32Khz, and use that as power input in the H-bridge? This would effictively give me about 0.4A on the stepper.
Or am I just way better off buying a A4988 driver? All it has to do is spin though, there's not a lot of force..
Or am I just way better off buying a A4988 driver?
Yes. If you don't need a lot of force, save yourself the time and trouble by buying this (or one like it): Pololu - A4988 Stepper Motor Driver Carrier, Black Edition You will have to set the current limit to 1.5 amps/winding (or a bit lower) to avoid overheating the motor driver. There don't yet seem to be any H-bridges on the hobby market that will handle more than about 2 amps/winding.
Inevitableavoidance:
I do have some small unipolar steppers, but they are way too slow to move a curtain nicely, so that won't work well..
As last try at making my own driver, would it be an idea to PWM a 12v supply at about 5%, 32Khz, and use that as power input in the H-bridge? This would effictively give me about 0.4A on the stepper.
No, unless you measure the current and use a feedback loop to maintain it as the back EMF
rises this won't help, the winding needs to be current driven.