I’m starting work on an electronics project, and I’m a bit rusty on some of the basics. Since I’m sure we have a few experts on such things around here, could you let me know if I’m on the right track.
I want to drive two stepper motors from a microcontroller (probably an Arduino). I want to make sure I’ve got the motors, driver, and power supply properly sized. If I go with a motor like this, 12V and 0.4A, would I need a 12V and 0.8A power supply and a motor driver board that can handle at least 12V and 0.8A? What if I can’t find that exact power supply?
The motors will be connected to the knobs of an Etch-A-Sketch. Any idea what sort of torque I’ll need for that?
You do not necessarily want the voltage rating of the motor coils to be the same as the voltage of your power supply. The driver boards will be current-limiting the motors, regulating the output voltage to the motor depending on the step angle and current limit set on the drive board. You actually want the motor to have coils with a rating that is a fraction of the voltage of your power supply so that you have adequate overhead to get full torque when microstepping. (For example, the stepper motors on my 3D printer are rated for 3V, but I’m using a 12V power supply. The driver boards keep the motor coils from ever drawing too much current). This is assuming that you’re going to be doing microstepping - if you don’t intend to use microstepping drivers, you can just use motors rated at the full voltage since you’ll always be driving the coils at 100%.
If you can’t find a power supply rated at exactly the current your motors draw, you can just use a higher-current one. A 12V 20A power supply shouldn’t mind if you’re only drawing 0.8A. (Usually, anyway. There are some with minimum power outputs to be stable, but if you have that problem you can just add a big power resistor for additional load to bring the current draw up.)
I have no idea how much torque it takes to turn the knobs of an etch-a-sketch.
I’ve seen specs for drivers that give a range of voltages, like 4.5VDC to 13.5VDC. If I connect 5V steppers to that, how does the driver know how much current (or voltage) to provide to the motors; are there jumpers on the board somewhere?
It depends on the driver board. Typically it will have one or more tiny trimmer potentiometers on the board to set voltage and/or current limit. Some of them can also be set through a digital interface by writing to registers in the chip.
From what I recall the voltage and current have to be within a certain range, not precise, and the timing and shape of the pulse from the controller is the critical factor.
This is a total guess, but a stepper that weighs .6 pounds probably has enough torgue.