Driving a stepper motor
So I recently got this stepper motor and since I'm temporarily lacking for a proper power supply, would it hurt to make a simple h-bridge for each coil out of some 2N4401 npn transistors and a 9V battery? It would be driven by an FPGA, with 3.3V logic. I read through the gecko paper that's always referred to, but I still feel ignorant in the design of stepper motor drivers.
Posted May 15, 2017, 7:54 PM UTC
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You can buy H-bridges that are much simpler and probably cheaper than building your own.
Last time I built an H-bridge myself was sometime in the 1990s.
Of course, I've found some that would be perfect for this. I'm actually preparing for a much larger project that I'll be working on in the fall, so I'm learning how to make the most basic parts of the driver first. I just happened to have those transistors laying around is the main reason I'm thinking of constructing my own. My primary concern is that this experimenting at lower current than the rating (1.5A) will damage something, though I can't think of why it would.
Have you read the Jones tutorial? I think everyone who works with stepping motors should read it.
Anyhow, I checked the Amazon link you posted and it says that motor draws 1.5A and that's definitely not in the range of what a 9V battery can supply. Don't count on anything over 10mA from a 9V battery.
I'll definitely give it a read, anything that can help. I know I can't pull much current from it, but would there be any harm to using such a low current to power the motor? Or is 1.5A like, a hard limit that it's going to try to pull?
It will be a hard limit. The motor windings have a certain resistance, the nominal current is equal to the nominal voltage divided by the nominal resistance.
Trying to use a 9V battery with that motor would be like shorting the battery. The motor probably won't move at all.
Minor quib: a 1.5A motor will draw 1.5A at the full rated load. More load or stalled rotor == more current. more importantly, at startup it will draw >>1.5A for some dozens of ms. It is essentially "stalled" until it starts spinning. I don't work with motors enough to really advise here, but I think 5x or so startup current is normal.