Stepper motor controller circuit

[Somethingrandom]

Hi All

I'm in the early stages of designing a motorised focusing device for my telescope. I will be using a stepper motor a pic microcontroller and a SN754410 motor driver.

Here are the specs for the stepper I have available:

Model 35BY412L
Rated Voltage(v) 24
Resistance(ohm) 5
Holding Torque(gf.cm) 450
Detent Torque(gf.cm) 90
Rotor Inertia(g.cm2) 7.8
Number of Phase 4
Step Angel 2-2 Phase Excitation 7.5
Step Angel Precision ±0.5
Height(mm) 22.2

My question is how do I know how much current the stepper will draw? I can see from the motor driver ic datasheet that it's rated to one amp. Is this chip able to handle this motor. The stepper is rated at 24v and has a resistance of 5 ohms. If v=ir does that mean the motor is capable of drawing (24/5) amps? Seems like a lot for this sized motor but I was never good at theory. If it is the case, I'll probably need a less current hungry stepper as the final design will need to be battery operated.

 

[Somethingrandom]

Ok, so after doing more research I believe i'm correct that it could potentially draw 4.8Amps under certain conditions. Way more than i'd expect from this small stepper but I've also found that it will probably work at a lower voltage so I'll try to drive it as low as possible. If I can drive it at 9v current draw should stay in spec with the SN754410. It will need only a small amount of torque to turn the focuser on my scope.

I've simulated the circuit in some spice software and it works exactly as intended. A few days ago I went to hook it up on a protoboard and managed to fry my PIC! I'm not sure what I did wrong exactly but I was not careful and was adding components while the power was live. Not good. I started smelling bad things right around the time I started hooking up the SN754410 to the 12v rail (I will be using 9v as opposed to 12v in the next test). That's when I noticed my 5v voltage regulator was super hot and I pulled the plug.

This failure has caused me to question something else since I am not really familiar with working with multiple power rails. In this circuit which I will attach a diagram of there are two positive power rails. +5v to control the PIC and the motor driver logic, and +12v to supply the power to drive the coils in the stepper to the motor driver. The SN754410 has two voltage inputs just for this and it also has multiple ground pins however the datasheet does not specify anything special about connecting these ground pins. This got me thinking about ground. Does it matter if I ground my controller to the 5v rail ground, or the 12v rail ground. Do I ground to both? Am I supposed to pick one? How do I pick one if I am using both. I was under the impression that it doesn't really matter, that ground was just ground. But maybe it does. My power supply simply consists of an unregulated brick power supply, tip chopped off, supplies around 12v, and I have an l7805 5v voltage regulator connected to this to supply the 5v.

I've attached a diagram, let me know what you think.

I should add the motor controller in the diagram is an l293d which is exactly the same pinout as an SN754410, but my spice didn't know what an SN754410 was.