I'm currently playing with small uni-polar steppers. 12v taken from 5.25 floppy drives and other small steppers from printers and hard drives. Really just trying to learn about it, not build the ultimate machine. At this time.
I have a 27v transformer that I wish to power the 12v stepper with. Therefore I'm learning choppers and PWM with my pics and stand-alone comparators and such.
Here are my questions/reasonings:
1. I measure about 68ohms across my unipolar, that is double coils for more torque. Therefore I could calculate the desired voltage between the ground side of the motor and a current sense resistor using the voltage divider formula?
2. I can also just run 12v across two coils of the stepper and measure the voltage at the bottom side before a current sense resistor.
3. Using the voltage measured at #1 or #2 I can set my comparator to turn off when it exceeds that voltage, i.e. 27v is applied to the stepper and it rises beyond the 12v value.
4. What is a good value for a current sensing resistor? Can I get away with only 1 ohm or is that too small?
5. Can I take my voltage measurement after the coil selecting transistors and still get an ok result?
Thanks in advance.
I'm not sure that I'm understanding your questions explicitly but if in general you're looking to devise a pic based driver circuit you might want to take a look at the Linistepper schematic and related discussion. Linistepper
If the driver is made to be current limiting the voltage isn't that critical (up to a point).
I'm probably misunderstanding something, I've done a lot of reading, just having trouble integrating some of it. Don't think I'll design a complete driver, just small ones so that I understand the concepts behind it. I really like the Linisteppers, OSS all the way.
As I understand it, to limit the current one should turn off the stepper when the voltage measured ahead of a current sense reisistor is going higher than you want. See attached schematic, maybe that explains a little better.
If each coil in your 12 volt motor is 68 ohms, then from ohms law, your motor is rated at about .176 amps(12/68=.176)
You calculate the voltage across the sense resistor from Ohms law. V=IR If you use a 1 ohm resistor, then 1 amp will give 1 volt across the 1 ohm resistor. For .176 amps the voltage will be .176 volts. Should work, but probably better to use a little higher sense resistor to minimize noise problems.
The circuit you show would work for wave drive where only one coil is activated at a time, but for half step or full step with both phases on, you will need separate sense resistors and comparators for each phase, ie 2 resistors and two comparators.
If you just want a driver for your motor, the linistepper design is probably a good one for this low current motor. If you wanted to understand how a chopper drive works, you might look at this thread for a unipolar chopper drive design. http://www.cnczone.com/forums/showthread.php?t=25361 (unipolar motor microstepping)
Jeffs555: working my way through that thread.
How about a sense resistor between 4-8 ohms? I can also use a comparator IC and some logic to run multiple comparators with no problem.
I'm doing this as a learning exercise, I don't really "get it" until after I've built something. I'm looking at CNC'ing a harbor freight micro mill and will use bi-polar steppers at that point. Just don't have a garage right now. I do want a gut level understanding of what is going on with the electronics and would like to build as much of it as possible. Otherwise why not just save up and buy a brand name CNC.
I'll use either purchased drivers or build from a an established design, but I want to figure it out first.