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Running a Unipolar Stepper Motor with a Bipolar Driver

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So I gutted an old printer last night and found a few rods, motors and gears for my parts stockpile.

I started typing motor details into Google and identified the one as being a 12V stepper, but the datasheet is very limited. From the 6 pins I realized it has to be a Unipolar motor. I started testing the resistances between the pins and realised that the 2 centre taps are connected together.

Since my driver has a H-Bridge I need a Bipolar configuration and started investating if the two centre taps are joined internally of externally.
 
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Step 1: Center Taps joined Externally

Picture of Center Taps joined Externally
Looking at the external PCB i noticed Pin3 and Pin4 are joined together. This was very good news since I could simply sever the track on the PCB to create two separate center taps.

Step 2: Separating the two Center Taps

Picture of Separating the two Center Taps
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I got a good sharp knife from the kitchen and cut the track joining Pin 3 and 4. I tested the resistances afterwards and it seems my operation was a success!

Step 3: Can it be used with a Bipolar Driver?

Picture of Can it be used with a Bipolar Driver?
The major question I have at this point is whether I can disregard the two center tap pins using only the two major coils.

Coil A of 8.1 Ohm between Pin 1 and Pin 2.
Coil B of 8.1 Ohm between Pin 5 and Pin 6.

Looking at the datasheet, I get the impression that the motor won't draw serious current with a 12V supply (rated around 230mA), but using Ohm's law, I calculate that an 8 Ohm load will draw 1.5A of current from a 12V supply.

I really hope that my calculations are wrong since my driver can handle current ratings up to 2A, but I'm not planning to configure the Current Sensing and Chopper parts of the circuits, so it might get a bit hairy if I run at around 1.5A... Any Comments?

See the schematic I'm planning to implement and please let me know if you think I'm creating fire-lighters in stead of a stepper driver...?
pfred21 year ago
Ah the joys of driving stepper motors, I know them well. To run a little motor like you have in this article I wouldn't have chosen a 297/298 solution. Today I'm not sure what I'd choose to use a 297/298 for. Why did you pick those antique chips? The only reason I can think of is there are a lot of data about them on the Internet today. Electrically they are many generations out of date now though. They were hot stuff about 20 years ago but today you can do better.

I picked up some little driver ICs that would run those printer motors you have. They're good for 350ma per phase. MC3479 which I like to think of as the modern replacements for the classic SAA1027 as those are very difficult to purchase at a reasonable price today. Still popular with the telescope crowd because of the projects on the net that use them. But in reality a total rip off for what they are.

I know this is a build it yourself project site but when it comes to stepper motor drivers I'm not sure how to rationalize that decision today. I put up an article about one stepper driver I built here, but I advise people to skip trying to build them themselves and just buy drivers for whatever project they happen to be working on. It is cheaper.

I'm in no way affiliated with this website, but they seem to have some interesting products to me:

http://www.pololu.com/catalog/category/120

There is still plenty of opportunity to do some building with what they offer, but they do some of the heavy lifting for you. Note how they do not say they are selling drivers, so much as carriers. Which is a more honest, but slightly confusing description. You could run a single stepper motor off of one of their boards, but you couldn't really integrate it into a multi motor system without some thought to noise suppression I don't think.

The other hot on the cheap product out there today is the imported drivers that use the same driver IC I did, the TB6560AHQ. I've heard of folks picking up 3 axis drivers for $22? That is a tough act to beat for what is effectively a plug it in and go solution.

Anyhow if you decide to press ahead with your 297/298 project good luck to you. I'm sure you'll learn a lot in the process. It will be a costly education for you in terms of finances and time though. I further question the ultimate outcome of it. I mean the best a 297/297 driver can do is half stepping. That is pretty rough by today's standards. Aren't those chips bipolar technology, like the transistors in them too? Primitive!
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