Stepper Motors..

I recently pulled two stepper motors out of a working IBM dot-matrix printer. One larger, 3v one with 1.8 steps. The other is a smaller, 9.5v one with 7.8 steps. I am more interested in the large one right now.
I managed to find out that the large one has center-tapped coils. I also found out that Coil 1 is red-white-brown, coil 2 is blue-black-yellow. I found the schematic below on

I was wondering how this could be adapted to control my motor from a PC program with the ULN2003 (that chip sounds like UT2003)?

Here are the complete residences of what I found on the wires with a multimeter, in case that helps.

Red to Brown: 8 ohms (or 7 some times)
Red to White: 4 ohms
White to Brown: 4

Blue to Yellow: 7
Yellow to Black: 4
Black to Blue: 4

Picture of Stepper Motors..
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l0cke (author) 10 years ago
No, that chip does not come on the motor, that is just the recommended one. I saw that site, but still can't really figure it out.
westfw l0cke10 years ago
The ULN2003 and similar chips are "low side" drivers. A small signal on the nB input will allow a large current to flow from a positive voltage (through a load) to ground; essentially it's can be an electrical switch to ground. In your schematic, any of the coils can be turned "on" by putting a signal on the appropriate nB pin; current flows from +12V through the coil, through the chip, and to ground. You get the stepper to spin by activating the coils in a certain sequence, usually one or two at a time. In the simplest driver, you just turn them on one at a time... Since the ULN2003 can only act as a switch to ground, but not to +V, you can only use it for "unipolar steppers" like yours, that have the center tap...
l0cke (author)  westfw10 years ago
Yeah, I did not make the schematic. Forgot to mention that, which might have made it confusing. I was looking for a way to modify that to work with my motor. But wow, I can not say that information will not come in handy at some time.
westfw l0cke10 years ago
For relatively low-current motors, the ULN2003 is approximately similar to this discreet circuit that might be easier to understand.
i am a low tech guy..and am am constantly blown away by the fluencey and the breadth of your knowledge in the arcane world of electronics... I am better with springs and levers
westfw10 years ago
Goodhart westfw10 years ago
Oh cool ! Some rather good ideas at that URL. And I do have a few steppers laying about, too. I just never thought I would be able to do much with them, thanks from me, too.
royalestel westfw10 years ago
Or westfw could pop up with something totally relevant!
royalestel10 years ago
Is the ULN chip part of the stepper motor? By the way, you could always try to PM Las Vegas. He's disappeared, but might answer your Q.
Goodhart10 years ago
Sorry, I have almost no experience with stepper motors.

That chip, though, is a Darlington Sink Driver (data sheet link to pdf)
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