Driving stepper motors / schematic comprehension help

Hi all... I'm a software guy trying to learn the ropes of electronics.

I have the goal of controlling a stepper motor using my PC parallel port to create +5v control signals. Shouldn't be that hard, right?

My understanding of 5-wire stepper motors is that there will be one common power wire, and four other wires coming out. Attach the power wire to the power source, and attach the other wires to ground (using some logic to connect the right ones to ground at the right time), and you're all set.

I bought a unipolar stepper motor... the first one listed on this page:

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One thing I noticed is that the spec sheet lists that it has a "drive circuit".

Some more investigation led me here:

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Since I bought a unipolar stepper, I think the first schematic on the page applies. I'm not quite sure how to read it. It appears that what I want to do is attach my parallel port data pins DIRECTLY to the bases of each of the transistors (which is what the four non-power wires coming out of the motor will be).

If that's the case, it looks like I use my parallel port control signals to activate each transistor according to activate the internal electromagnets in the proper order. Being a smart software guy, I can handle that. When I do that, the power will go through the coils, through the transistors, and into the little black boxes.

What are those little black boxes? Some sort of internal ground?

Sorry for the newbish questions, but every description of a unipolar stepper motor seems to imply that I need to build or buy my own drive circuitry. So the idea that this one might come with its own already built right in is throwing me for a loop.

If someone could confirm my suspicions, or point me in the right direction, I'd appreciate it.

Also, I don't actually have the motor in my hand yet. It's being shipped. I'm just trying to get things figured out before it gets here. :)

Thanks! Adman

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See if this kids course doesn't help:

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I believe they used BASIC print commands to move their stepper ...

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Thanks for your response! That site looks really helpful. I'll check it out.

You didn't actually read my post, did you? The site you referenced specifically uses a ready-made IC to convert signals to drive the stepper motor. My question is, given the stepper motor that I bought, do I need to use a similar chip myself. Or does the schematic described in the following link:

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... prove sufficient.

Thanks again! Adam

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NPN transistors (NOT POINTING N)


base is +, collector and emitter are -

so, a small positive voltage on base (the flat line sticking to the left)

causes voltage to flow from the emitter (the lead with the arrow that is NOT POINTING IN) to the base

this biases the transistor, and will allow collector current to flow through the emitter...

so, small positive voltage on base (control) ends up giving your stepper motor GROUND (negative)

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Gotcha... I guess I expected "ground" to be a wire coming out of the whole system. It appears from the schematic that there is some sort sort of "internal ground".

That is, if I apply a control signal to the base of one of the transistors, the current flows from the source, through the coil, through the transistor, and into the internal ground... ?

I think we're on the same page. thanks so much for your reply!


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No, the drive circuit is not part of the stepper.

All I see on the spec sheet is the "Drive IC" as 2SC3346 which seems (when I look it up on the internet) to just be a a high power switching transistor you could use to build a drive circuit. I read that as just a suggestion of the type of drive circuit you could use to drive it.

If the stepper actually included a drive circuit, it would give you far more detailed specs on the drive circuit instead of just listing the part number of a single transistor.

No, that's not how I read it.

The transistors and diodes in that schematic are NOT part of the stepper. They are just shown as a _typical_ drive circuit and how you would connect a _typical_ drive circuit to the stepper. The wires coming out of your stepper will be the black, red, brown, etc writes connected directly to the coil.

You still need to buy or build some type of drive circuit because the signals on your parallel port do not have the right voltage or current to directly drive your stepper.

That's right in theory. But you have to get the voltage and polarity and current right. That is, the drive circuit you end up using has to be compatible with the signals coming from your parallel port. In that schematic, which uses NPN transistors, you would have to apply a positive voltage to the base to make the transistor "turn on" and a ground (or negative, or just open) to make the transistor turn off. The parallel port would need to supply enough current at a positive voltage to make that work. "enough" will be a function of the amplification of the transistor (it's beta) and the current you need to drive the stepper. I don't remember how parallel ports work, but you might need something like a pull-up resistor in there as well. Which is why it's best to just buy a drive circuit designed for working with a parallel port or to find a schematic of one for your need and build it if you want to have fun learning to do that.

I'm sure you can get some suggestions here on where to find a drive circuit for your needs.

That's just a generic symbol which means ground. It's where the negative of the power source driving the coils are connected.

It doesn't have it's own internal circuit. You do need to build or buy a drive circuit. I've never played with these so I don't have any suggestions on what to get, but I suspect you can find one made to be driven by a parallel port - lots of people do that. Or at least they used to back when all PCs had parallel ports. :)

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Curt Welch

Not "internal ground"... just grounded to the case... there is probably a terminal on the case, or a terminal on the PCB that ends up being connected to all the appropriate spots (places in the circuit where "ground" is needed")

The NEGATIVE side of the parallel port (GND) would be connected to the GND of the stepper controller... this flows to the "little black boxes" you see in the schematic.

ground is shown as a block, or often as a T with reducing width lines on top:

(view with monospace font) =2E._.. =2E___. _____ =2E.|..

The reason: to clear up the schematic. a ground symbol means "connected to negative side of power supply". If you had a schematic with all the grounds shown, they end up crossing over all of the positive lines, and the schematic ends up being very hard to read.


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I suspected this myself but wasn't absolutely sure. It looks like circuit Adman provided the link to is really just a generic diagram showing what the external circuitry might look like (without values, resistors, etc). It is not a circuit actually provided with the stepper motor.

The part you are buying is indicated on the drawing by the dotted line box, which indicates what part of the diagram is the actual motor you are purchasing.

Joe Dunfee

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Great. Thank you all very much for your comments.

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1985 called. It wants its PC/AT back.

John Nagle

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John Nagle

Hi there.

Here are 2 great books to get you started:

1) Google for "Parallel Port Complete" and you can see most of the book online. 2) Google for "Controlling The World with Your PC" to see much of it online, too.

These are both old enough to be found in libraries or very cheap online.

also, here's a nice page:

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Good luck! JCDeen

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This doesn't look that complicated.... right?

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So far, you haven't actually told us what you want to do with the stepper motor. Stepper motor control can range from really simple to rather complex depending upon the application.

For the simple stuff, the key is not to exceed the coil current for the stepper motor and/or driver circuit. The stepper motor spec. sheets rarely specify the maximum coil current. They do specify a nomial voltage and (sometimes) a coil resistance. If the coil resistance is not specified, break out a volt/ohm meter and measure it directly. Use Ohm's law to find maximum current.

Voltage = Current x Resistance

For the drive circuits, some of them have protection diodes built in and some do not. Make sure that all components in the drive circuit are rated for the specified current.

For beginners, I recommend using drive circuits that are designed for driving inductive loads, rather than building them from individual transistors and diodes. The L293D has protection diodes built in and comes in a 16-pin DIP package. The SN754410 is a more modern equivalent for the L293D. For a higher current, the L298 is good device, but it needs external protection diodes.

Everything starts from the current requirements of the stepper motor.


Reply to
Wayne C. Gramlich

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