Running a Unipolar Stepper Motor with a Bipolar Driver

Written by: roboguru

Picture of Running a Unipolar Stepper Motor with a Bipolar DriverIMG_0046.JPG

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.

http://www.instructables.com/files/orig/FMC/434B/HD4VOTHX/FMC434BHD4VOTHX.pdf

Step 1: 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 TapsM55SP-1N Resistence After.png

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...?

Step 4: Vero Layout for the Board

Picture of Vero Layout for the Board

I decided to plan the build on Vero and this is the most efficient layout I can come up with. If you see any errors and/or ways to compact or simplify it further, feel free to comment...

In the next couple of days I will start the build and post further steps on whether it worked or not.

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