Unipolar Steppers
October 26, 2007 07:15AM
Has support for unipolar steppers fallen by the wayside?

I've been away for a while... and it seems none of my old stuff works with the new stuff! The software crashes with my circuitry, and my axis stop working when I try and upgrade to the latest pic code, so I decided to 'get with the times' and ordered the rrrf circuit board kit...

I see instructions for how to populate the board for bipolar steppers, and I see a 5 'pin' connection at P10 marked 'stepper' 'slave', but I don't see anything in the wiki about populating it for unipolar steppers... I don't see any unipolar circuitry on the wiki anymore...

I know everyone's all about driving these big machines with powerful $40 bipolar steppers... but the design I'm working with runs just fine with $5 unipolar steppers, and I really don't want to replace my motors with something that's overkill!
Re: Unipolar Steppers
October 26, 2007 09:24AM
As long as the centre taps of the two windings are separate, i.e. 6 wires not five you can just wire to the centre tap and one end of each winding and operate your unipolar motors in bipolar mode.

Reversing the current in one half of the coil is just the same as passing current through the other half.

Re: Unipolar Steppers
October 26, 2007 12:45PM
(in)conveniently enough... I've got 5 wire steppers smiling smiley
Re: Unipolar Steppers
October 26, 2007 02:38PM
You could leave the centre tap floating and connect the other four wires. Each half winding would see half the voltage, hence half the current, but as they are both on at the same time the number of ampere turns would be the same, so same torque.

The fact that the centre taps are linked should not matter because they will both be at half supply voltage or floating depending on whether the coil is on or off.

Unfortunately, you would have four times the inductance so your top speed will be limited.

Re: Unipolar Steppers
October 26, 2007 02:59PM
Alternatively, just connect the centre tap to 12V and it should work. One half of the h-bridge will drive low powering the coil as it would with unipolar. The other side drives high but the centre tap is at 12V so the other half of the coil is off as it would be with unipolar.

Re: Unipolar Steppers
October 27, 2007 03:08PM
Herewith I expose my ignorance of stepping motors. I have some six-wire motors. I speculate that they could be used as either unipolar, by connecting the center-taps to ground, or as bipolar by ignoring the center-taps. True?
Re: Unipolar Steppers
October 27, 2007 03:21PM
... True ...
Re: Unipolar Steppers
October 27, 2007 04:15PM
Tried a couple different ways with both my unipolar stepper motors, without luck, and then gave up when i found the bipolar steppers i had originally ordered! They weren't much more (still under $10 a piece), and they run much better than the unipolar ones I was using... I just gotta mount then properly and compensate for the added weight, and I'm in business!

I gotta say, these boards are coming together nicely... much easier than the strip board i was working with last time!
Re: Unipolar Steppers
October 27, 2007 07:43PM
When connecting 6 wire unipolar as bipolar, to get equivalent performance you use half the coil, i.e. connect to one end and the centre tap. If you use the whole coil you need to drop the current by root 2, you get root 2 times more torque at low speeds but the torque drops of four times faster as speed increases.

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