# Unipolar and Bipolar Stepper motors

I would like to universal stepper motor driver circuit. Is it possible for one circuit to be able to drive unipolar and bipolar motors with only a firmware change or do they require much different circuits? I know that bipolar motors use 4 wires and unipolar uses 6 so if i use a connector with 6 pins would it be possible?

## 2 Answers

You can drive a unipolar stepper with a bipolar stepper circuit-- just ignore the center taps. In fact, it should be more efficient to drive a unipolar stepper this way. The only reason (that I know of) to drive a unipolar stepper in the traditional way is to simplify the hardware; if you've already got a pair of H-bridges, you may as well use them.

• i almost said 'ignore the taps' but then freaked out that maybe the field sequence inside the motor would be wrong, but i think you are actually right. so +1 for that! Oct 6, 2010 at 3:38
• This will not work on motors where the center taps are connected together (They do exist). Oct 6, 2010 at 9:37

Sometimes.

There are actually three stepper motor topologies. "Bipolar", "Unipolar", and "Universal" (the parlance I have always heard, anyways).

Bipolar motors are 4-leaded, and have two field separate field coils.
Unipolar motors are 5 leaded, and effectively have 4 field coils, with one end of each connected together (The "Common" lead) See Here.
Universal motors have 6 leads, and are effectively a center-tapped bipolar motor.

A 5-leaded unipolar motor cannot be driven with a bipolar stepper motor driver. A bipolar driver can only drive a universal or bipolar motor.

• the usual way to drive a 5-wire motor is to put the common at +V and then use open collector drivers to pull the coil ends low one at a time in sequence, isn't it? So if you have a bipolar driver that is a pair of H-bridges, it has four low-side drive transistors; where's the problem? Oct 6, 2010 at 21:59
• Hmmm. I suppose if you're doing full-stepping, and the unused winding is off and floating, it might work. However, if you're doing microstepping, or the driver clamps the unused winding, all sorts of weird stuff would happen. Basically, you would always be driving 1 1/2 windings, and possibly not in the right direction. Oct 7, 2010 at 10:55
• ok, now what if you're half-stepping a regular bipolar motor; assume each coil is driven identically (except in phase, of course), and think about the voltage at the exact center of the windings. At some moments you have one coil hi-Z and one driven, other moments both coils are driven. With one coil off (hi/floating/etc) the floating coil is effectively out of the circuit. No prob. With both coils driven, no matter which direction, the voltage at the centers of the coils should be the same, and it wouldn't matter if you wired them together. Oct 7, 2010 at 11:23