# 4.2 - Uni-polar stepper motors

A uni-polar stepper motor looks like this:

The only difference is that each of the coils now has 3 wires and will normally there have 6 leads. The new wire on each coil is called a ‘centre tap’ and is connected to the middle of the coil. If you have no documentation on your motor then use your ohmmeter to check the connections. By checking for infinite resistance then you should be able to identify the leads that are for one coil, and the other leads for the other coil. Given a group of 3 leads you can tell which is the centre tap because, for coil 1, the resistance between ‘1a’ and the centre tap will be same as that between the centre tap and ‘1b’. The resistance between ‘1a’ and ‘1b’ will be double this value. NB the resistance tend to be very low, a few ohms, so you will need to select the appropriate resistance scale on your meter.

# Uni-polar stepper motor driver

As mentioned earlier: you can drive a uni-polar motor ‘as if’ it was a bi-ploar motor. To do this you just ignore the centre tap and then use the other two leads per coil as if it was a bi-polar.

Otherwise: you want to use the centre tap and, assuming you connect it to ground, then you will need two switches to dictate which direction the current flows through the coil.

Note that this mode of operation means that you are only using half of the coil in each direction. This will mean the ‘half coil’ only has half of the total resistance. Using Volts = Amps x Resistance (and assuming your supply voltage is the same) then if the resistance is halved then the current drain has doubled.

So why would you choose uni-polar over bi-polar if it requires twice as much current?

Compared to the bi-polar H-Bridge driver, which requires ‘4 switches; per coil then the uni-polar circuit only needs ‘2 switches’ per coil. So less electronics!

The price you pay is that you may be using twice as much current and because you are using half of the coil at a time then you may only get half the torque. Despite its name (uni versus bi) it sounds as if it is less capable but remember you can always use a uni-polar motor as if it was a bi-polar by ignoring the centre tap. So a uni-polar can be thought of as a bi-polar with extra choices.