Re: [CAD_CAM_EDM_DRO] Uni-polar or bi-polar, which??

At 05:14 PM 4/2/2007, you wrote:

>Hi,
>
>I have found uni-polar and bi-polar stepper driver boards.
>
>What's the difference and why is one better than the other.

You don't ask the easy ones, do you?

Simple answer: Bipolar drivers run current into the coils both
ways. Unipolar drivers run the current one way only.

In general, a unipolar driver looks for either a tapped or a two coil
per pole motor with the tap going to the positive voltage. The
outputs are pulled to ground.

A bipolar driver usually pulls either side up or down and doesn't
want a tap connected to anything.

Now about current limiting and choppers. If you have a 2 Volt
stepper and run it on 2 Volts, it will stop having enough torque to
turn the motor at a low speed. Drive it with a current source and it
is MUCH faster. You can generate the current source many ways: A
big series resistor will help, but you will be able to heat the room
with it. Since the motor is an inductor, rapidly switching the
voltage (like a computer power supply) will allow the current to be
limited without the room heater.


David G. LeVine
Nashua, NH 03060


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