Re: Xylotex or Gecko step driver for my Mill/Drill?

---
I tried to run a small lathe with a very popular, at the time, 5amp
bipolar board with very dissappointing results. I used 6 wire
(center to end tapped) surplus motors recommended and purchased from
the supplier of the drive kit.
I gave up with it for several years than changed to an allegro
SLA7044 based unipolar board and the difference was astounding in
both smoothness and speed.
I cannot understand why some of the well intentioned advice I have
read goes against what I have actually experienced. Could it be that
many of the prejudices are based on the crude boards that were the
only ones affordable for the hobbyist at that time?

CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

>
>
> Roy J. Tellason wrote:
>
> >On Saturday 15 May 2004 11:05 pm, Jon Elson wrote:
> >
> >
> >
> >>>but I ruled that one out because it's a unipolar only board.
> >>>
> >>>
> >
> >
> >
> >>Good move, as most unipolar drives are very poor performers at

higher

> >>speeds
> >>
> >>
> >
> >Why is that?
> >
> >Would it have something to do with a bipolar setup energizing

coils in the

> >reverse direction rather than just turning them off?
> >
> >
> >
> Yup, that is basically the problem. With the typical unipolar

drive,

> the power
> source is connected to the center tap, and transistors ground one

end or the

> other of the winding. When you ground one end, there is still

circulating

> current in the other half, fighting the field you are trying to

build in the

> newly powered winding. This energy has to go somewhere, and it

ends

> up being burned off in some diode-resistor arrangement. It is a

hard

> topology to harvest this energy constructively. If there is no

resistor,

> the current in the shut-off winding dies very slowly. If there is

a

> resistor,
> then it gets quite hot at high step rates.
>
> With a bipolar drive, the power source is applied across the whole

winding,

> and is reversed to reverse the field. The field already in the

winding

> opposes the power source at the moment the transistors are

switched,

> and the reversed voltage across the winding collapses the existing

field

> and quickly builds it again in the opposite direction. The energy

mostly

> ends up back in the power supply, so there are no big resistors to

burn

> off the left-over field.
>
> There are ways around this, but they are generally more

complicated than

> the bipolar drive scheme. The only advantage to unipolar is it

saves a

> 4 transistors per motor.
>
> Jon