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

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, May 16, 2004 12:08 PM
Subject: Re: [CAD_CAM_EDM_DRO] Xylotex or Gecko step driver for my
Mill/Drill?


>
>
> 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
>
>
>
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