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

Harvey,
Yes you are correct. I was referring to the current limit type Ron was
mentioning versus a bipolar design. I'm SURE the Xylotex uses current sense
resistors in the same way as the unipolar chip. Can't vouch for Geckos
though. Never seen one up close.

Regards,
Dave Rigotti
HobbyCNC.com

----- Original Message -----
From: "Harvey White" <madyn@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, May 16, 2004 1:42 PM
Subject: Re: [CAD_CAM_EDM_DRO] Xylotex or Gecko step driver for my
Mill/Drill?


> On Sun, 16 May 2004 12:29:25 -0400, you wrote:
>
> >Jon,
> > This chip doesn't use resistors!
> >http://www.allegromicro.com/sf/97060/
> >Now what?
>
> Looks like it needs two sense resistors to ground to sense motor
> current. Typical values are 0.5 ohms each. Motor current is
> controlled by a variable voltage into the sense input, I think. The
> internal DACs allow for up to 16th step operations, which is why there
> are DACs in there, and not a direct run from the sense input to the
> comparator.
>
> Harvey
>
>
>
> >
> >Regards,
> >Dave Rigotti
> >HobbyCNC.com
> >
> >----- 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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sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.
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DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
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> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
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