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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>OFF Topic POSTS: General Machining
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>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.
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>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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