RE: [CAD_CAM_EDM_DRO] Re: Allegro 3977SED questions

My 2c worth here. I recently built 4 identical 3 axis boards using these
IC's. They work quite well.. I have however, been able to destroy a chip by
hooking up 3.6mH motor and winding the current limits right up. I've also
found that I just can't get the same top speeds out of them when running in
half, quarter and eighth step modes..

PK

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of lomcevaktumble
> Sent: Thursday, 22 December 2005 1:27 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: Allegro 3977SED questions
>
>
> > The value of the current sense resistors is determined by the
> maximum
> > input voltage on the SENSE pins. This is 0.5 volts absolute
> maximum.
> > Higher voltages are more immune to noise, but will result in more
> > power dissipation in the sense resistors. Selecting 0.125 ohms
> would
> > result in 0.3125 volts at the SENSE input for 2.5 amperes of coil
> > current and 0.78125 watts maximum dissipation in the sense
> resistor.
> > This allows the use of a 1 watt resistor and keeps the sense
> voltage
> > well inside the safe zone.
> >
>
> Steve,
> Thanks so far for all your excellent help. I checked out your
> website and its clear you have a good understanding of the 3977 and
> CNC. I was very impressed!
>
> I am getting close to understanding all that I need to try and get
> this chip up and running - I'm just trying to make a simple setup
> with the minimum amount of parts that Allegro recommends.
>
> So you're recommending a .125 sense resistor...but I also notice in
> the diagram there is a capacitor. I'm assuming this is to help
> cancel out noise in the ciruit. Do I need a 0.1 uF or greater on
> this as well...or is it some other value??
>
> I feel like I should try and draw up what I plan on using where and
> seeing if you think it will work that way.
>
> Thanks again for all the help from everybody! I'm learning this
> slowly but surely!
>
> Michael
>
>
>
> >
> >
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "lomcevaktumble"
> > <lomcevaktumble@y...> wrote:
> > >
> > > I'm hoping someone can help me figure all this out. I've figured
> > out
> > > quite a bit on this chip...but I have some questions that I
> can't
> > > answer myself.
> > >
> > > I went ahead and bought myself a few Allegro 3977 SED chips.
> Most
> > of
> > > the stuff on the diagram I understand. But there are some things
> > > that I need elaboration on.
> > >
> > > There is something called Percent Fast Decay or PFD. It shows a
> > > place for Vpfd to be entered...and the info tells you the
> allowable
> > > voltage range...but the info also makes it sound like this is a
> > > voltage that comes from monitoring something. I just can't
> figure
> > > out where to connect the Vpfd so it works correctly.
> > >
> > > Also Allegro is pretty good at telling you what components you
> > > need...but there are 2 sense circuits that don't show what value
> of
> > > sense resistor or capacitor you need. Does anyone have a clue on
> > > this?
> > >
> > > http://www.allegromicro.com/sf/3977/ has the functional diagram
> > that
> > > I've been looking at.
> > >
> > > Any help, clues, suggestions much appreciated!!!
> > >
> > > Thanks!
> > >
> >
>
>
>
>
>
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@...,
> timg@...
> Moderator: pentam@... indigo_red@... davemucha@...
> [Moderators]
> URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
> 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........
> bill
> List Mom
> List Owner
>
>
> Yahoo! Groups Links
>
>
>
>