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!
> >
>