Re: [CAD_CAM_EDM_DRO] Re: Allegro 3977SED questions

If you want to do it yourself, then maybe you want to consider something
besides the 3977 chip. I mean, it ertainly is a matter of preference, but if
you are building a controller yourself, first see what kind of requirements
you have, what kinds of voltages and currents you need to drive your motors,
and then pick your circuitry.

My first prototype controller was also based on the 3977, but after making
it and trying it out, I abandoned that design (still have a few spare 3977
chips lying around). It did not do quite what I wanted to do (a little
low-ish power), and I had a really hard time picking the heat off of it (but
maybe that's just me not being able to fit a heatsink well onto that thing).

I am not saying it's bad -- it is a really nice
everything-and-a-kitchen-sink-integrated chip, but if you want to dive in
and build something yourself, then you may want to research and see what
else is out there.

Vlad

On 12/21/05, lomcevaktumble <lomcevaktumble@...> wrote:
>
> So basically the PFD is something that is selected by me? I could
> use 0.21V or 0.6V or something in between to set it to whatever
> decay speed I want? I thought that maybe it automatically sensed a
> voltage in the circuit and then adjusted the operation on its
> own...but if its a steady state signal then I just pick the voltage
> I need to set it in that mode. Am I understanding it correctly now?
>
> Thank you also for the resistor values. I'm just trying to make this
> as simple as possible. I'll be driving bipolar 2.0Amp motors...so I
> should be able to make everything work with your suggestions and
> Allegro's values for the chip.
>
> I know there is a company out there making PCBs for this board...but
> I want to do it myself. I would like the satisfaction of figuring
> all this out! Its much more fun that way and makes me feel like I've
> accomplished something!
>
> Michael
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Steve Stallings"
> <stevesng@n...> wrote:
> >
> > The PFD (percent fast decay) pin is an input to the A3977 chip. It
> > has three possible modes depending on the voltage. The voltage can
> be
> > established using a resistor divider, but must have a 0.1 uF or
> > larger capacitor to suppress noise on the input. If the voltage is
> > less than 0.21 volts, the chip uses fast decay mode. If the
> voltage
> > is greater than 0.6 volts the chip uses slow decay mode. If the
> > voltage is between these two values the chip uses mixed decay
> mode.
> > Think of it as a "trinary" logic input signal. It is a steady
> state
> > input, not a signal that must be changed as the circuit operates.
> >
> > 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.
> >
> > Regards,
> > Steve Stallings
> > www.PMDX.com
> >
> >
> >
> >
> > --- 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!
> > >
> >
>
>
>
>
>
>
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--
Vlad's shop
http://www.krupin.net/serendipity/index.php?/categories/2-metalworking


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