Re: Allegro 3977SED questions

>
> Vlad -
> The 3977 seems to fit what I need it to do pretty well. The
> steppers I'm looking at are in the 1.7 - 2.0 Amp range. The machine
> I'll be building won't need very heavy stepper motors.
> I picked the 3977 because it was recommended and its seems pretty
> simple. I'm not an expert with electronics and I need something that
> can be slapped together by me. I also need something fairly quickly.
> Do you have any recommendations for something that might fit those
> parameters? I'm always open to looking at something else. And of
> course if I ever build a machine that needs more powerful motors
> I'll have to leave the 3977 chip for something else.
>
> Thanks for all the tips and info everybody.
>
> Michael
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Vlad Krupin <vlad.cnc@g...>
> wrote:
> >
> > 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@y...> 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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