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!
> > > >
> > >
> >
> >
> >
> >
> >
> >
> > Addresses:
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