RE: [CAD_CAM_EDM_DRO] Re: Allegro 3977SED questions

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Steve Stallings
> Sent: Thursday, 22 December 2005 10:46 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: Allegro 3977SED questions
>
> Paul,
>
> The A3977 chip depends on the normal chopping action to prevent
> excess current from harming the chip. A short circuit will kill the
> chip because the chopping circuit is not fast enough to prevent a
> deadly current spike. This killer spike can be less than 1
> microsecond if the inductance is low enough. Normally the motor
> inductance will limit the current rise time and give the chopping
> circuit time to react. A short circuit of course has almost no
> inductance. That being said, 3.6 mH inductance in within the normal
> operating range and we have not seen problems with such motors.

Hmm might have been a voltage spike. Many thanks for that info.

>
> The A3977 can also be easily damaged by overvoltage. While we usually
> assume that chips are conservatively rated the voltage rating on the
> A3977 is pretty tight. Allegro specifies 35 volts maximum and in our
> testing the "smoke comes out" pretty dependably at 36.5 volts. Given
> that the typical AC mains voltage can vary 10% from nominal, one
> should be careful not to push the supply voltage too high without
> overvoltage protection.
>
> Having different top speeds for different step modes (half, quarter,
> and eight) is common when top speed is limited by motor resonance.
> Eight step would be expected to allow faster operation in this case.
>

Hmm, I see the reverse... fastest is full step, speeds drop as microstepping
increases..

> Another issue that will affect top speed differently for different
> step modes is the ability of the driving system to generate
> accurately timed step pulses. A slow computer trying to generate
> step pulses in software will have more difficulty with eight step
> mode than for half step mode for the same motor RPM. This is because
> it has to generate four times as many pulses in the same time frame
> while not distorting the pulse to pulse timing. It may even reach its
> limit and not be able to generate the pulses as fast as required. I
> have seen this happen with 20 TPI lead screws and eight step mode on
> a 1 GHz Pentium computer.

Another good data point. I'll get the DSO out and look for glitches...

Thanks Again..

PK

>
> Regards,
> Steve Stallings
> www.PMDX.com
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Paul Kelly" <tech@c...>
> wrote:
> >
> > 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
>
>
>
>
>
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