RE: [CAD_CAM_EDM_DRO] Higher power from L298 etc.

Hi Tony
I have four controllers (Avistep) that I bought surplus. Each controller has
four 298's and only one 297. Claimed current per phase is 4A. I could never
test this as none of my setups is that power hungry. My point is that it is
possible to get 4A / phase with 4 298 and one 297. Unfortunately I do not
have the circuit diagrams for the controllers only the physical controllers.

BTW : Has anyone ever heard of Avistep controllers? Net searches don't seem
to be able to find any references to these controllers.

Cheers
Coert

-----Original Message-----
From: Tony Jeffree [mailto:tony@...]


I was musing about the problem of getting better than 2A/phase out of
L298s (or better than 2.5A/phase out of Allegro 3977s). The datasheet
for the L298 suggests that it is possible to use 2 L298s, and deliver
3A/phase, by paralelling the two driver stages in one 298 to drive
one motor coil, and paralelling the other driver stages to drive the
second coil. Presumably, you can't get the full 4A/phase when doing
this because of imperfect matching of the driver characteristics,
which would cause one driver to take more of the current, and the
current sense operates on the sum of the currents through each
driver, leading to the danger of thermal runaway & release of the
magic black smoke.

It occurred to me that there is an alternative approach that could
work with 8-wire motors; instead of wiring the L298 outputs in
paralell, you use one half of a 298 to drive the A coil, and the
other to drive the B coil; similarly, use one half of the second 298
to drive the A' coil, and the other half to drive the B' coil.
Unfortunately, you would have to double up on L297's as well, in
order to control all four coils independently; the step-and-direction
signals into these would simply be wired in paralell.

All four drivers should then be able to operate up to their full
2A/phase, as they would be generating their own independent current
sense signals back to their respective 297s. In effect, you would
then have the same result as driving the 8-wire motor in bipolar
paralell configuration, and would set the current limit on the
drivers to 1/2 the value you would normally use for bipolar paralell.

Presumably this same approach could also be used with other driver
chipsets such as the Allegro 3977, etc.

So this would seem allow driving up to 4A/phase into 8-wire motors
using two sets of 297/298, or 5A/phase using two 3977s.

Can anyone see a problem with this approach (other than the cost of
an extra chipset & ancilliary components per motor, and the
limitation of using 8-wire motors)?

Regards,
Tony



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