Re: Chopper circuit - follow up to motor woes

Steve,

Took a look at your ckt.

(1) Make sure you parallel the outputs exactly as the mfg recommends.
Bipolar transistors to share current well when paralleled. That is
because the transistor that is ever so slightly warmer will conduct
current better, making it even hotter, causing it to carry even more
current until it destructs. Notice the maximum rated current set by
the mfg is 3.5A, not 5A as one would expect for that reason.

(2) Re-think the value of your current sense resistors. 3.5A squared
times .51 ohm is 6.25W. That's a big, hot resistor. Use non-inductive
(no wirewound) resistors.

(3) The 8 return path diodes should be fast recovery (<100 nS), high
efficiency (<1V @ 3.5A) rectifiers. To keep forward losses low, use
6A, 100V rated devices.

(4) Keep your board layout tight, particularly in the power
distribution area. That means short wide traces, "copper-pours" being
best. Return the ground side of your sense resistors, L298 ground
pins and supply by-pass caps to as close to a single point as
possible (<0.25" apart). That is important. Use double-sided board
with plated-thru holes for best results. Do not even think of wire-
wrapping (bread-boarding) the circuit.

(5) Make provisions for serious heatsinking. You will need it. Use
the "MultiWatt" 15-pin package.

(6) Do not run the drive with more than 36V to 40V supply. Ringing
transients on the supply will kept below 46V that way.

Good luck with your project.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Steve Blackmore <steve@p...> wrote:
> Hi All - I've sent the snail motors back and am going to go the
> Bipolar/Chopper route. I've drawn a circuit using an L297 and two
> L298's but am not sure if I got it right. Before I go any further
can
> some kind soul peruse the following link and tell me if the
> schematic's right or not. It's the paralleling bit I'm really unsure
> about.
>
> http://www.pilotltd.net/chopper2.jpg
>
> Thanks in advance!
>
> I've not got the motors yet, and I'm hopefully waiting for a reply
> from the link that Paul sent.
>
>
> Steve Blackmore
> --
> Pilot Consultancy Limited ***** Computer Consultants
> Telephone 01744 752256 Mobile 07831 768478
> Email: steve@p... WWW: http://www.pilotltd.net