re:Re: Chopper max voltage?

Mike writes:

>Dan Mauch claims the L298 won't run over 18 volts, even with a big
>heatsink, without getting too hot if you're running anywhere close
>to 2amps with it. He sells a L298 based board so I assume he'd know?

<snip>How much current are you running at 35 volts?

Mike,

I have 3axis 297/298 boards running at 35v ,2A since 1993. The
heatsink is a 1.062 x 3/4 alum. extrusion with 5 fins .062 thick.The
thick part is about .120 and the 298's screw to its face. ascii art
below:
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MOST IMPORTANT(IMO), the heatsink is screwed directly to the PC bd
and sits ON the ground plane, effectively making the H/S part of the
gnd plane:Also removing mechanical stress from the 298 leads. Shields
for the output wires are connected TO the 298 securing screw.

The 297/298 combo is VERY SENSITIVE to board layout!!!
The 297 chopping will be erratic and often noisy with poor bd layout.
(This was probably the problem described in an earlier thread.)
The 298 will self-destruct(sometimes over a period of time) at these
higher volts/amps combos without GOOD trace design.

Tom Hopkins at SGS-Thompson in Schaumberg,IL is knowledgeable and
personable. VERY familiar with the 297,298 and their eccentricities.
Wrote many of the articles in the SGS power products design guide.

Best book I've found so far on layout(and wiring) issues (relating to
our mixed mode usage): The Circuit Designers Companion by Tim
Williams. The art of electronics and the ARRL handbook(ask any radio
guy) are also good.

I should mention that my boards include modified half-step(where the
current is increased by 1.4 when only one phase is active, to "smooth
out" the motor and reduce resonance. Also included auto-idle.

In the last 6 months I have set the auto-idle time VERY short and
increased the current during single phase half-step to get an AVERAGE
current of 2A. No problems so far, but this is (technically)over
driving the 298.

Hope this helps.

Ballendo

P.S. No, I do not currently want to get in the business of selling
boards. Paul at Alzanti(do a web search) and AMSI both have good,
reliable hi-current 298 designs. As usual,YMMV. And,IHNCWTPOP.

P.S.S. Almost forgot, the heatsink in turn is screwed to the .062
alum box which has a forced air cooling fan. This effectively
increases the radiating area of the h/s, as part of the case aluminum
is now part of the cooling scheme.