Schmidtn:
Must have been a problem with Amps... just weird that it would start off working and then quit like that.
Not weird at all - that's the thermal overload protection on the L293 kicking in. Piggybacking another L293 on helped to eliminate the issue because the current draw was being shared (however, while this is working now, I'm not sure how long it will work in the long run - the output drivers on the L293 are (I believe) bipolar NPN darlingtons, which don't typically take well to such paralleling (whereas with mosfets you're OK).
Did you measure the current needs of the motors? If you note in the "mounting instructions" in the L293 datasheet, it reads:
The Rthj-amp of the L293 can be reduced by soldering the GND pins to a suitable copper area of the printed
circuit board or to an external heatsink.
Which pins these are depend on the package (ref the datasheet for details). Basically, what the wording means is that if you can attach a heatsink (see the datasheet for an example of a clip-on heatsink) to these pins (or provide a large enough copper pour for them on the PCB - the size of which is specified in the datasheet - best size being 35mm on a side for maximum heat dissipation), you'll be able to run the chip to its specs (600 mA for the L293D, which the Adafruit shield uses).
Now - looking at the pics of the Adafruit Motorshield here:
I see a couple of issues with the board:
- The copper pour doesn't look large enough to me (difficult to say without a scale, though - I like it that SparkFun does this in their pics).
- The copper pour is on the -bottom- of the motorshield, where the heat generated would be trapped, at best.
So - if you measure your running current (and loaded current), and find that it is below 600 mA (per motor), then all you may need to do is add one of those clip-on heatsinks to the top of the chip. Maybe add a tiny muffin fan too, if you want.
Or leave it as-is; I can't guarantee that the L293s will survive for a long time, but maybe they will be OK. Worst case, you fry it and have to implement another solution - happens to everyone sooner or later...
