[AustinSchuh]

I guess I forgot to explicitly say one design choice. I'm trying to get the most bang out for the buck. I'd like to make ~10+ of these, and if they cost $35 per board in parts...

Originally Posted by Gdeaver

The motor controller chip does not have current monitoring. It's ashame to have all that CPU power and nothing for it to measure. You might want to consider the Freescale MC33887. It has current monitoring and some additional protections.

Sounds like a better chip than what I had chosen. It's more expensive, but you are probably right that the current sensing is helpful. I should then be able to sense if the motor is hitting something, and do better diagnostics of how it's working. And it'll have a pretty good safety factor for the current that my motor will stall at. Your point about CPU power hits home too. I'll probably implement a PID loop on each controller, along with higher level logic so that the master controller on the bus can do some pretty cool things.

Does anyone know how to solder a 20-hsop package that the MC33887 comes in? It has a large pad underneath it, which makes it harder. Do I put solder on the pad, place the chip on the pad, and then warm it up with the hot air until it melts? The naive way is to just apply lots of heat until everything is hot enough, and then it should be soldered on, though it might not work afterwords...

Originally Posted by Gdeaver

You could separate the controller parts from the drive. Hope fully one board will survive instead of frying the whole thing . After you get it working the boards could be stacked.

Interesting idea. Would standard .1" headers stand the 5 amp current of the motor? That will also help me make the board smaller when I add the MC33887. The MC33887 has a larger footprint than the DIP footprint of the other chip.

Originally Posted by EricVanWyk

I'm having a hard time reviewing it because not all the part numbers are shown. Capacitors are notorious for being a pain in the butt for reviews - their rated capacitance only tells you 20% of what you need to know. Also, more capacitance is more better, especially when you are prototyping.

Part of the problem is that I was under the impression that all capacitors are created equal, so I thought I'd worry about that later. Thanks for the heads up. I'll work on figuring that out.

Originally Posted by EricVanWyk

In the world of current sharing, 1 + 1 hardly ever equals 2. One leg will end up taking slightly more of the load than the other, so 1+1 might equal 1.8 or 1.9. This uses a BJT (Darlington) output, which doesn't share current as well as FETs do.

Upgrading to the MC33887 will fix that. Yet another reason to use the better controller.

Originally Posted by EricVanWyk

Page 22/32 of http://www.nxp.com/acrobat_download/...01_02_03_3.pdf indicates that you can handle up to 5.5V on an input - a standard margin. You should be fine with 5V inputs - the encoder will be fine. Driving 5V outputs is another issue.

Thanks! I feel better about reading 5 volt inputs now. I read the spec sheets, and at least for the old motor controller I was looking at, 3.3v registered as high, so "5V" outputs aren't a problem.

Originally Posted by EricVanWyk

Why not use CAN or I2C? No need to go custom here.

I looked around online for about 2 weeks looking for an inexpensive micro-controller that would support CAN. I couldn't find any ones that were below $10, if I remember correctly. Any suggestions? $10 x ~10+ adds up a lot faster than $2-3 x ~10+. And I'll learn a lot implementing the bus

Originally Posted by Al Skierkiewicz

Also check the requirements for the bypass caps on the LM1117. High output capacitance near the chip might be detrimental. The spec sheet shows a 22mmf. Something larger may cause current foldback when starting.
Let us know how it works.

Good to know. I'll go back and reread the data sheet carefully.

Thanks for all the feedback Al, Eric, and Gdeaver! Now time to act on it and update the schematic, build a better parts list, and come back to do it again.