Progress is being made.
The boards came back, and I could Dremel out the bad connection. Unfortunately, I initially created the footprint for the main power screw terminal with too small (default) through holes. I updated the footprint before I sent the boards off, but apparently forgot to merge that update into the actual board -- something KiCad doesn't do on its own. Thus, I couldn't fit the power connector into the holes without shaving down the pins.
Everything else so far seems to work, so that's good. Except I had assumed 3mm resistors all around, but the parts box only contains 6mm parts for some values I'm using, so I have to inventory those and make a Digi-Key order for the final version.
Another thing I've noticed is that, when running with 16V supply, the electrolytic capacitors on top of the OpenCM-9.04 run very hot. My guess is that's not actually ripple current, but heat that goes through the vias/leads from the power regulators, which are linear. The maximum input voltage for the regulator is 20V, but the input capacitor is only rated at 16V, and the output is rated at 6V. Given that a 4S battery maxes out at 16.8V, I think that's quite marginal! (Similarly, MX servos are rated for "4S input" but they declare over-voltage if the voltage goes over 16V.)
The OpenCM draws a fair bit of current as these things go, so using a linear regulator to drop from 16V down to 3.3V (as well as 5V) generates a lot of heat! Maybe it's their way of feeling like MechWarriors :-)
Options on the table:
- Let it run like this; heat just makes the electrons flow faster, and nothing should live forever!
- Drop an adjustable linear regulator into the board, pushing perhaps 7.5V into the OpenCM board ++ pins. (Power for the DXL already runs through fat separate traces.)
- Drop a switching regulator onto the board, similarly pusing perhaps 7.5V into the OpenCM board, with less wasted current.
Most of the time was spent clearing up the workspace so I could actually solder up the board, though. After all was said and done, four full trash bags of junk and old finished or failed projects made it towards the landfill and metal recycling (!)