stan2004 posted:gunrunnerjohn posted:I did order a different regulator with the same footprint to try in this board, it'll be interesting if it has the same issues with motors.
Was it a CMOS regulator (vs. a bipolar)???
Here's a scope shot of a 78L05 (bipolar) driving an MTH smoke motor during startup. This was with NO capacitor on the 78L05 output. This wild negative voltage excursion (green trace) of about -2V is from a commutation spike as I mentioned earlier. The red trace is the current into the motor. This is NOT a result of back-emf. I don't know what you said in your inquiry to Microchip/T.I. but hopefully you did NOT refer to back-emf as the culprit as you will lose credibility with an App Engineer. The 78L05 was in current-limiting in this transient capture (the entire screen is only 5 microseconds of time!).
The intrinsic protection diode structure in CMOS devices (if in fact this is what's in-play with the MCP1804) is what I believe is the issue...but a Microchip App Engineer would be able to say for sure; they should be able to tell you how much "clamping" energy can be handled.
Sure, you can spend the time to analyze clamping energy, currents, choose capacitance, blah blah blah, but why bother? If you really need LDO or the lower quiescent currents offered by CMOS technology, then that's one thing and we can get into it (yawn!). But the 317 bipolar devices are tried-and-true and probably even cheaper. Your LED module using the 317 remains the gold-standard of voltage regulation for O-gauge AC!
I wonder if this dip is what causes issues with Lionel's newer Legacy board. Especially if a motor is going bad. G
