Stan corrected me on my use of the term back EMF. 
I was just referring to the inductance of the windings causing a negative spike. Actually, the lack of back EMF at start up would create in-rush current that puts the voltage regulator into current limit. When the current through the regulator is interrupted, the current through the windings then flows through the internal catch diode. This diode, I'm sure, can handle very little energy. The diode is probably intrinsic, not there by design to protect the IC.
gunrunnerjohn posted:BTW Stan, love the current probe, I wish I had one for my scope, it would be very useful. I keep looking at them, and they're just too expensive!
When I retired, I found one of these at a hamfest for around $50. They can be fragile but that one worked ok. Most resellers want to make it all on one sale, but keep looking...
Interesting Chuck, I haven't been to a hamfest in a very long time, maybe I'm missing a bet. 
I downloaded the manual for that one, I'd sure hate to have to re calibrate it! 
I see the Hantek CC-65 A DC Current Clamp 20kHz Bandwidth 20mA to 65A for reasonable prices, but the 20khz puts me off, I wouldn't see what Stan sees using that. The 50 Mhz of the Tektronix P6042 would probably be sufficient, maybe I should consider that.
The probe is the fragile part, but there are working ones out there. And I think there are newer maybe improved versions of that same model number. I actually had two of them , one with a busted but fixed probe which worked. I sold them a few years ago intending to buy a stand alone dc/ac probe but didn't.
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
gunrunnerjohn posted:I did that a few minutes ago, and it promptly destroyed my last regulator just as fast as the others.
I don't think I order any more of these, I chalk it up to a good idea gone bad.
Curious who made these parts (reputable firm or "some weirdly marked or named firm) and all from same date/lot code. Its been my experience that not all regulators of same part number from different vendors act exactly the same. The pass transistor may be beefier or there may be phantom diodes that react when they should be "dormant" etc.
We had an avionics design that worked beautifully in lab and went to production then had failures that we traced to a particular vendors IC. For whatever reason one brand just would not work over temperature and voltage levels. Manufacturer swore design and process was exactly like what they had licensed from original. Finally we just had to specify that this vendor part was disqualified for this build. All other vendor same number parts worked perfectly except this vendor. So guess like Animal Farm: "all 74xx parts are same, but some are more same than others"
I'm thinking that I just picked poorly, I should have gone for the bipolar part.
