LOLbooster Update: Overcurrent Protection Issues
Having abandoned the track-level voltage regulation, my recent testing has centered on the short-circuit and over-current protection. These tests are fun, as they involve loading down the booster as heavily as I can (a combination of 12V bulbs, LEDs, and non-DCC motor cars can get me to about 2A, according to my multimeter). I’ve found that the present configuration is not very tolerant of inrush currents, refusing to light more than about eight 50mA bulbs at once (since my multimeter is very slow, I can only guess that represents about 4A of inrush current); five if there are any motors idling on the track (non-converted locomotives draw a not-inconsiderable current at rest). The solution has thus far been to place a simple RC circuit on the current sense line to add a kind of filter/delay. My calculations suggested a 50–100nF cap and a 100ohm resistor should do, but I’ve also been experimenting with a 10uF cap as well (because I had one handy).
The results have been mixed. I haven’t been able to fit a proper heatsink to the LMD18200, because the breakout board that I’m using has no provision for fitting anything that’s very large. So, running a bunch of lights and trains at 2A for a few minutes gets it nice and warm. And by warm I mean hot (but not overheating). And when it’s nice a warm, something very weird happens. When it is cool, putting a quarter on the tracks under any conditions immediately trips the over-current circuit. When it is hot, this does not happen. Instead, if there is a considerable current being drawn, the quarter causes some kind of oscillation, the source of which I haven’t been able to identify, that makes everything behave quite odd (bulbs dim, motors stop, but still buzz), and reduces the current consumption to about 100 mA less than before the quarter was placed on the track. Needless to say, this is no good. Moreover, once hot, under normal loading conditions, the booster will randomly shut down because of phantom short circuits. Also no good.
Since the current sense line on the LMD18200 has a very high output impedance, and the RC circuit I’m using as a buffer probably does not, I’m going to guess that some kind of impedance mismatch is the issue, and stick a voltage buffer between the LMD18200 and the rest of the overcurrent protection circuitry and hope that fixes the issue. Tests to resume tonight, with luck.