gunrunnerjohn posted:...The resistor does the trick, but I'm open to a more elegant solution, especially if it costs significantly less. Let's talk about the constant current and limiting the charge to 5V maximum. How were you visualizing accomplishing that? Is there a simple trick to add to the LM317 circuit to cutoff at a specific voltage? I was thinking along the lines of a Zener diode circuit.
Well, as charging circuit go, I don't think you can get cheaper than a resistor! I'd go with that if it does the trick.
If you want to talk about constant current charging, I'm still unclear on where the 100 mA charging current comes from. Is this from the drooping of the half-wave input with "only" a 150uF cap? As shown, with the 7810 and 22 ohm resistor, on power-up into an empty supercap you're drawing ~500 mA (10V / 22 ohms) albeit dropping rapidly...but still attempting to source over 100 mA half a minute later (time constant R x C = 22 ohms x 1 Farad = 22 seconds). So if you've done-the-math and the choice of "only" 150uF purposely starves the 78xx regulator which then limits the charging current, then that's a clever current limiting method! Sort of like the trick of inserting a 5 cent diode into the AC path to dim passenger car lamps on command-voltage tracks. Of course this applies to TMCC which as I understand it only operates on AC track voltage (vs. PS2/3 where you might have DC on the track).
So if the numbers work, then it seems you could have a 7805 charging a single 5V supercap with a single resistor.
Getting back to the constant current. I'd still like to hear the duty-cycle requirement on how many interruptions must be handled, how quickly, how often, whatever. I have little experience running TMCC engines so don't have a feel for how often the 9V battery is called into service or what the expectations are. A constant-current 100 mA into a 1 F cap has a charge rate of "only" 0.1 V/sec. So charging from 0 to 5V would take 50 seconds; I don't have a sense of whether this is OK. Obviously once fully charged, I suspect the 9V battery applies to conventional operators pressing DIRECTION that get the long dropout while command operators might get fraction-of-a-sec dropouts?
