You may have noticed a frothy amount of progress with the next version of #Yapolamp in June and July this year, and then a big pause. Summer holidays? No. Leaky circuit. When I tested the "full on" run duration of the test setup, which uses a 3F capacitor pair (in series to achieve 5V handling), I also tested the "always on" mode, where we put the microcontroller to sleep between very short, spaced out pulses that allow you to find the torch in the dark without depleting the capacitors significantly.
The problem was that I couldn't get 24h from this, even though in theory it was using waaaay less power, so I dug in a little deeper. It appears that my carefully thought-out constant current charging circuit, with anti-backfeed diode (to avoid zapped tongues if the charging connector ends up in someone's mouth...) was discharging my supercaps at a noticeable rate. In the following diagram from the schematic, you'll have to forgive the error in placing the caps in a 1S2P arrangement - we needed at least 2S for these supercaps to take 5V. Also, J4 and J11 would be closed/connected and J7 would be open/disconnected. They are test points.
So where's the leak? I had my suspicions but it took a little while to be sure. It's the Schottky.
Everyone will tell you about Schottky reverse leakage current but I didn't hear them or blithely ignored any mention of it. In the case of the diodes I was using, the ON Semi MBRA210LT3G, this is what is claimed:
- A better Schottky - how hard could it be to find one?
- A normal diode
- Some other anti-backfeed device, akin to the MOSFET-in-reverse protection against reverse polarity
So, I couldn't find anything simple as an ideal diode that wasn't a bit expensive for this application (rail to rail opamp and a PMOS perhaps). A normal diode would be fine except that it means I can't charge the capacitors to as close to the 5V supply. This is annoying because I heard the higher voltage you go on a capacitor the more energy you store... But seriously, we can keep this as a backup version. I tried a normal diode and it did a grand job at preventing reverse current leakage. So, back to Schottky's.
One of the sad things about the parametric searches is that there is rarely a checkbox for "reverse current leakage at 1.8-4.8V between 10C and 25C". I noticed that you can have decently low forward voltage (tick!) but poor leakage (untick!). Or you can have "low leakage" diodes that have nA Ir (tick!) with Vf in the 1V range (untick!). Not to mention needing current carrying capacity. I could put a couple in parallel for my attempted 500mA charging current but I'm loathed to buy three or more for every torch, so that rules out another subset of parts.
I think I now have a decent contender: the ON Semi MBR0520LT. Here are the claims:
5-8uA reverse leakage current (tick!)
And 0.5A max current, so we may use a pair in parallel for our charging at half that max rate. Or perhaps charge more slowly.
Anyway, I'm off to update the board and maybe one day soon we'll have a supercapacitor version of Yapolamp on the go!