Computing stuff tied to the physical world

Tiny solar cell – part 2

In Hardware on Apr 28, 2012 at 00:01

The tiny solar cell chip presented a couple of days ago has been doing some indoor sun-bathing:

DSC 3073

I’ve left it alone for some 3 days, just to give it a chance to charge up the 0.47 F supercap as far as possible. The voltage after all that time (partly sunny on most days) is now only just over 2.78 V, so this isn’t really going to work indoor, I’m afraid. Nor outside during the winter, probably – it’s just too weak.

The other solar cell I tried is also a very small one, rated at about 5V but only 1 or 2 mA, IIRC:

DSC 3083

It’s currently in the shadow, but during these same 3 days it has had its share of sunlight (still indoor, and again behind double-glazing). Much better: about 4.75V on the second day, and unchanged since then.

This might actually do the trick. I’ll wait for another experiment to finish and will then hook up the JeeNode running my radioBlip sketch to see how it goes.

  1. Looking at the datasheet for that Clare CPC1824, it looks like its output current goes to zero at light intensities less than 1000 lux. This implies its leakage (dark) current is 1/5th of its rated 100 uA output at 6000 lux, or 20 uA. That’s awful. It implies that (in the absence of a blocking diode) it needs an hour of full sunlight to make up for charge lost by one hour in the dark.

    • (edit, after coffee: that would we one hour of full sunlight needed to make up dark current lost after five hours in the dark)

  2. Solar cells are pretty cheap now. For under US$4 shipped(!), you can get a 6×6 cm cell, claims 4.3V at 81 mA.

  3. hi JC – – i have this one at home the 6V 120mA Solar Panel Power Cell Charger – – if you like i could send it to you by mail

  4. Hi JC, duplicating your setup in the first picture, I charged a 10 µF to AIO3 level 1023 in 15 seconds. My chip is at 45° South, though. The supercap charges slower, but it ticks up 1/1023th every 20 seconds or so when exposed to full sunlight. Only difference: I did not put a diode in the circuit. As long as Vcap<Vsolar, that will work ;-) You probably have the math to show I did something wrong…

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