Computing stuff tied to the physical world

Tiny solar cell

In Hardware on Apr 25, 2012 at 00:01

Got a tip from Lennart Herlaar a long time ago about a tiny CPC1824 solar cell from Clare with 4V output:

CPC1832N sml

It’s packaged as a SOIC-16 chip, so clearly the light collecting capabilities of this thing will be limited. But with all this ultra-low power stuff going on here at JeeLabs, I thought I’d give it a whirl anyway. It’s trivial to hook up:

Screen Shot 2012 04 22 at 15 27 51

In bright sunlight, you get over 4V with a 100 µA short-circuit current according to the datasheet.

I added a BAT34 Schottky diode in series (which has a low voltage drop) and placed it all on a little breadboard together with a 0.47 F supercap – the solar chip is mounted on a little SOIC breakout board:

DSC 3069

The initial voltage was under half a volt, but rising (very) slowly and steadily while exposed to light.

Let’s just leave this thing exposed to light near a south-facing window for a week or so, eh?

  1. There is something great about not seeing anymore wires…nothing going to a ‘battery.’ Just a solar source and a big cap! Staying tuned!

  2. Interesting. Would you be prepared to do the same test while the cell is installed in a place where it has only access to artificial light? Given the size of it and the fact it is connected directly on the circuit, a lot of typical applications will not allow to provide access to direct sun light.

    • One step at a time, please – first baseline will be to see it will work in bright daylight. It’s not a given that it can, given the fact that the energy-collecting area is less than half a finger nail!

  3. Series of BPW34 also works well.

  4. @buslik, it seems BPW34 has a much lower voltage of 350mV. So you need to set a few in series as you basically lose that voltage on the Ge protection diode.

    Now maybe jcw could explain why you need this diode ? Wait, I’ll try to remember my old electronics courses. IIRC, the solar cell is actually a diode, and the light generates some potential difference (i.e. voltage) in the reverse zone. This means that WHEN (not IF, as there are clouds ;-) the voltage would drop on the photocell, any voltage above around 0.7 V just flows thru the cell … and make it burn. Not sure this is correct though.

    • Sounds about right. I doubt any damage would happen to the cell, it might warm up a fraction of a degree, but any reverse leakage is energy being lost from the supercap. Energy you have far better uses for :-)

    • A quick check told me that the reverse leakage from connecting 5V over a solar cell is 10 µA, so on a long winter night too much energy might end up drainign back through the solar cell without the diode.

      More testing needed. A diode can have leakage as well, and the voltage drop over it also “wastes”energy. A perfect diode (in the form of a MOSFET) might be an option. As with the ultra-low power AC mains experiments, my goal is to first establish some sort of working baseline – and then try and optimize for efficiency, parts count, cost, etc. With this level of nano-power, and while sailing so close to the edge, you can’t really predict where the trouble-spots are going to be, IMO.

  5. There are quite cheap garden solar lamps out there, just grab one and use the solar cell. I think those would be better candidate for such project.

    I also wonder how will the AA power board behave with a Solar Cell, would it pump-up the voltage to 3V

    • Agree on cheap solar cells, but I’m interested in trying several options. In some cases, size will matter.

      As for the AA board – its quiescent current draw may well(and sub-100% efficiency) may well lose too much of at these tiny amounts of energy.

      Lots of things to try out still. Rome wasn’t built in a day either, you know!

  6. @mharizanov: the stuff in garden lamps, like yesterday’s “TD – Solar light”, is bulky and there are often just chips of solar cells. While not mass-produceable on an SMD production line like the CPC1824, DealExtreme item 23805 has a complete tiny setup with solar cell, diode, rechargeable (3.7V) battery, no extra pumping circuitry. Taking off the LEDs and connecting a JeeNode is about the cheapest way to solar power your stuff.

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