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Solar-based PSU for low-power wireless board

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Junior Member level 2
Oct 25, 2005
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I recently built a solar-based system to power a custom (low-power) wireless board. This solar system was quite simple:

Solar panel --- NCP1400-based PSU --- Wireless board

The whole system works well under direct sunlight. However, out of direct sunlight and even with strong indirect sunlight the solar panel is not capable to provide the minimum output voltage to make the NCP1400 work.

The NCP1400 is a very efficient step-up voltage regulator, capable to provide 3.3 VDC (and other outputs too) and up to 100 mA starting from an input voltage of 0.8V. Once the NCP1400 starts regulating, input voltage can go down to 0.2 V without problems.

My solar panel is a cheap one bought from eBay. It provides 2V under strong sunlight and up to 40 mA. However, voltage goes down to 0.18 V when removed from direct sunlight and then the NCP1400 stops regulating. Since I need my "solar wireless mote" to work indoor, I need it to make it work under indirect sunlight and even under artificial light if possible. For that purpose, I can imagine three different improvements:

1. Get a higher voltage solar panel (4V instead of 2V) so that it should be able to provide higher voltages under indirect sunlight. Here I can't go too far since the NCP1400 doesn't tolerate input voltages above the nominal output (3.3V) well.
2. Get a better, more efficient solar panel, capable to provide higher voltages under indirect sunlight. Any suggestion for better, more efficient solar panels is welcome.
3. Replace the NCP1400 by a more sensitive step-up regulator, as the LTC3108, capable to start regulating from voltages as low as 20 mV. Suggestions are welcome too.

Thanks guys for your comments and suggestions!!

Hi mkkeee. Please fins attached the schematics:

SRC is where the 2V solar cell is connected.
BAT, same for the supercapacitor
OUT: 3.3V regulated output


I've discarded the LTC3108 IC since it accepts a very limited input voltage range. Moreover, for optimum use of the energy stored in the supercapacitor, I want to connect the supercap to the input of the step-up converter (as showed in the precedent diagram). Using the LTC3108 would force me to place the supercap at the output, having to use a bigger supercapacitor compatible with the 3.3V output.

Another alternative is the LTC3105. This boost converter accepts a 250mV-to-5V input range but, inexplicably, all of the application examples place the supercap at the output. Maybe this has to do with the nature of the LTC3105 input. Maybe this regulator only accepts voltages from high-impedance sources as PV cells, so placing a supercapacitor in the middle may cause some kind of mismatch.

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