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Solar Powered +5 and +3.3 Power Supply

This design was generated as part of a remote weather station project. One of the requirements of the design is that it have a solar-powered supply with rechargeable batteries. This design is based on a photovoltaic array available from Radio Shack called a BatterySAVER (part number 980-1045). It was designed to charge your car battery during the day. They apparently aren't very popular because you can pick them up for $15. The documentation claims an output of 1 watt but I found that at noon in Atlanta, the power was a little les than half a watt. Maybe at the equator during the autumnal equinox...

For my application, the average drain off the batteries would is much smaller than the charging current (50mA). If your application has higher average current requirements, then R1 will need to be adjusted accordingly:

R1=64mV/(50mA+Average Load Current)

Don't try to pull too much current though because the solar panel can only provide 500mW and the batteries need 40% of that. That leaves 300mW for your load.

The design is based on four functional blocks: 1) a 50mA current source driven by the solar cells, 2) three AAA NiCAD batteries with a 500mAHr capacity, 3) a 5V, 400mA switchmode regulator, and 4) a low dropout 3.3V 150mA linear regulator. If you have no need for 3.3V then U3 and C4 can be eliminated. If you only need 3.3V then U3 and C4 can be eliminated and pin 2 of U2 can be connected to ground - this will convert U2 to a 3.3V switchmode regulator.

All the capacitors on the switchmode power supply should be tantalum with C2 being connected directly to pins 6 and 8 if possible.

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