Solar Piston V2

Solar Piston V2

The continuing story of battery charging circuitry. V2 is, of course, an improvement on V1. The new version uses fewer parts, and uses less power that the old version. Here’s the Schematic, explication below.

The Seiko Instruments S-808 OUT pin is ON and connects to GND when voltage on its Vdd pin is under the threshold (in this case, 3.5V) When it is above the threshold (3.5V + hysteresis of 0.175V) the OUT pin is OFF and in high Z (a.k.a Not Connected To Anything). So, as the solar panel is charging the capacitor, the OUT pin pulls the gate of the N-channel MOSFET (BS170) to GND, turning it off and not allowing a path for the cap to discharge. When the charge on the capacitor gets to 3.67V (threshold + hysteresis), the OUT pin on the S-808 turns OFF, the gate of the N-MOSFET feels +V through the 10M resistor and turns on, connecting the ground of the capacitor with the ground of the battery and the capacitor discharges into the battery. As the capacitor discharges, the voltage drops, and when it reaches 3.5V, the OUT pin turns ON again, disconnecting the capacitor and we take another spin on the wheel.

Some interesting things to note:

Internally to the S-808 the OUT pin is an open drain MOS device. I could connect the battery ground directly to this pin, and use it to make the capacitor (-) to battery (-) connection, but the S808 Absolute Maximum current rating on the OUT pin is only 50 mA. It’s better to interface the Power MOSFET, which can allow 500mA continuous. An improvement to this design would be to use the CMOS output version of the S808. That would delete the 10M resistor, and keep the quiescent currents to a minimum.

I hope you’re enjoying this as much as we are. In the next issue we’ll bring it all together with pictures and diagrams and datalogging oh my!

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