At 11:16 PM 12/2/2009, Forrest W Christian wrote: >Use a P Channel FET... connect it such that the body diode is in the >same direction as a traditional diode would be, and then connect the >gate to ground. You'll need one with appropriate ratings - probably Vgs >would be the most worrisome, but you would need to look at the whole >picture. > >-forrest You do realize that probably won't do what the Original Poster wants? Here's why: when the FET turns ON because the solar cell is supplying energy, it can't turn OFF - because its on. In other words, once the FET turns ON, the Source and Drain are effectively connected together and even when the solar input goes away, the FET remains turned ON. You need to control the gate somehow. I use a series FET switch in our solar chargers but the FET gate is driven with an op-amp that monitors a current sensor. The FET is turned ON only when the solar input exceeds some small value and turns OFF when the solar current drops below some lower value (a little bit of hysteresis). One section of a really inexpensive quad op-amp and a ZXCT1008 current sensor along with a few resistors is all that we used. The remaining op-amp sections were for the solar voltage regulator (with temperature compensation) and some other analog functions that this project required. We spent quite a lot of time on that particular charger because it needed to run under Arctic conditions for many years without anyone visiting the sites. That includes time periods several weeks long with effectively no solar energy available (winter solstice). So far, so good . dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist