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The D1 solar engine
The original nocturnal solar engine

The D1 solar engine is the original nocturnal solar engine design.


The design is simple, tuneable, but no longer terribly useful (I include it here just for purposes of education and folks' historical interest). This design has a number of weaknesses; courtesy of Wilf Rigter, here are a few:

  1. In order to turn on the FET the gate terminal needs at least +3V with respect to the source terminal and since the gate senses the difference between the solar cell voltage and the capacitor voltage it needed a relatively high charging voltage for the capacitor and a very dark solar cell to turn on and then only tolerates a small voltage drop on the cap as it discharges into the rest of the circuit before the FET turns off because the gate voltage is starved!
  2. When the FET finally turns on in the dark, it does so slowly and for a period of time the FET is only partially on, discharging the capacitor at a low current which is not enough to do any work but nonetheless consumes stored energy. This design gives a "mushy" turn on point and no hysteresis and a significant amount of energy wasted in the forward voltage drop when partially turned on.
  3. FET are not as common as other components such as 1N34 Ge diodes or 2N3904 / 2N3906 transistors available from Radio Shack.

Either the SmartCap or one of Wilf's SIMD1 (simplified D1) designs will do quite nicely at addressing these deficiencies.

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Page author: Eric Seale
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