The Zero Volt Diode (ZVD) is a circuit useful in a variety of applications
including  solar chargers of all types. It is a novel circuit in which
a power mosfet acts like a very  low voltage drop diode which 
switches state at 0V! and which is used to conduct NEGATIVE! current from
drain to source.

In D1 type solar engines, a low loss diode can be used to charge a cap
to the open circuit voltage of the solar cell and used in solar battery
chargers, the battery is charged at the maximum  rate when the source
voltage is highest.   The diode must be used  in series
with the solar panel or else  the cap or battery would discharge through
the solar panel when the panel voltage drops below the stored voltage.

The diode or equivalent polarity sensitive switch is therefore essential
to solar chargers.

Most diodes used in BEAM SEs and solar chargers are Silicon diodes
like the 1N4001 which have a voltage drop of 0.6V to 1V at currents up
to 1A. More efficient diodes for currents form >100mA to tens of amps applications 
are the  Schottky type rectifiers with a voltage drop of from 200mV
to 1000mV depending on the current level. For <100mA applications a
Germanium diode can used with 200mV or less drop.

This voltage drop issue is important in competition solar engines 
since you would like to have the maximum voltage to charge the cap and
supply the load (low diode drop) and keep the charge stored on the cap
when  the lightlevel drops (leakage current cut off).  

and the SE  triggers. Moreover, since the energy in the cap is
proportional to the square of the voltage even the small voltage drop of
a  diode reduces available energy.

One obvious simple improvement over the original D1 design  is
to substitute a Ge 1N34A diode (Radio Shack)  instead of the Si 1N4001
diode.

An ideal diode would  have zero voltage drop. While a straight
hookup of the solar cell has minimum voltage drop it leaks if the light
drops and any real diode has a forward voltage drop.   
What to do?

The solution is to use a mosfet as a rectifier just like the synchronous
rectifier applications in volatge converters. The mosfet should be switched
ON when the solar voltage is larger than the capacitor or battery 
voltage and  switch OFF when the solar voltage is lower than the stored
voltage.

Here is a little design for charging capacitors from solar cells with
zero voltage drop at the end of the charge cycle. It can be easily scaled
to higher currents by changing the 2N7000 for a larger mosfet.  If
a parallel load is present, the circuit also delivers maximum voltage with
minimum insertion loss from the solar cell. The mosfet turns on when the
voltage difference is zero and turns off when the solar voltage drops less
than 100mV below the cap or battery .

The NPN transistor is normally ON when the cap voltage is more than
.6V and this clamps the gate of the 2N7000 which is turned  OFF. The
PNP tarnsistors is connected to the negative terminal of the solar panel
and when the voltage on that terminal drops below 0V the PNP turns ON.
This in turn turns the NPN OFF and the 2N7000 turns ON.   MOSFET
have an interesting characteristic in that they act like bidirectional
switches, so the 2N7000 is perfectly happy to have it's DRAIN conduct a
negative current  to the 0V line. When the voltage on the negative
terminal of the solar panel is more positive than 0V the PNP turns OFF
and the NPN ON and the 2N7000 turns off with the drain voltage positive
with respect to the source voltage and 0V line.  Since the 2N7000
does not turn on until the gate voltage is more than 2V (in practice: 
higher according to the data book) a logic FET with a lower gate turn on
voltage would be preferred. In any case the mosfet has a integral reverse
diode from  drain to source which will carry the current until the
voltage on the cap reaches 2V at which point the mosfet turns on and the
forward voltage drops to a few 10s of mV.

Anyway it's a start to revisit some of the more primitive solar circuits
and get the technological edge on your competitors in the WCRG solar races!

enjoy

wilf

 

Bumper314@aol.com wrote:

I'm looking for a solarengine that works better then
the D1. The D1 is so

twitchy and needs alot of tweeking so I'm looking for something that
will

just charge up all day and kick off for the night..I have found that
there is

always plenty of power left in the cap when the D1 turns off (after
about 10

minutes even with a .47F cap) Does anyone have a solution

Steve