Justin Richards wrote:
> > Keep in mind that with series-connected panels, the total voltage is th=
e
> > sum of the individual panel voltages, but the current through the strin=
g
> > is limited to the current of the least-illuminated panel.
>=20
> I had considered the current would be limited but that implies that
> the shaded panel's resistance is greater than zero.  I accept that the
> panels do have a reistance, but if they are partially illuminated and
> generating some power them I suspect that resistance would be low.
>=20
> I have some panels, I should go and measure OC Voltage, CC Current and
> Non illuminated Resistance.  Also perhaps voltage across a panel
> connected to perhaps a 20 ohm resistor and measure the current and
> voltage at various illuminations and plot.

The concept of "resistance" in a solar cell is a tricky one.

A solar cell is fundamentally a diode, and this diode is pointed the "wrong
way" in terms of drawing power from the cell. In other words, when function=
ing
as a solar cell, positive current comes out of the anode, rather than going
into it. The only way the cell can pass current in this direction is by mea=
ns
of the electron-hole pairs that are created by the incoming photons. This
means that the number of electrons available to carry current -- including =
the
current driven by other cells in series -- is directly proportional to the
number of photons striking the cell.

In other words, once the reverse saturation current level is reached in the
cell, including the current generated by photons, the cell functions as a
current source, with effectively infinite resistance. In all other regimes =
of
operation, the V-I curve of the cell is that of a diode, where the effectiv=
e
resistance varies widely, depending on the current.

-- Dave Tweed
--=20
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