That diode in solar panels...

[Externet]

Hello all.
Some solar panels recommend the use of a diode in series for the charged battery to not dump its charge back into the panel at darkness, or something like that.
Why not all the panels manufacturers suggest its use, is it depending on the technology/chemistry ?
If the solar cells are actually PN junction diodes, why is such extra diode needed ?
Miguel

[russlk]

I imagine that some low quality solar cells have significant reverse leakage. The ones I have tested have no measurable reverse leakage and I do not use a diode.

[MrAl]

Hi there,


The extra diode is used to prevent the battery from dumping it's
power into the solar panel when there is no sunlight, for example.

The solar panels are PN junctions, but they would be wired in
parallel with the battery where the battery would forward bias
them if there was no sunlight...in other words, the *anode* of the
panel connects to the battery positive terminal and cathode to
the battery negative terminal so that the panel would normally
charge the battery.
The protection diode, on the other hand, connects with its
*cathode* connected to the battery positive terminal and its
anode connected to the anode of the solar panel. Thus, when
the sunlight doesnt allow the solar panel to reach a voltage
as high as the battery the protection diode is reversed biased and
prevents current flow through the solar panel.
Without the protection diode since the battery connects directly to the
anode of the solar panel it pushes current through all the PN junctions
if the total sum of all the PN junction voltages isnt as high as the
battery voltage. This is the same way we forward bias a pn diode
such as the base emitter of a transistor or even a simple diode,
so the pn conducts current. This could be bad for the panel, depending
on the current level.
What is the cost of doing this? It costs more than the simple protection
diode used to protect the array. If a Si diode is used and the array
puts out 70 volts and the array costs 7,000 dollars, connecting a diode
for reverse current protection means a drop of 0.7 volts in the array,
and since the array is made of many PN junctions in series/parallel,
we loose a complete row of PN junctions (0.7 volts worth) which comes
out to a loss of 70 dollars. So, that 50 cent diode costs a total of
$70.50 to install. Lucky if the voltage is high so the small drop doesnt
matter as much.
Perhaps a Schottky would be better, where the drop might not go
over 0.5 volts. This is especially true if the normal array voltage is low,
like 12 volts or something like that. The diode must be rated for the
full array voltage and of course for the full rated current.

SUMMARY
The protection diode PN junction is wired opposite to that of the solar panel
PN junctions, so it prevents current flow back through the array during
periods of low sunlight.


ADDED LATER:

Here's a little drawing to show the connection of the diode and its
effects:

[Externet]

Clear as can be Al,
Thanks one more time. I did not think of the PN junctions polarity.
Miguel

[dacflyer]

they also use dioads in large panels as a bypass so that if there happend to be a panel shadded by a tree or what ever in an array, the power from other panels in series will bypass that panel untill the shaded panel get sun.
the panels on my roof are 100 watts each, and they have 3 dioads in their junction boxes. they are 12/24 volt selectable.

[jwax]

Also, every solar panel has its operating point of maximum power output.
As the sun power or load varies, the operating point of the panel varies, as does its efficiency. A solar charge controller "tracks", and makes best use of power vs load under most conditions.

[Bob Scott]

I don't have solar panels. The sun does not appear 90% of the time in this rain forest climate, so using solar panels would be a big waste of time and money.

I bet Philba in Seattle is in the same situation.

If I lived in the desert climate near Las Vegas, I'd cover my property with solar panels and live 100 feet underground in a cool deep basement in order to avoid the 110+ temperatures on the surface.