Questions about solar cells

[royalmp2001]

I am considering a solar cell powered project but I have some
questions.

What happens during low light levels. Does the rated volatage decrease
or only the current, or both. If I want to implement a battery backup
what kind of circuit will switch between the two. Are solar cells
shortcircuit proof?
Thanks

 

[John Popelish]

royalmp2001 wrote:

I am considering a solar cell powered project but I have some
questions. What happens during low light levels.
Does the rated volatage decrease or only the current, or both.

Both the voltage and the available current go down. The voltage
roughly proportional to the log of intensity (so small decrease) and
the current in proportion to intensity, if you use power at the
optimum rate.

If I want to implement a battery backup
what kind of circuit will switch between the two.

Usually, the circuit just runs from the battery, and the solar cells
connect to the battery through an anti reverse diode, or regulator.

Are solar cells
shortcircuit proof?
Thanks

Sure. They just get as warm as if they were laying in the sun with
nothing connected. This allows them to be regulated with a shunt
dummy load that soaks up all the excess current once the battery
reaches full charge. For a reliable operation over many months, the
cells have to provide a full day's charge in just a few hours, to
allow for a run of rainy weather.
--
John Popelish

 

[mike]

royalmp2001 wrote:

I am considering a solar cell powered project but I have some
questions.

What happens during low light levels. Does the rated volatage decrease
or only the current, or both. If I want to implement a battery backup
what kind of circuit will switch between the two. Are solar cells
shortcircuit proof?
Thanks

You might find this article useful.
http://www.elecdesign.com/Articles/Print.cfm?ArticleID=9699
mike

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[royalmp2001]

John,
Is an anti-reverse diode a special type of diode. Can I use a IN4148?
Any advantage of using a schottky?

 

[John Popelish]

royalmp2001 wrote:

John,
Is an anti-reverse diode a special type of diode. Can I use a IN4148?
Any advantage of using a schottky?

It is nothing special. It must stand the battery voltage in reverse
(when the cells are in the dark) and carry the solar cell current in
the forward direction. The schottky type wastes about half as much
voltage compared to a junction diode with the same current rating, but
will also leak more current when the solar cells are in the dark. A
1N4148 is fine for up to about .1 amp maximum cell current.

You may not need an anti reverse diode at all, if your cell array
leaks a small enough current when it is in the dark, and still
connected to the battery. Put an ammeter in series and measure the
leakage or put a small resistance in series, measure the voltage drop
and calculate the current with ohm's law. For example, if you put a
1k resistor in series and had 0.1 volt drop across it when the cells
were dark, that would mean that the cells leaked about .1/1000=.0001
ampere (100 microamps).

If that current times the number of hours of darkness is a small
fraction (say, .01) of the forward current times the number of hours
of daylight, then the leakage will not use a significant fraction of
the charge energy. Adding the anti reverse diode would lower the dark
leakage, but would also lower the daylight charging current, so it
might be a net loss to have it. Measure the two cases and find out.

--
John Popelish