# Solar Panel regulator

Discussion in 'Electronic Design' started by bxdobs, May 12, 2007.

1. ### bxdobsGuest

I was recently give a Solar Regulator that had been thrown out; its made by
Go Power! Electric Inc.

Doing a quick check I determined that the possible reason it had been tossed
is it had a short across the Solar inputs ... not knowing how these
regulators work I decided to do a bit of reverse engineering. To my dismay
it appears that this device shorts the solar panel out when the battery is
deemed fully charged. Because they also have a pair of blocking diodes this
would certainly stop the charging however I wonder if this is potentially
damaging to the solar panel.

Attempting to find an answer to that by reading specifications on PV panels;
most specs give a short cct current, and an open cct voltage and specify
that the operational voltage/current is somewhere in the middle of that.
Being that they provide a Short cct spec sort of implies they can be
shorted.

To make a long question short;

I am trying to understand;
A) Why shorting a panel is an acceptible way to shut off the charging
B) Is there a Solar Cell Equivalent CCT (similar to a battery) that can be
used to mathematically predict the characteristics of the cell.
C) Why shorting out a cell doesn't fry the cell

2. ### John PopelishGuest

I think it is no problem. It just adds a tiny temperature
rise to the silicon.
Since the panel is completely power limited, it is cheap and
reliable.
A current source proportional to light intensity in parallel
with a string of silicon diode junctions. Instead of doing
this, mathematically, I suggest you simulate it with
LTspice, a free circuit simulator. It has built in
functions to display voltage, current and power dissipation.
http://www.linear.com/designtools/software/switchercad.jsp
Count the number of cells in series in your panel and use
that number of diodes. Set the current source to the panel
short circuit current rating.
They are so inefficient that the electric energy is only a
small part of the total heating caused by the absorbed
light. If you want to eliminate that small additional
heating, you could add a power resistor in series with the
short that drops just enough voltage to keep the blocking
diode from conducting. That will move that extra power to
the resistor. A better way might be to have that resistor
be the heating element in a water heater, so that you would
make some use of the energy.

3. ### Martin RiddleGuest

Its a shunt type regulator.

Solar modules are current sources. Get the data sheet of a module to model
it.

Cheers

4. ### boBGuest

It is also acceptable to simply open the PV to stop charging.

boB

5. ### mpmGuest

Many solar regulator designs simply dump the excess current to a load
when the power is not needed for charging. Others will open the
circuit.

Is the short present without the solar panel connected?
-mpm

6. ### Chris JonesGuest

High-efficiency (20% +) concentrator cells (intended for use with a concave
mirror and a big heatsink) will run measurably cooler when they are under
load. This is of course required by the conservation of energy, but can be
a bit surprising when you see it in practice. If you measure the
temperature carefully, then this should be observable with more common
solar cells too. The manufacturers would almost certainly make sure that
each individual module can be run open circuit or short circuit or anywhere
in between, but see below for arrays made of several panels.

Regarding the possibility of damage, I would only caution that if there are
a lot of cells (and a lot of panels) in series then make sure that you have
the right arrangement of power diodes connected backwards across the cells
(or modules) according to any requirements of the panel manufacturer. The
reason for this is that if you have say 50 volts worth of cells in series,
and you short circuit them, and one of the cells is slightly in the shade
(or slightly less efficient), then the less efficient cell will be subject
to a reverse voltage produced by all of the other cells in the series
string. So in this example, one cell could get up to 50 Volts across it
the opposite polarity from normal (the p-type semiconductor would be
negative and the n-type semiconductor would be positive). If the cell
can't handle that much reverse voltage (and usually they can't), then it
will start to leak current like a so-called Zener diode, and then it will
get more hot and more leaky, and current crowding will take place because
the hottest parts of that cell will hog the current, and in the worst case
the cell could be damaged. This can be prevented by placing a diode across
each cell with the p and n regions of the diode connected the opposite way
around to the p and n regions of the solar cell. (For less protection but
for lower cost, a diode can be placed in reverse across each group of cells
or each module.) The diode should be rated for the max current of the
solar cells. In normal operation the diode won't conduct, but if a bird
s(h)its on one cell then the diode will prevent damage and will also help
to maintain the output power of the array. As John says, you can simulate
all of this in some SPICE type thing.

Chris

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