Figure 6. Multimeter Block Diagram for Measuring Current.
free to use your own solar panels, but make sure they do
not go over five volts open circuit or else you will swamp
the A/D converter chip or PICAXE microprocessor’s A/D
inputs and perhaps damage them. Refer to the Experiment
Components sidebar for your source.
Measuring Current with
Current is always measured using the voltage drop
across a sense resistor. This is true even in the best multimeters. Once you understand this principle, you will have
more appreciation for and understanding of the technique.
When measuring current with a normal multimeter,
you must interrupt the circuit by placing the multimeter
between the voltage source and the load as shown in
Figure 6. What you are actually doing is placing a very
small [value] sense resistor in series with the load. The
multimeter measures the voltage drop across the sense
resistor using an A/D converter much like the one we
are using for the experiments. Based on Ohm’s Law, its
internal microprocessor computes the current as follows:
I = V / R
I = current in amps
V = voltage in volts
R = resistance in ohms
In order to determine the voltage drop, the multimeter
Solar Panel History Time Line
1839 Edmund Bacquerel — a French physicist
who studied the solar spectrum, magnetism, electricity,
and optics — is known for his work in luminescence and
phosphorescence. He discovered the photovoltaic effect
which is the basic physics behind the solar cell.
1877 Later in the 19th century while
investigating this effect, Adams and Day noted an
anomaly they thought could be explained by the
generation of internal voltages. They made selenium cells
that were 1-2% efficient.
1904 Albert Einstein — most known for this
Theory of Relativity — published a theoretical explanation
of the photovoltaic effect. In 1916, Robert Millikan
experimentally proved Einstein’s theoretical explanation.
1916 The Czochralski method is the
technology used to grow the large crystals required in
today’s chip industry. The method was named after the
Polish scientist Jan Czochralski who discovered it by
serendipity after accidentally dipping his quill into a tub
of molten metal instead of an inkpot. Accidents are the
basis for many scientific discoveries.
1950s The first attempts to commercialize solar
panels are begun prompted by prior research during
World War II for military applications. Once again, the
government is needed to fund basic research into new
technology in order to spin off commercial applications.
1958 The first commercial solar panel is
used with NASA’s Vanguard I satellite. The panel was
manufactured by Hoffman Electronics, a major
commercial and military electronics firm now essentially
out of business.
1980s Solar panels are first used in conventional
consumer applications for powering calculators, digital
watches, battery chargers, and even small buildings.
1990s More efficient solar panels are developed
using materials other than silicon. Mechanical solar
tracking devices gain popularity to increase total
2000 to present Solar energy’s promise of
independent, grid-free power is slowly becoming a reality.
High cost and low efficiencies below 25% are still the
senses the voltage at both places on the sense resistor —
at the voltage source and at the load. The current is then
computed based on the voltage drop across the sense
resistor as follows:
I = V / R
V = Vsource – Vload
R = sense resistor value
In modern multimeters, the sense resistor is typically
well below one ohm in order not to add to the load.
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