A/D converter receives a voltage of 100 millivolts that is
well within the limits of a good A/D converter.
For our experiments, it was decided to save on the
cost and complexity of a voltage amplifier and go, instead,
with a one ohm sense resistor. If the actual load resistance
is also low (like 10 ohms, or less, as in most of these
experiments), there is ample voltage drop across the
resistor — especially when applied to a 10- or 12-bit A/D
converter which can measure down to 4. 88 and 1.22
millivolts, respectively, with a + 5 volt reference.
The downside to using a one ohm load resistor is that
it adds to the load itself. So, for a 10 ohm load and a one
ohm sense resistor, the total load is actually 11 ohms.
Craving more “green” stuff?
Check out what the Nuts&Voltsstore
has to offer on Pages 90-93 in this issue!
There are lots of alternatives!
Experiment #2: Determining the
Maximum Power Point
This experiment demonstrates how solar panels in
either series or parallel have a maximum power
operating condition known as the Maximum Power
Point or MPP. The maximum power point is where the
solar panels can deliver the maximum power into a
load. MPP is a dynamic condition that varies based on
external influences such as light intensity, tilt angle, heat,
and either a series or parallel arrangement of the panels.
You are shown that the MPP is achieved when the
resistance of the solar panels matches the load
resistance. You will discover this when you vary the
load resistance to produce maximum power with solar
panels in series and parallel configurations. Satellites in
space must constantly adjust their solar arrays to acquire
the MPP, which is why this is such an important concept
Wire the solar panels in series and adjust the 100
ohm potentiometer from full resistance to a lower
resistance until the maximum power is reached.
Continue to adjust it for lower resistance to see what
happens. Then, rotate it back to the maximum power
level. A plot similar to the one in Figure 9 is displayed.
Notice the large dip in voltage, current, and power as
the heaver load resistance overwhelms the solar panel’s
ability to source power.
Next, wire the panels in parallel and repeat the same
procedure with the potentiometer. While the voltage is
noticeably lower due to the parallel wiring configuration,
the voltage does not dip as much and the output current
and power remain much steadier under the varying load
(Figure 10). Thus, solar panels in parallel have a better
means to supply more stable power over varying loads
as compared to an equivalent series arrangement.
Therefore, be aware of the fact that the resistance values
displayed on the computer are the “sum” of the one ohm
sense resistor plus the actual load resistance.
This first brief look into solar photovoltaic experiments
gave you a glimpse into what desktop solar panels are
capable of doing and how their performance is measured.
Commercial solar panels behave in much the same way
which is one of the main points of these experiments; that
is, to make the connection between what you learn in
model form in order to apply the same techniques to
Next time, we will investigate the effects of tilt angle,
heat, and shading on solar panels which will give you even
more background information. In the meantime, conserve
energy and “stay green.” NV
Figure 9. MPP for Solar Panels in Series.
Figure 10. MPP for Solar Panels in Parallel.
August 2009 73