turn-on voltage for a green LED is lower than that for blue.
See if you can acquire an I-V curve for a green LED, and
predict how it might behave in Figure 11 instead of the
Second, the emission wavelength of an LED can be
discovered using the trend lines found for the “on” region
of an LED’s I-V curve to precisely determine the turn-on
voltage of an LED (use the trend line equation to find
what V makes I=0).
From this V, find the emission
wavelengths for a green, blue, and
red LED. (Hint: For a photon, E=hc/l,
where E is the photon’s energy, h is
Planck’s constant, c is the speed of
light, and l is its wavelength. Note
also that E is related to the turn-on
voltage of an LED). Compare your
results with those on the Wikipedia
Third, the slope of the blue LED’s
trend line is 0.122W and for the red
is 0.08W. Thinking of the LEDs then
loosely as “resistors” of these values,
what behavior would you expect as
the voltage was slowly turned up
from zero if they were connected in
Can you explain Figure 11’s
behavior based on what you now
know about junctions and these
Fourth, acquire a “varistor” and
“resettable fuse” (see the Parts List)
and study I-V curves for them.
I hope you saw the value in
acquiring I-V curves for the light
bulbs, and particularly for the LEDs. I
always find it enjoyable to figure out
some way of generating raw data
that I can then study to learn more
My original motivation for this
article came years ago, when I was
working on a project with different
Putting them in parallel across a
power supply was simple but
puzzling because the different
colored LEDs wouldn’t light properly.
I now know why, and it appears
that driving (different colored) LEDs
in parallel directly from a DC power
supply was not a good idea.
Lastly, by seeing the steepness of an LED’s I-V curve,
hopefully you see why LEDs are most often used in
conjunction with a series resistor in an actual circuit.
Typically, a safe operating current for an LED is
determined, and the value of the resistor is chosen to drop
enough voltage across itself (at that current as per V=IR)
to limit the voltage drop across the LED to some safe level.
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February 2018 35