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Forward Current versus Forward Voltage
for Light Emitting Diodes
Anyone who has worked with light emitting diodes has
noticed that there's a relationship between color and the forward voltage drop
for the same current.
Intuitively, this makes sense, because higher frequency
(shorter wavelength) light has higher energy than lower frequency light.
The relationship between the frequency of a photon
and its energy is given by the Planck–Einstein equation:
E is energy
h is Planck's constant
v is the frequency
Numerically, Planck's constant is 4.136 x 10-15
eVs (electron-volts seconds)
The image below, from
http://en.wikipedia.org/wiki/Electromagnetic_spectrum shows the wavelength
and frequency of the visible spectrum as well as the neighboring infrared
and ultraviolet.
If we say red light corresponds to 675 nm (4.44x1014
Hz), we can compute the associated frequency as photon energy as 1.8 eV.
If we pick blue as 450 nm, the associated photon energy is 2.76 eV. Near
infrared at say 900 nm has a photon energy of 1.38 eV.
Thus, we expect the range of expected forward voltage for
an LED to range between 1.4V or so for an IR diode and 2.8V for white, based
purely upon the physics of the emitted light. |
