4.2 Diode I-V Characteristic Curves and Load Lines
4.2 Diode I-V Characteristic Curves and Load Lines
The following graph shows part of the curve for the red LED in the ECE361 electronics kit. This curve was obtained from the data sheet for this LED, and the URL for that data sheet can be found in section 1.4 of this book. The curve reveals that a forward current (that is, current flowing from anode to cathode) of ~0 to ~ 45mA would flow through the diode when an external voltage of ~ 1.5V to ~ 2.0V is applied across the diode between anode and cathode. The “knee” or turn-on voltage, above which significant current begins to flow, is ~ or ; an applied voltage less than this value results in negligible current flow, whereas an applied voltage results in increasing current flow through the device. Note how the current increases rapidly from ` mA to mA as the applied voltage increases from to just under volts. The data sheet for this device specifies that the maximum safe current through the red LED is . Above this value, the LED would be damaged due to internal heating.
Figure 4.12, also part of the data sheet for the red LED, characterizes the luminous intensity, which is a measure of the radiated light brightness, versus forward current. The luminous intensity varies between and mcd (millicandela) as the forward current varies between and mA. For comparison, cd (candela) is the luminous intensity of a common candle, so the LED with mcd = cd is as bright as a common candle at a forward current of mA. (The LEDs in the ECE361 kits are typical low-brightness lights used to indicate that a device is on or off; other LEDs are designed with significantly higher brightness values. For example, the Cree PLCC6 LED, used in outdoor transportation and roadway signs, has a luminous intensity as high as mcd.)
The curve shown in figure 4.12 indicates that the radiated light intensity is approximately doubled as the forward current increases from mA (luminous intensity of ~ mcd) to mA (luminous intensity of ~ mcd). [Note that these luminous intensity values are only approximate as we are reading them from the plot of figure 4.12]. If we had access to a variable DC voltage supply that could be connected in parallel with the red LED, we could chose the desired relative intensity we wanted to achieve from the LED and use these two curves to determine the appropriate voltage to apply across the diode to generate a desired current and, hence, a desired brightness.
Examples
Example: What applied LED voltage would be needed to achieve a luminous intensity of mcd for the red LED described by figures 4.11 and 4.12?
Solution: The luminous intensity vs. forward current curve shown in figure 4.12 indicates that a luminous intensity of ~ mcd corresponds to a forward current of ~ mA. The forward current vs. applied voltage curve of shown in figure 4.11 indicates that a forward current of mA corresponds to an applied voltage of V. Thus, setting the voltage across the red LED to V would cause a forward current of mA to flow, an