Developed by J. Fiore, edited by V. Borra 1
Experiment 4: Light Emitting Diodes
Objective: This exercise examines the general performance and use of light emitting diodes. This includes
forward bias and reverse bias characterization along with brightness variation.
Components needed: Adjustable DC Power Supply
Digital Multimeter
Each of standard LEDs of various colors (red, blue, green, yellow)
High brightness white LED
1 k Ω resistor ¼ watt
Schematic:
Figure 1 Figure 2
Procedure:
Forward Curve
1. Consider the circuit of Figure 1 using R = 1 kΩ. For any positive value of V, the diode
should be forward biased. Once V exceeds the knee voltage, the difference between the
source and the knee drops across R. Thus, as V increases, so does the LED current and
hence its brightness.
2. Build the circuit of Figure 1 using R = 1 kΩ and the red LED. Set V to 0 volts and measure
both the LED voltage and current and record the results in Table 1. Note the relative
brightness level. Repeat this process for the remaining source voltages listed.
3. From the data collected in Table 1, plot the current versus voltage characteristic of the
forward biased LED. Make sure VD is the horizontal axis with ID on the vertical.
4. Repeat steps 2 and 3 for the blue LED using Table 2.
Developed by J. Fiore, edited by V. Borra 2
5. If other colors are available repeat steps 2 and 3 for them using Table 3 (create other tables
as needed).
High Brightness
6. Replace the LED of Figure 1 with the high brightness white LED. Set the supply to 12
volts. Record the LED voltage, current and brightness in Table 4.
Reverse Curve
7. Consider the circuit of Figure 2 using R = 1 kΩ. For any positive value of V, the LED should
be reversed biased. In this case, the LED should always be open causing no current to flow.
If no current flows, the LED produces no light. Also, the voltage across R should be zero,
and thus the LED voltage should be equal to the applied source voltage. Note that the LED
voltage polarity is negative with respect to that of Figure 1.
8. Build the circuit of Figure 2 using R = 1 kΩ using the red LED. Set V to 0 volts and
measure both the LED voltage and current and record the results in Table 5. Repeat this
process for the remaining source voltages listed.
9. From the data collected in Table 5, plot the current versus voltage characteristic of the
reverse biased diode. Make sure VD is the horizontal axis with ID on the vertical.
Data: Table 3-1: Red Color LED