Experiment 4: Light Emitting Diodes

14 Mar

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:
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 V
D 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 V
D is the horizontal axis with ID on the vertical.
Data:
Table 3-1: Red Color LED

 V (volts) VD ID Brightness 0 1 2 3 4 6 12

Developed by J. Fiore, edited by V. Borra 3
Table 3-2: Blue Color LED

 V (volts) VD ID Brightness 0 1 2 3 4 6 12

Table 3-3: Green Color LED (if available)

 V (volts) VD ID Brightness 0 1 2 3 4 6 12

Table 3-4: High Brightness LED

 V (volts) VD ID Brightness 12

Table 3-5: Reverse Curve

 E (volts) VD ID 0 1 3 8

Developed by J. Fiore, edited by V. Borra 4
Is the forward knee voltage of an LED comparable to that of ordinary switching and
rectifying diodes?
Report:
Write a report that summarizes this experiment. Your report brief must include:
a) Cover sheet.
b) Objective – a short paragraph stating what the experiment is about.
c) Procedure – a description of the process of conducting the experiment. It should
not be a step-by-step account, but rather an overview of what was done.
d) Discussion – a complete discussion of the results of the experiment and the
principals involved. Include the outputs and also the screen captures as needed
to support the statements made in this section. Relegate extensive lists of raw
data and detailed computations to an appendix as appropriate.
e) Conclusion – a summary of what you have learned from the experiment.
f) Appendix – original raw data (initialed by the lab instructor) and details not
placed in the discussion section