Knee voltage of led

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

V (volts) VD ID Brightness