ELEC 242 Lab

Experiment 1.1

LED Characterization

Equipment

Components

We'll start our characterizations with the LED. We do not have the equipment for an accurate optical characterication, but can perform an electrical characterization.

For this experiment we will use the same apparatus as we did in Experiment 1.1, Part 5 of the ELEC 241 Lab, where we measured the I-V characteristics of an incandescent light bulb.

Part 1: Light Emitting Diode



Step 1:

 Set up the power supply: turn both voltage controls to zero and set the meter selector to the 6V supply. Don't connect the supply yet.

Step 2:

 Using a 100 ohm (brown-black-brown) resistor and your jumbo LED, wire the following circuit.

First wire the resistor and LED on the breadboard.

There are two ways to connect the power supply to the circuit (use one or the other, not both):

The first way: Plug your BNC-banana adapter into the 6V supply terminals.



Note
There is a bump on one side of the adapter to denote which prong is connected to ground. Be sure to plug this prong into the black terminal of the power supply.

Then use the clip leads to connect to the LED and resistor.

The other way: Use the BNC adapter as above, but use a BNC patch cord to connect the power supply to P3 on the interface board. Use two pieces of wire to connect ground (pin 31) to the LED and the P3 signal (pin 30) to the resistor.



Step 3:

 Turn on the power supply. Slowly increase the voltage until you see the LED just begin to glow. If the LED doesn't light by the time the meter on the power supply reads 3 V, check your circuit to make sure the diode is wired in the correct orientation. Unlike a resistor or light bulb, the LED is polarized. The anode must be positive for it to glow. Reverse the LED and verify that this is the case.

Step 4:

 Vary the supply voltage (as read by the front panel meter) between 0 and 6 volts in steps of 0.5 V. At each step measure (with the DMM) and record the voltage across the LED and the voltage across the resistor. Also record the brightness of the LED (subjective evaluation).

Step 5:

 From your measurements, compute the current through the LED for each value of voltage. Make a plot of current vs. voltage for the LED. Does it have the exponential shape we expect?

Question 1:

 In a flashlight, the battery (a voltage source) is connected directly to the light bulb. What would happen if we connected the LED directly to a voltage source? Consider several different values of voltage for the source.