LAB: Exploring Light Emitting Diodes (LEDs)

Transcription

1 Your Name: Your Lab Partners Name: LAB: Exploring Light Emitting Diodes (LEDs) Summary: In this lab, we will be exploring the characteristics of Light Emitting Diodes (LEDs). In order to light up our LEDs, we will set up a simple circuit that allows us to control how much current we put into the LED. An important characteristic of individual LEDs is the current-voltage curve (abbreviated I-V curve). The I-V curve is a graph that plots the current (I) passing through the LED vs the voltage (V) across the LED. We will be working with different colored LEDs, and will explore the relationship between the colors of light emitted and the voltage, current, and power needed to light them up. Background Information: All visible light is a form of electro-magnetic (EM) radiation. The entire EM spectrum is shown below: Different colors of light have different wavelengths and corresponding frequencies. In this lab, we will be using five LEDs in five different colors. The colors and corresponding wavelengths are shown below: Color Wavelength (nm) Blue 470 Green 525 Yellow 589 Red 660 * Remember, a nm (nanometer) is equivalent to 10-9 meters. In addition, you will be working with two mystery LEDs. The colors and wavelengths of these mystery LEDs are not disclosed to you at this time. The material that is used to make the diode determines the color of the LED. In class, we learned that GaN is used to make blue and white LEDs. Examples of materials used to produce different colored LEDs are Gallium Arsenide (GaAs), which produces red LEDs, and Gallium Phosphide (GaP), which produces green LEDs. 1

2 The white LEDs that we are working with are actually blue LEDs. The blue LEDs shine blue light onto a phosphor that emits yellow light, and these colors combine to produce light that appears white. If you look at the white LED from above, you should be able to see the yellow phosphor. If you look at all the other LEDs, you will not see any yellow phosphor. Equipment: We will be using some lab electronics to power our LEDs and measure their current and voltage characteristics. 1. Breadboard: The board that the LEDs and wires are connected to is called a circuit board or a breadboard. Many components can be directly plugged into the holes and wires can be used to form interconnections between them. The holes in the vertical column to the right of the red line are connected (shorted) together, as are those in the column to the left of the blue line. (In the figure below, these columns are also designated by + and symbols.) For the rest of the board, the holes are connected across horizontal rows. In the Figure below, the five holes in the top row (Row #1, Column a, b, c, d and e) are connected together. This means that if we apply a voltage to hole b1, the rest of the holes in the row (a1, c1, d1, e1) will have the same voltage. 2. DC Power Supply: In this lab this is used to supply the DC voltage The DC power supply can provide a constant voltage and/or constant current to your circuit. In other words, it is a large variable battery; we simply turn a knob to increase or decrease the voltage or current. The DC power supply can vary the voltage from 0 to 20V and the current from 0 to 1A. Relevant Features 1. The meters on the front of the DC power supply show the approximate current and voltage that the supply is putting out. DO NOT USE these numbers for you report use the readings from the DMM s (see below). To Use the DC Power Supply 1. Make sure that the knob under METER is set to 18 V (top left knob). 2. Make sure that the VOLTAGE +18 knob is turned all the way down (counterclockwise) (bottom left knob). This is the knob that you will use to adjust the voltage. When adjusting this knob, turn it slowly. 3. When the red lead is in the +18 V 2

3 connector and the black lead is in the COM you will be supplying + voltage to the system. To supply negative voltage put the black lead in the +18V connector and the red lead in the COM connector. 4. Attach the alligator clip on the other end of the red lead to the red connector on the breadboard (labeled Va) connect the alligator clip on the other end of the black lead to the black connector on the breadboard (see picture below). 5. Push the toggle switch up to turn on the DC power supply. 2. Digital Multi-meter (DMM): General notes about DMMs Measuring Voltage When measuring voltage with a multi-meter: 1) make sure that the leads are plugged into the voltage (V) and common (com) connectors and 2) make sure that the knob is turned to voltage. Note: some multi-meters require you to select direct current (DC) or alternating current (AC). AC is symbolized by a wavy line and DC is symbolized by a straight line. DC is for measuring things like batteries and AC is for measuring things like the voltage in a wall socket. To measure voltage touch the lead to any exposed wires on either side of the area in which you wish to measure the voltage drop (see below). Measuring Voltage Measuring Current Measuring Current When measuring current on multi-meters, have the leads plugged into the common (com) and amp (A) connectors. Turn the knob on the multi-meter so that the line is pointed at the A setting. Note: some multi-meters require you to select DC or AC. In order to measure the current you must insert the multi-meter into the circuit. This is done by breaking the circuit and hooking it up to either end of the multi meter (see above). 3

4 Table-top Digital Multi-meter (DMM): In this lab the table-top DMM will be used to read current It can also be used to measure DC voltage, current and resistance. Using the table top DMM 1. Have the red lead plugged into the 2A max connector that is white and have the black lead attached to the 2A max connector that is black. 2. Make sure the AC/DC button is fully extended (first on the left in the row of buttons). If the button is pushed in, push the button and it will pop out. In the out position DC measurements will be taken. 3. Make sure the 20mA button is depressed (4 th button from the left in the row of buttons). Important do not change this setting for this lab. 4. Hook the other end of the red lead (step 1) to the wire coming out of row 35 (± 3) on the breadboard. 5. Hook the other end of the black wire (step 1) to the long leg of the LED (if you are measuring the current in the resistor it does not matter which leg you use) see below. 6. Turn on the power by pushing the button in the top left corner. 7. Goes to white connector on table top DMM see step 1 Goes to the com connector on the portable multi meter Goes to black connector on table top DMM see step 1 Note: This is attached to the long leg of the LED Goes to on the V connector on the portable multi meter. 4

5 Portable Digital Multi-meter (DMM): In this lab the portable DMM will be used to measure voltage The operation of this portable DMM is similar to the desktop DMM above. We will measure DC voltage (marked V --- ) using the portable DMM. We will not measure any voltage greater than 20 V, so we will use the scale setting of 20, as shown in the figure. We will also use this DMM to measure the resistance of the resistor. Resistance is measured in units called ohms, whose symbol is Ω. So to do this, move to the area marked Ω and select the 2000 scale since we will not be measuring any resistances greater than 2000 Ω. Using the portable DMM To measure DC voltage 1. Turn the dial to the 20 DC voltage mark (marked V--). 2. Have the black lead plugged into the common connector and the red lead plug into the VΩmA connector. 3. Attach the other end of the red lead to the red wire on the breadboard (wire located in top row of the breadboard marked +) see above. 4. Attach the other end of the black lead to the black wire on the breadboard (wire located in the second row of the breadboard marked -) see above. To measure resistance 1. Turn the dial to the 200 Ω mark. 2. Attach the two lead to the different sides of the resister. Procedure 1. Read all of the information on pages Check your lab space and make sure you have 7 LEDs (white, blue, green, yellow, red, Mystery LED #1, and mystery LED #2), 1 resister, the breadboard, a portable digital multi-meter (DMM), and 4 connectors. 3. The breadboard will be set up initially with a 222 ohm resistor (it s the solid brown one). 4. Verify the resistance of this resistor using the hand-held DMM. DO NOT turn on the power supply when you measure resistance. 5. By applying voltages to the circuit, fill out the table for the resistor in the results section. You will need to measure the current in the circuit at -3V, -1.5 V, and 0V (all of these values can be ±0.1V) from the power supply. To apply a negative voltage see the DC power supply section on page 3. Then adjust the power supply voltage until you measure 5, 10, 15, and 20 ma current (all of these values can be ± 1mA). Try to get the 10 ma reading between 9.7 ma and 10.3mA. Make sure to record the actual voltage and current on your chart. For data set you will need to calculate the power (mw) = current (ma) voltage (V) 6. Remove the brown resistor from the breadboard and insert one of the known LEDs in its place. Note that longer leg of the LED is the positive end, also known as an anode. The shorter leg is the negative leg, also known as a cathode. For the forward bias (light emitting) condition described in class, you will need to connect the longer leg of the LED to the wire that is connected to the black connector on the tabletop DMM. WARNING: Adjust the power supply slowly and carefully. Do not apply more than -3 V (3V in the reverse direction) or 20 ma of current in the forward direction, or the LED will burn out. 5

6 Current (ma) Lab: Exploring LEDs 7. Fill out the tables for all the known LEDs. You will take the same measurements as you did for the resistor, but in addition, there is another line where you should fill in which is the voltage and current needed when you first see the first bit of light. Now as the turn on voltage (V turn-on ). You will have look carefully from above the LED to see that first glimpse of light. 8. Fill in the table for Mystery LED #1. You should notice that this LED gives off light that is not visible to our eyes. 9. If possible as you fill out the tables for each device construct a current vs. voltage curve similar to the one seen below. Make sure to include all resistor/leds on one plot, have appropriately labeled axis, have a title for your plot, have lines of best fit for each device, and have a legend (try using appropriate colors to distinguish between LEDs on the legend ex: make the red LED red points). If you need help with excel come and talk to Darby Put Title Here Voltage (V) Device X Linear (Device X) y = 1.5x Now try Mystery LED #2. You do not need to fill a table out for this LED. You only need to identify the color it will be one of the questions due with this report. What happens when you put this LED the other way around (reverse the cathode and the anode in the circuit)? 11. With the materials provide try to make a device that functions like the mystery LED #2. 6

7 Results: Things to remember: 1) Adjust the voltage very slowly or you will burn out your LEDs 2) The numbers on the chart are what you should shoot for. You should be able to get voltages to be within ± 0.1V. For the current you should be able to get ± 1 ma. Record the actual values read from the DMMs on the chart. 3) Try to get the 10 ma reading between 9.7 ma and 10.3 ma Fill out the following tables for measurements on the resistor and the LEDs 222 Ohm Resistor Green LED Voltage (V) Current (ma) Power (mw) Voltage (V) Current (ma) Power (mw) V turn-on = Yellow LED Blue LED Voltage (V) Current (ma) Power (mw) Voltage (V) Current (ma) Power (mw) V turn-on = V turn-on = White LED Red LED Voltage (V) Current (ma) Power (mw) Voltage (V) Current (ma) Power (mw) V turn-on = V turn-on =

8 Mystery LED #1 Voltage (V) Current (ma) Power (mw) V turn-on = Lab Report (Turn in all answers in class on April 18, 2011). 1. Hand in the tables showing current, voltage and power consumed for each LED and the resistor. 2. At home or in lab, either in Microsoft Excel or another graphing program, plot the I-V curves for all the known LEDs, mystery LED #1, and the resistor as describe in step 9 of the procedure. 3. Make a bar chart comparing power at 10 ma for each LED color. 4. Make a chart comparing the turn-on voltage for each LED color. Provide typed answers to the following questions 5. Comment on your I-V curves, in particular comment on the difference between LEDs and the resistor and on the differences between the graphs for different LED colors. 6. Comment on your chart comparing turn on voltages. What is the qualitative relationship between the LED color and the turn-on voltage? 7. What wavelength do you think the mystery LED #1 is? You must generate an evidence based explanation for this. Prizes will be given to the person who gives the most accurate wavelength and also to the person with the best reasoning for his/her wavelength estimate (even if the actual answer is not the most accurate). 8

9 8. What color is mystery LED #2? (Hint: This may be a trick question of some sort) How do you think the LED is designed to give the color(s) you saw? What happens when you turn this LED around? How did you make a device that functioned like mystery LED # 2 9. In class we made the claim that white LEDs are just blue LEDs that stimulate phosphorus causing both light from the LEDs and phosphorus to combine to make white light. Based on your data do you believe this statement? Come up with an evidence based explanation of why or why note. You are encouraged to discuss things in groups while working on this lab report, but all answers must be your own. Ask Darby any questions if you are stuck or need help with Excel. 9