SampleLabWriteup

SampleLabWriteup - LIGHT EMITTING DIODE...

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1 LIGHT EMITTING DIODE CHARACTERISTICS (SAMPLE LAB WRITEUP) John A. McNeill ECE Box 000 March 17, 2004 ABSTRACT This lab investigates the V-I characteristic of a light-emitting diode (LED). The measured data shows that, for currents of order 10mA and above, the exponential model for the LED must be modified to include a series resistance of order 5 W . EQUIPMENT Wavetek 2015 DVM HP3458A DVM HP33120A Function Generator Tektronix PS2521G Power Supply HP54602B Oscilloscope INTRODUCTION The junction diode is a familiar component which is widely used for circuit applications such as rectification. When the diode is fabricated in silicon, the forward voltage drop is approximately 0.7V and the v D -i D characteristic relating diode voltage and current can be described by an exponential relationship: i D = I S exp v D nV T ( ) (1) where I S and n are scale factors, and V T is the thermal voltage kT/q e 25.4mV at room temperature. The LED is a junction diode that emits light when forward biased. To achieve the light emitting property, it is necessary to fabricate the LED from materials other than silicon. As a result, the forward voltage drop of the LED is greater than 0.7V; usually of order 1 to 2 volts. The purpose of this lab is to investigate the v D -i D characteristic of the LED, and determine how well the model of Eq. (1) applies. Be sure the title block includes your name (and, if appropriate, your lab part- ner's), the ECE box number you want the writeup re- turned to, and the date of the report. The abstract is a concise description of the material covered in the lab, as well as a summary of the important results. Don't just repeat the introduction from the lab handout -- be sure to include some specific result(s) from your work. Spell out acronyms (LED) the first time they are used. Be sure to record the equipment used, since (as we'll see later in this report) the characteristics of the instrumentation may affect the accuracy of your measurements The introduction should provide the theoretical background for the work you will be presenting Important equations should be set on their own line. Be sure to number every equation so you can refer to it later in the text if necessary. If there are any new symbols introduced, explain them in the text immediately following the equation. Your introduction should concisely state the purpose of the lab, motivating the reader to understand the work to be described
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2 CIRCUIT DESCRIPTION Two circuits were used to measure the V-I characteristic of the LED. The first circuit, shown in Fig. 1, was used to display the V-I characteristic on an oscilloscope. V X V Y LM741 R 1 1k W i 1 i D V D + - LED SCOPE "X" CHANNEL SCOPE "Y" CHANNEL v in + - FUNCTION GENERATOR Fig. 1. Circuit for X-Y oscilloscope measurement. The operation of this circuit can be understood by noting that the op-amp is in the inverting configuration with negative feedback; therefore the op-amp inverting input is at virtual ground. The input current is then given by i 1 =v Y /R 1 . Since the current into the op-amp input is (ideally) zero, the LED current i D =i 1 .
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