100S11_PS3 Soln

100S11_PS3 Soln - ENES 100 Introduction to Engineering...

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ENES 100 Introduction to Engineering Design Spring 2011 1/7 Problem Set #3 SOLUTIONS Assigned: Feb. 11, 2011 Due: In class on Feb. 21 (M) or Feb. 22 (Tu) Reading: Chapter 4: Basic Circuits and Sensors To receive full credit, you must show and explain your calculations. Problems: 1. Imagine that on your hovercraft, you will be using a downward-pointing light-emitting diode (LED) to illuminate the floor surface. The figure below shows how you might control the LED using an Arduino, and the equivalent circuit diagram.
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ENES 100 Introduction to Engineering Design Spring 2011 2/7 The LED that you are using is model SLR-343, made by Rohm Semiconductor. The following graph was taken from the data sheet of this LED: The maximum current that this LED can handle (without damage) is 20 mA. To be safe, you have decided to operate the LED at a current of 10 mA. (a) Using the data shown in the graph above, what resistor (R) should you choose in order to obtain a current of 10 mA? From the graph provided by the manufacturers data sheet, we see that when the LED current is I F = 10 mA, the corresponding LED voltage will be approximately V F = 1.88 V. Applying Kirchhoff’s voltage law to the circuit, we conclude that the voltage drop across the resistor R must therefore be (5 V – 1.88 V) = 3.12 V. Finally, Ohm’s law tells us that in a resistor, the current and voltage must be related to one another by V = IR. The current is 10 mA, and the resistor’s voltage drop is 3.12 V. We therefore calculate that R = (3.12 V)/(10 mA) = 312 . GRADER: Any value in the range 310 – 315 is acceptable. (b) Under these conditions, how much electrical power is consumed by the LED?
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ENES 100 Introduction to Engineering Design Spring 2011 3/7 The power dissipated in the LED is given by: P = IV = (10 mA)(1.88 V) = 18.8 mW GRADER: Any value in the range 18.5 – 19 mW is acceptable (c) How much power is dissipated in the resistor? The power dissipated in the resistor is likewise given by:
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This note was uploaded on 04/03/2012 for the course ENES 100 taught by Professor Jacob during the Spring '08 term at Maryland.

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100S11_PS3 Soln - ENES 100 Introduction to Engineering...

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