ECE3042 WA1 - Writing Assignment 1: Application Note...

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Writing Assignment 1: Application Note General Use of an Electro-Optical Isolator Clinton Plaza ECE 3042-L08 Brewer/Rogers/Leung Due: 15Feb2007
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In many modern electronic systems, it is often advantageous to electrically isolate two interdependent circuits. This isolation can prevent shock while providing stability and increased efficiency to circuits by preventing ground loops. While transformers have often been used to create this isolation, an increasing number of systems are implementing electro-optical isolator circuits. These circuits incorporate a phototransistor, or other light sensitive component, driven by a light emitting component, often a light emitting diode (LED), to interconnect two circuits without direct electric interaction. Due to the minimum number of parts needed, these circuits can often be implemented cheaper and with less space consideration than their older and often- large transformer based counterparts. A pulse train generating electro-optical isolator should produce two distinct waves that are electrically isolated. If built using both the 2N3904 NPN bipolar junction transistor, and the MCT6 phototransistor optocoupler, the output pulse should be approximately 180 degrees out of phase with the input signal. This small pulse train generating circuit than could be used for controlling a larger more powerful system without the risk of control signal corruption caused by the more powerful signals around it. With the ever-increasing field for electronics demanding more circuits to operate in close proximity to each other, and smaller and more portable size requirements, electro-optical isolators provide circuit designers the ability to make smaller, safer, and more all encompassing devices. Design The design of electro-optical isolators are a key to the performance and functionality of many electronic systems. They can be tailored for a specific required application, such as the production of two electrically isolated digital pulse trains. It would make sense that, in order to implement and fully analyze an electro-optical isolator the circuit must designed to use its components to their full potential. Assuming a basic isolator circuit uses three resistors, a MCT6 optocoupler and a 2N3904 BJT, there are several considerations that must be made for proper functionality, performance and component safety. First, a reasonable current level must be selected for the components to provide the best possible signals. A user would need to look to the data sheet for the parts being used to find that the maximum current allowed to safely operate the part. In the case of the MCT6, this value is 30mA. Further investigation of the data sheet leads to the discovery of the Normalized CTR vs. Forward Current graph seen in Figure 1. Figure 1.
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This note was uploaded on 04/07/2008 for the course ECE 3042 taught by Professor Brewer during the Spring '08 term at Georgia Institute of Technology.

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ECE3042 WA1 - Writing Assignment 1: Application Note...

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