Bates-OaktonCommunityCollege.ppt

Good to Know Maximum allowable current for any resistance setting is calculated as: P and R are the rated value of rheostat Q. What is Imax of a 5-KΩ 2-W rheostat? Maximum voltage which produces the rated power dissipation can be calculated as: 2-6: Resistor Troubles Resistors can become open or they can drift out of tolerance. Some controls (especially volume and tone controls) may become noisy or scratchy-sounding, indicating a dirty or worn-out resistance element. Due to the very nature of their construction, resistors can short out internally. They may, however, become short-circuited by another component in the circuit. An open resistor measures infinite resistance. An example of an out-of-tolerance resistor: 2-6: Resistor Troubles 1 kW,5% nominal 1.5 kW ? W 2-6: Resistor Troubles Resistance measurements are made with an ohmmeter. The ohmmeter has its own voltage source, so voltage must be off in the circuit being tested. Otherwise the ohmmeter may become damaged. 2-6: Resistor Troubles All experienced technicians have seen a burnt resistor. This is usually caused by a short somewhere else in the circuit which causes a high current to flow in the resistor. When a resistor’s power rating is exceeded, it can burn open or drift way out of tolerance. Critical Thinking Problem A manufacturer of carbon-film resistors specifies a maximum working voltage of 250V for all its ?-W resistors. Exceeding 250 V causes internal arcing within the resistor. Above what minimum resistance will the maximum working voltage be exceeded before its ?-W power dissipation is exceeded? Resistors Topics Covered in Chapter 2 2-1: Types of Resistors 2-2: Resistor Color Coding 2-3: Variable Resistors 2-4: Rheostats and Potentiometers 2-5: Power Ratings of Resistors 2-6: Resistor Troubles Chapter 2 ? 2007 The McGraw-Hill Companies, Inc. All rights reserved. 2-1: Types of Resistors The two main characteristics of a resistor are its resistance, R, in ohms and its pow