Voltage, Current, and Resistance. Objectives

Transcription

1 Voltage, Current, and Resistance ELEC 111 Objectives Define voltage and discuss its characteristics Define current and discuss its characteristics Define resistance and discuss its characteristics 21 January 2004 ELEC Andrew H. Andersen 1

2 Categories of Materials Conductors readily permit current flow, due to a large number of free electrons in the material Conductors are characterized by 1, 2, or 3 valence electrons in their atomic structure Semiconductors have 4 valence electrons Insulators have few free electrons, and tend not to permit current to flow through them 21 January 2004 ELEC Voltage The unit of measurement for voltage is the volt (V) It is the source of electrical energy in a circuit By definition: One volt is the potential difference (voltage) between two points when one joule of energy is used to move one coulomb of charge from one point to the other Schematic Symbol for the DC Voltage Source Anode Cathode 21 January 2004 ELEC Andrew H. Andersen 2

3 Batteries A battery is a voltage source that converts chemical energy into electrical energy The way cells are connected, and the type of cells, determines the voltage and capacity of a battery 21 January 2004 ELEC Other Voltage Sources Solar Cells convert light energy into electrical energy Generators convert mechanical energy into electrical energy Electronic power supplies do not produce electrical energy, but they convert the ac voltage from an outlet into a constant dc voltage for use in our circuits 21 January 2004 ELEC Andrew H. Andersen 3

4 Current Electron Current Flow is the movement of free electrons from negative terminal of the source to positive terminal We may also define Conventional Current Flow as the flow of positive free charges from positive terminal of the source to negative terminal Both are called electrical current (I) which has the Ampere (A) as the unit of measurement By definition: electrical current is the rate of flow of charge Q C I = A = t s 21 January 2004 ELEC Ampere: The Unit of Current One ampere is the amount of current that exists when a number of electrons having a total charge of one coulomb pass a given cross-sectional area in one second 21 January 2004 ELEC Andrew H. Andersen 4

5 Resistance The property of a material that restricts the flow of electrons is called resistance (R) By definition: Resistance is the opposition to current Where there is current through any material that has resistance, heat is produced by the collisions of electrons and atoms 21 January 2004 ELEC Factors effecting Resistivity ρ l R = A 21 January 2004 ELEC Andrew H. Andersen 5

6 Ohm: The Unit of Resistance By definition: One ohm of resistance exists if there is one ampere of current in a material when one volt is applied across the material The unit of measurement for resistance is the ohm (Ω) Schematic Symbol for a Resistor 21 January 2004 ELEC Resistors Resistors are used to limit current or divide voltage, and in some cases, generate heat Common resistors are carbon-composition, carbon film, metal film, and wirewound surface mount resistors are available as small resistor chips wirewound resistors are used where high power ratings are required 21 January 2004 ELEC Andrew H. Andersen 6

7 Common Fixed Resistors 21 January 2004 ELEC Color-code Bands on a Resistor This coding is used for 5%, 10%, and 20% tolerance resistors The first three bands are used to denote the rated value The fourth denotes how much the device may vary from the rated value 1 st band is the first digit of the resistance value 2 nd band is the second digit of the resistance value 3 rd band is the multiplier (number of zeros) 4 th band indicates the tolerance 21 January 2004 ELEC Andrew H. Andersen 7

8 Precision Resistors Precision resistors will have a tolerance of 2%, 1%, 0.5%, 0.25% or 0.1% Some precision resistor values are identified using the color code with 5 color bands, the first 3 bands indicate the rated resistance value the 4 th band is the multiplier the 5 th band indicates the tolerance Other precision resistors use an alphanumeric coding The cost of the device is directly related to the precision higher precision = higher cost 21 January 2004 ELEC Color Coded Precision Resistors 21 January 2004 ELEC Andrew H. Andersen 8

9 Alphanumeric Labeling Two or three digits, and one of the letters R, K, or M are used to identify a resistance value The letter is used to indicate the multiplier, and its position is used to indicate decimal point position 21 January 2004 ELEC Cutaway View of a Resistor 21 January 2004 ELEC Andrew H. Andersen 9

10 Fixed IC Resistors Often it is convenient to use multiple resistors mounted in IC packages in many of today's modern circuits 21 January 2004 ELEC Variable Resistors Variable resistors are designed so that their resistance values can be changed A potentiometer is a three terminal variable resistor used to divide voltage A rheostat is a variable resistor used to control current 21 January 2004 ELEC Andrew H. Andersen 10

11 Linear vs Logarithmic Taper Potentiometers 21 January 2004 ELEC Power Resistors Used when the device must accommodate high currents Physically large to surface can dissipate heat into the environment Heats the air around the device may be very hot 21 January 2004 ELEC Andrew H. Andersen 11

12 The Power Rating of Resistors The power rating is the maximum amount of power that a resistor can dissipate without being damaged by excessive heat buildup Power rating is determined by physical composition, size, and shape of the resistor The power rating is not related to the ohmic value 21 January 2004 ELEC The Power Rating of Resistors Metal-film resistors are available in standard power ratings from 1/8 W to 1 W When a resistor is used in a circuit, its power rating should be at least 25% greater than the maximum power that it will have to handle 21 January 2004 ELEC Andrew H. Andersen 12

13 The Power Rating of Resistors When the power dissipated in a resistor is greater than its rating, the resistor will become excessively hot the device should be expected to prematurely and unexpectedly fail the resistor may burn up its resistance value may be greatly altered damaged resistors may be visually detected by charred or altered appearance of the surface, When a device power is exceeded, the suspect resistor should be removed from the circuit and checked with an ohmmeter 21 January 2004 ELEC Resistor Power Rating. The power rating of a resistor increases as its physical size increases 21 January 2004 ELEC Andrew H. Andersen 13

14 Relative Size of Metal-film Resistors 1 / 8 W ¼ W ½ W 1 W 21 January 2004 ELEC Example of Power Resistors The graphics above are for power resistors, typically 2W or larger. The size of each resistor in the graphic is not to scale. 21 January 2004 ELEC Andrew H. Andersen 14