ELEC 103 Voltage, Current, and Resistance Objectives Define voltage and discuss its characteristics Define current and discuss its characteristics Define resistance and discuss its characteristics Identify the rated, minimum, and maximum resistance value using the resistor color code 23 July 2005 ELEC 103 2 Andrew H. Andersen 1
The Bohr Model of an Atom 23 July 2005 ELEC 103 3 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 23 July 2005 ELEC 103 4 Andrew H. Andersen 2
The Simplest Atoms, Hydrogen And Helium. 23 July 2005 ELEC 103 5 Attraction & Repulsion of Electrical Charges. Charge (Q) has the Coulomb (C) as the unit of measurement Current is charge in motion Any body may have one of three types of charge Positive Charge Negative Charge No Charge 23 July 2005 ELEC 103 6 Andrew H. Andersen 3
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 23 July 2005 ELEC 103 7 Ampere: The Unit of Current One ampere of current is flowing when one coulomb of charge passes a given cross-sectional area in one second 23 July 2005 ELEC 103 8 Andrew H. Andersen 4
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 23 July 2005 ELEC 103 9 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 23 July 2005 ELEC 103 10 Andrew H. Andersen 5
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 23 July 2005 ELEC 103 11 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 23 July 2005 ELEC 103 12 Andrew H. Andersen 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 23 July 2005 ELEC 103 13 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 23 July 2005 ELEC 103 14 Andrew H. Andersen 7
Common Fixed Resistors 23 July 2005 ELEC 103 15 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 23 July 2005 ELEC 103 16 Andrew H. Andersen 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 23 July 2005 ELEC 103 17 Color Coded Precision Resistors 23 July 2005 ELEC 103 18 Andrew H. Andersen 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 23 July 2005 ELEC 103 19 Fixed IC Resistors Often it is convenient to use multiple resistors mounted in IC packages in many of today's modern circuits 23 July 2005 ELEC 103 20 Andrew H. Andersen 10
Cutaway View of a Resistor 23 July 2005 ELEC 103 21 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 23 July 2005 ELEC 103 22 Andrew H. Andersen 11
Examples of Variable Resistors 23 July 2005 ELEC 103 23 Linear vs Logarithmic Taper Potentiometers 23 July 2005 ELEC 103 24 Andrew H. Andersen 12
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 23 July 2005 ELEC 103 25 Andrew H. Andersen 13