ELEC 103 23 July 2005Andrew H. Andersen 1ELEC 103Voltage, Current, and Resistance23 July 2005 ELEC 103 2Objectives• 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 codeELEC 103 23 July 2005Andrew H. Andersen 223 July 2005 ELEC 103 3The Bohr Model of an Atom23 July 2005 ELEC 103 4Categories 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 themELEC 103 23 July 2005Andrew H. Andersen 323 July 2005 ELEC 103 5The Simplest Atoms, Hydrogen And Helium.23 July 2005 ELEC 103 6Attraction & 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 ChargeELEC 103 23 July 2005Andrew H. Andersen 423 July 2005 ELEC 103 7Current• 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 chargeQ C I = A = ts23 July 2005 ELEC 103 8Ampere: The Unit of Current• One ampere of current is flowing when one coulomb of charge passes a given cross-sectional area in one secondELEC 103 23 July 2005Andrew H. Andersen 523 July 2005 ELEC 103 9Voltage• 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 otherSchematic Symbol for the DC Voltage SourceAnode Cathode23 July 2005 ELEC 103 10Batteries• 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 batteryELEC 103 23 July 2005Andrew H. Andersen 623 July 2005 ELEC 103 11Other 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 circuits23 July 2005 ELEC 103 12Resistance• 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 atomsELEC 103 23 July 2005Andrew H. Andersen 723 July 2005 ELEC 103 13Ohm: 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 materialThe unit of measurement for resistance is the ohm (Ω)Schematic Symbol for a Resistor23 July 2005 ELEC 103 14Resistors• 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 requiredELEC 103 23 July 2005Andrew H. Andersen 823 July 2005 ELEC 103 15Common Fixed Resistors23 July 2005 ELEC 103 16Color-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 value1stband is the first digit of the resistance value2ndband is the second digit of the resistance value3rdband is the multiplier (number of zeros)4thband indicates the toleranceELEC 103 23 July 2005Andrew H. Andersen 923 July 2005 ELEC 103 17Precision 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 4thband is the multiplier– the 5thband indicates the tolerance• Other precision resistors use an alphanumeric coding• The cost of the device is directly related to the precision – higher precision = higher cost23 July 2005 ELEC 103 18Color Coded Precision ResistorsELEC 103 23 July 2005Andrew H. Andersen 1023 July 2005 ELEC 103 19Alphanumeric Labeling• Two or three digits, and one of the letters R, K, or Mare used to identify a resistance value• The letter is used to indicate the multiplier, and its position is used to indicate decimal point position23 July 2005 ELEC 103 20Fixed IC Resistors• Often it is convenient to use multiple resistors mounted in IC packages in many of today's modern circuitsELEC 103 23 July 2005Andrew H. Andersen 1123 July 2005 ELEC 103 21Cutaway View of a Resistor23 July 2005 ELEC 103 22Variable 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 currentELEC 103 23 July 2005Andrew H. Andersen 1223 July 2005 ELEC 103 23Examples of Variable Resistors23 July 2005 ELEC 103 24Linear vs Logarithmic Taper PotentiometersELEC 103 23 July 2005Andrew H. Andersen 1323 July 2005 ELEC 103 25Power 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