Unformatted text preview: Basic Circuit Definitions, Elements and Properties – DC Circuits EE 201
DePiero / CalPoly State University Common Components Appearing in Electrical Circuits Independent Active element supplying power at a constant voltage. Polarity specified. Voltage • Analogy: Hydraulic pump providing constant pressure Source • Practical Examples / Approximations: 1) Power supply used with consumer electronics. 2) Battery (weaker approximation). • Ideal Internal resistance: Zero • Practical Limitations: Real voltage sources have a maximum current limit and a non
zero internal resistance. Voltage not exactly constant. Independent Active element supplying power at a constant current (& given direction) Current • Analogy: Hydraulic pump providing constant flow. Source • Practical Examples / Approximations: Real sources can be constructed as small sub
circuits, not single elements. Current not perfectly constant. • Ideal internal resistance: Infinite • Practical limit: Real current sources have a maximum voltage limit. Resistor Passive element converting electrical energy to heat. Resistance in Ohms. • Analogy: Flow restriction. • Practical Example: Volume control for audio (variable resistor) • Ideal Internal resistance: Finite • Practical limit: Resistance value not exact
has some tolerance. Power limit on rate energy can be converted to heat. (1/8 to 1/2 W common, 1%
10% tolerance). Switch Passive element controlling current flow • Analogy: On/Off shutoff valve. • Practical Examples: Rotary, toggle, momentary (doorbell, fridge) • Ideal Internal resistance: Zero or infinite • Practical limit: Current limit / Voltage limit Wire Passive element conducting current flow • Analogy: Piping __________ • Ideal Internal resistance: Zero • Practical limit: Current limit / Voltage limit Dependent V or I Source Similar to an independent source, but voltage or current is determined by another circuit quantity (either another voltage or current). What do these devices look like? Resistors • Fixed value, non
variable • Color bands indicate resistance value and tolerance (accuracy) Variable Resistors • Turning knob changes resistance. • Often used for control, such as setting the volume for a music amplifier Figure 1. A simple circuit. Resistors in the above circuit have values in Ohms. Common Structures in Electrical Circuits • Node: A connection between circuit components. These connections are assumed to have zero resistance. Hence the voltage at a node is constant. • Loop: Any closed path starting and ending on a given node. Fig 1 has ___ loops. • Mesh: A loop that does not contain any other loops. Fig 1 has ___ meshes. • Branch: A portion of a circuit between two distinct nodes, containing one or more elements in series. Basic Relations and Units Describing Electrical Components (DC Perspective) • Voltage. A potential difference that causes current to flow. Measured in Volts, ‘V’. Analogous to a pressure difference causing fluid flow or a temperature difference causing heat flow. • Current. The flow of electrons through a conductor. Measured in Amps, ‘A’. Current
carrying electrons are in the valence band of a molecule, permitting them to move freely thru the conductor. o 1 Amp = 1 Coulomb (C) of charge / sec. 1 C has charge of 6*10^18 electrons o (Note: semiconductor devices are covered in EE 321). o Also: current flows, voltage does not! • Resistance. A restriction that limits the flow of current. Measured in Ohms, Ω. o V = I R • Conductance. G = 1/R. Measured in Siemens (1/Ohms). Convenient for describing conductivity in materials such as wire, with low resistance. €
• Power. Measured in Watts, W. P = V I. This is the actual power available to do work. • Energy. Measured in Joules, J. The integral of power, P. Elements that store energy will be considered later in the course (inductors and capacitors). Water Analogy: We can make the analogy between current & water flow, and between voltage & pressure. One stipulation is important: Electrons do not spill from a conductor when it is removed from a circuit. (Unlike water from a hose). Thus there is no analogy to sputtering as water rushes into a dry hose before encountering a restriction to flow. Another perspective on this is to consider a piping system with bleeder valves that are used to remove air bubbles before operation. Combining Resistors in Series eq
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Note: Resistors in series share the same current. R = R + R + ... + R € Combining Resistors in Parallel eq
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Note: Resistors in parallel share the same voltage. R= € 1 11
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 Spring '08
 ARAKAKI

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