Electrical circuits, power supplies and passive circuit elements 1

Electrical Circuits We want to transfer electrical energy to perform a task We want to supply energy to some load Charged particles want to “move” when an emf applied Apply emf and constrain the path of the charged particles Force charged particles to supply energy to the load in order to do work (no path through load  no useful work done!) 2

Electrical Circuit – example 3

Types of Circuit Elements Circuit components are generally classified as control elements, passive elements, and active elements Control Elements – direct and modify the current (e.g. switches) Passive Elements – total energy delivered to the element by the rest of the circuit is nonnegative (e.g. resistors, capacitors, inductors) Active Elements – can provide energy to the circuit (e.g. batteries, generators) 4

Control Elements Examples: Switches and transistors (MOSFETs, BJTs) We will present only switches here – MOSFETs and BJTs will be introduced in a later lecture 5

Control element example -- switches Switches can be used to direct and control the flow of current The switch can act as an insulator or a conductor 6

Switch operation Time t<0: Time t>0: 7

Power Supplies Power supplies provide a source of electrical power The power source is typically a non-electrical process Electro-mechanical sources: generators typically convert a rotational motion to electrical power by moving magnets relative to one another. The rotational motion is induced by mechanical means, such as flowing fluid through a turbine. Chemical sources: batteries convert energy created by a chemical reaction to electrical energy Piezoelectric materials produce a voltage when they are deformed Solar cells convert light to electrical energy 8

Conceptual types of power supplies Power supplies can be modeled in a number of ways: Voltage, current sources Independent, dependent sources Ideal and non-ideal sources 9

Voltage sources Independent voltage sources provide a specified voltage Regardless of the current provided Can provide infinite power! Ideal, independent voltage source symbols: 10

Voltage sources – continued Dependent voltage sources provide a voltage which is based on some other parameter in the system Example dependent source symbol: Often used as control elements 11

Current sources Independent current sources provide a specified current Regardless of the voltage provided Can provide infinite power! Ideal, independent current source symbol: 12

Current sources – continued Dependent current sources provide a current which is based on some other parameter in the system Example dependent source symbol: Also often used as control elements 13

Common types of source signals Time-varying signals DC (Direct Current) signals Constant with time AC (Alternating Current) Vary sinusoidally with time 14

Passive Circuit Elements Examples: Resistors Capacitors Inductors 15

Passive circuit elements - resistors Resistance models the fact that energy is always lost during charge motion Electrons moving through a material “collide” with the atoms composing the material These collisions impede the motion of the electrons Thus, a voltage potential difference is required for current to flow. This potential energy balances the energy lost in these collisions. 16

Resistance 17

Resistors Circuit symbol: R is the resistance Units are ohms () Voltage-current relation (Ohm’s Law): 18

Resistance analogies Sliding mass with constant velocity on surface with friction Energy added by force applied to mass Energy dissipated by friction as heat Pressure loss in horizontal pipe Energy added by pump Energy dissipated by friction in pipe 19

Note that, in the two previous cases, none of the energy being supplied to the system is stored. The energy supplied by the force and by the pump are dissipated by the friction as heat. The kinetic and potential energies in the systems are not changing. Resistance is purely and energy dissipation mechanism 20

Demos: Show types of resistors (power resistor, low power resistor) Apply voltage to power resistor, measure current. Calculate power. Note that power resistor heats up as power dissipation increases. Apply voltage to low power, high-resistance resistor. Calculate power dissipation. Apply voltage to low power, low-resistance resistor. Calculate power dissipation, burn out resistor. 21

Passive circuit elements – capacitors Capacitors store energy in the form of an electric field Typically constructed of two conductive materials separated by a non-conductive (dielectric) material 22

Capacitors Circuit symbol: C is the capacitance Units are Farads (F) Voltage-current relation: Capacitors can store energy 23

Capacitors Notes: Capacitors can store energy The voltage-current relation is a differential equation Capacitance limits rate of change of voltage If the voltage is constant, the current is zero and the capacitor looks like an open-circuit 24

Capacitance analogies Stretched spring Energy added by force in spring Energy stored in spring: Pressurized accumulator Energy added by pressure change Energy stored by pressure change 25

Demos: Show types of capacitors, note sizes Demo of energy storage with Bob Olsen’s super-capacitor/wheel device Analogous systems: Stretched spring Toilet tank 26

Passive circuit elements - inductors Inductors store energy in the form of a magnetic field Often constructed by coiling a conductive wire around a ferrite core 27

Demo: Simple electromagnet 28

Inductors Circuit symbol: L is the inductance Units are Henries (H) Voltage-current relation: Inductors can store energy 29

Inductors Notes: Inductors can store energy The voltage-current relation is a differential equation If the current is constant, the voltage difference is zero and the inductor looks like a perfect conductor 30

Inductor analogies Increasing velocity of sliding mass (with no friction) Applied force increases energy applied to mass Energy stored in change of velocity of mass: Increased flow rate in fluid system Applied pressure adds energy Energy stored in increased fluid flow rate: 31

Demo: Electromagnet to illustrate magnetic field Show practical inductors Analogies: Sliding mass Slug of moving fluid 32