Introduction to Electrical and Computer Engineering. International System of Units (SI)

Transcription

1 Introduction to Electrical and Computer Engineering Basic Circuits and Simulation Basic Circuits and Simulation (1 of 22) International System of Units (SI) Length: meter (m) Mass: kilogram (kg) Time: second (s) Current: Ampere (A) Voltage: Volt (V) Temperature: Degrees Kelvin (ºK) SI Prefixes (power of 10) Tera (T) 10 9 Giga (G) 10 6 Mega (M) 10 3 kilo (k) 10-3 milli (m) 10-6 micro (µ) 10-9 nano (n) pico (p) Basic Circuits and Simulation (2 of 22) 1

2 SI Examples A few examples: 1 Gbit = 10 9 bits, or bits, or one thousand million bits 10-5 s = s; use closest SI prefix s = s or 10 μs or s = s or 0.01 ms Basic Circuits and Simulation (3 of 22) Typical Ranges Voltage (V) 10-8 Antenna of radio receiver (10 nv) 10-3 EKG voltage produced by heart 1.5 Flashlight battery 12 Car battery 110 House wiring (US) 220 House wiring (Europe) 10 7 Lightning bolt (10 MV) Current (A) Nerve cell in brain 10-7 Integrated circuit memory cell (0.1 µa) Threshold of sensation in humans Fatal to humans 1-2 Typical Household appliance 10 3 Large industrial appliance 10 4 Lightning bolt Basic Circuits and Simulation (4 of 22) 2

3 Electrical Quantities Electric Charge (positive or negative) (Coulombs, C) - q Electron: x 10 Current (Ampere or Amp, A) i or I Rate of charge flow, 1 1 Basic Circuits and Simulation (5 of 22) Electrical Quantities (continued) Voltage (Volts, V) w=energy required to move a given charge between two points (Joule, J) Joule is the work done by a constant 1 N force applied through a 1 m distance. so 1 J = 1 N m 1 V = 1 N m/c Basic Circuits and Simulation (6 of 22) 3

4 Circuit elements (Sources) Voltage Source (V) Current Source (I) Basic Circuits and Simulation (7 of 22) Circuit Elements (Passive Elements) Resistor (R) Ohms, Inductor (L) Henrys, H Capacitor (C) Farads, F Basic Circuits and Simulation (8 of 22) 4

5 Simple Circuit Active Sign Convention Passive Sign Convention Notice that current is shown in the direction of positive charge flow, even though the actual flow is the movement of electrons. V S = V R because both elements are connected to the same points electrically (from the top wire to the bottom wire). Basic Circuits and Simulation (9 of 22) Passive Components In a load or passive component, such as a light bulb, resistor, or electric motor Electric current (flow of positive charges) moves through the device under the influence of the voltage in the direction of lower electric potential, from the positive terminal to the negative So work is done by the charges on the component; potential energy flows out of the charges; and electric power flows from the circuit into the component Basic Circuits and Simulation (10 of 22) 5

6 Active Components In a source or active component, such as a battery or electric generator, current is forced to move through the device in the direction of greater electric potential energy, from the negative to the positive voltage terminal This increases their potential energy, so electric power flows out of the component into the circuit Work must be done on the moving charges by some source of energy in the component, to make them move in this direction against the opposing force of the electric field E Basic Circuits and Simulation (11 of 22) Passive Sign Convention Defining the current variable as entering the positive terminal means that if the voltage and current variables have positive values, current flows from the positive to the negative terminal, doing work on the component, as occurs in a passive component So power flowing into the component from the line is defined as positive; the power variable represents power dissipation in the component. Therefore Active components (power sources) will have negative resistance and negative power flow Passive components (loads) will have positive resistance and positive power flow Basic Circuits and Simulation (12 of 22) 6

7 Active Sign Convention Defining the current variable as entering the negative terminal means that if the voltage and current variables have positive values, current flows from the negative to the positive terminal, so work is being done on the current, and power flows out of the component. So power flowing out of the component is defined as positive; the power variable represents power produced Active components will have positive resistance and positive power flow Passive components will have negative resistance and negative power flow This convention is rarely used, except for special cases in power engineering Basic Circuits and Simulation (13 of 22) Resistance The most basic circuit equation is Ohm s Law (passive convention): V = IR The voltage across a resistor is equal to the current flowing through it times the resistance value The unit for resistance is the Ohm (named after a German physicist, George Simon Ohm) with a symbol of. A reminder: you need to be careful when using Ohm s Law in a circuit with multiple resistors or multiple sources This law refers to a single resistor and refers to the voltage across that particular resistor and the current flowing through that particular resistor. Basic Circuits and Simulation (14 of 22) 7

8 Resistance Equivalence If the polarity of a voltage is reversed, the magnitude also has to be negated. The branches are equivalent. In the same way, if a current direction is reversed, the magnitude has to be negated. See below. Each representation is equivalent for both voltage and current. Basic Circuits and Simulation (15 of 22) Special Cases When R = 0, referred to as a short circuit, V = IR = 0. You can think of this situation as a perfect conductor that is capable of carrying current with no voltage drop across it. When R =, referred to as an open circuit, I = V/R = 0. You can think of this situation as a perfect insulator that is capable of supporting a voltage without permitting current flow. Basic Circuits and Simulation (16 of 22) 8

9 Power Another important equation is the equation, P = IV. Power is measured in Watts (named after the Scottish engineer, James Watt) The unit of Watts can also be broken down further. I (Coulomb/s) and V (Joule/Coulomb) multiplied together result in Coulombs cancelling out and therefore, the unit of Watt is equivalent to Joule/s (rate of energy expenditure) P = IV = I*(IR) = I 2 R P = (V/R)*V = V 2 /R Basic Circuits and Simulation (17 of 22) Example To find I in the circuit, use Ohm s Law: V = IR. I = V/R I = 6V/3 = 2 Amps. If you change the resistor value to 3 k, I = 6/( ) = or 2 ma Increasing the value of resistance decreases the current flow Basic Circuits and Simulation (18 of 22) 9

10 Example (continued) Now look at power, P = I 2 R = 2 2 (3) = 12 W P= V 2 /R = 6 2 /3 = 12 W P = IV = 2(6) = 12 W All forms of the equation yield the same magnitude for power We can distinguish if an element is absorbing or supplying power by using the passive sign convention In this circuit, positive current is entering the positive voltage terminal of the resistor. We use the sign of the power to show that a positive power indicates that the element (resistor) is absorbing power Note in this circuit, positive current is entering the negative voltage terminal of the power supply. A negative power (2 * -6) = -12 W indicates this element is supplying 12 W of power Basic Circuits and Simulation (19 of 22) Ohm s Law Simulation Basic Circuits and Simulation (20 of 22) 10

11 Ohm s Law Simulation (continued) Basic Circuits and Simulation (21 of 22) Basic Circuit Simulation Start OrCAD Capture CIS Lite Select new project Select project name and location Project type should be Analog or Mixed A/D Select Create a blank project Create circuit by placing components Place->PSpice Component After circuit is complete, create a PSpice simulation profile Pspice->New Simulation Profile The analysis type should be Time Domain (Transient) Run simulation using PSpice Run Tool will allow you to display parameters such as currents through each device, voltage across each device, power supplied/dissipated by each device Basic Circuits and Simulation (22 of 22) 11