Introduction to Electrical and
Computer Engineering
Basic Circuits and Simulation
Electrical & Computer Engineering
Basic Circuits and Simulation (1 of 22)
International System of Units (SI)
•
•
•
•
•
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Length: meter (m)
Mass: kilogram (kg)
Time: second (s)
Current: Ampere (A)
Voltage: Volt (V)
Temperature: Degrees
Kelvin (ºK)
Electrical & Computer Engineering
• SI Prefixes (power of 10)
–
–
–
–
–
–
–
–
1012 Tera (T)
109 Giga (G)
106 Mega (M)
103 kilo (k)
10-3 milli (m)
10-6 micro (µ)
10-9 nano (n)
10-12 pico (p)
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SI Examples
• A few examples:
• 1 Gbit = 109 bits, or 103106 bits, or one thousand
million bits
– 10-5 s = 0.00001 s; use closest SI prefix
• 1×10-5 s = 10 × 10-6 s or 10 μs
or
• 1×10-5 s = 0.01 × 10-3 s or 0.01 ms
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Typical Ranges
Voltage (V)
Current (A)
• 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)
• 107 Lightning bolt (10 MV)
• 10-12 Nerve cell in brain
• 10-7 Integrated circuit
memory cell (0.1 µA)
• 10×10-3 Threshold of
sensation in humans
• 100×10-3 Fatal to humans
• 1-2 Typical Household
appliance
• 103 Large industrial
appliance
• 104 Lightning bolt
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Electrical Quantities
• Electric Charge (positive or negative)
– (Coulombs, C) - q
– Electron:
1.602 x 10
• Current (Ampere or Amp, A) – i or I
– Rate of charge flow,
– 1
1
Electrical & Computer Engineering
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Electrical Quantities (continued)
• Voltage (Volts, V)
– w=energy required to move a given charge between two
points (Joule, J)
–
– 1 1
1 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
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Circuit elements (Sources)
Voltage Source (V)
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Current Source (I)
Basic Circuits and Simulation (7 of 22)
Circuit Elements (Passive Elements)
Resistor (R)
Ohms, 
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Inductor (L)
Henrys, H
Capacitor (C)
Farads, F
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Simple Circuit
Notice that current is shown
in the direction of positive
charge flow, even though the
actual flow is the movement
of electrons.
Active Sign
Convention
Passive Sign
Convention
VS = VR because both
elements are connected to
the same points electrically
(from the top wire to the
bottom wire).
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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
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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
Electrical & Computer Engineering
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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
Electrical & Computer Engineering
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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
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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.
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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.
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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.
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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) = I2R
P = (V/R)*V = V2/R
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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/(3 × 103) = 2 × 10-3 or 2
mA
• Increasing the value of
resistance decreases the
current flow
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Example (continued)
• All forms of the equation yield the
same magnitude for power
Now look at power,
P = I2R = 22(3) = 12 W
P = V2/R = 62/3 = 12 W
P = IV = 2(6) = 12 W
– 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
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Ohm’s Law Simulation
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Basic Circuits and Simulation (20 of 22)
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Ohm’s Law Simulation (continued)
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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
Electrical & Computer Engineering
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