Download Learning Basic DC Circuit Techniques

Basic DC Circuits Review
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Prefixes
Prefixes come in handy when trying to express high or low
numbers.
Prefixes
Symbol
Value
atto
a
10-18
femto
f
10-15
pico
p
10-12
5.68 m
nano
n
10-9
56 µ
micro
µ
10-6
milli
m
10-3
kilo
k
103
mega
M
106
giga
G
109
tera
T
1012
EXAMPLES:
1,000
5.68×10-3
0.000056
1,212,000,000
1k
1.212 G
0.000000000005
2.5×10-10
5p
250 p
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Voltage (V)
Voltage is an electrical pressure which causes current to flow
through a resistance.
It is measured in volts (V).
Two common DC voltage supplies are shown below:
(Batteries)
The “long side” or + terminal of a battery is called the anode.
The “short side” or – terminal of a battery is called the cathode.
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Voltage
Continued…
Voltage can be compared to the pressure of water in a tank. As
the height of water in a tank increases, so does the water
pressure. This increase in pressure causes more water to flow
out of an opening in the bottom of a tank, much like how a
higher voltage (higher electrical pressure) produces more
current through a resistance.
123 A
369 V
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
3Ω
Voltage Polarity
Voltage polarity is denoted by a + and – symbol. When
Here we get -9.15
9.15 V,
V,since
sincethe
thered
redlead
leadisishooked
hookedtotothe
the
connecting the positive (red) lead of a multimeter to the
negativeterminal
positive
terminalofofthe
thebattery
batteryand
andthe
theblack
blackisisconnected
connected
positive terminal of the battery with the negative (black) lead to
to the negative
positive terminal.
terminal.
the negative terminal of the battery, a positive
of voltage
Vo value
= -9.15
9.15
VV
will be displayed. However, if you were to connect the red lead
to the negative terminal and the black lead to the positive
terminal, a+-negative voltage would be displayed.
-9.15
9.15VV
Vo
Digital Multimeter
+
+
Let’s look at the following source:
9.15 V
Now let’s see what happens when polarity is reversed.
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
The
multimeter is
essentially an
open circuit
when
measuring
voltage.
Current (I)
Current is the movement of electrons through a conductor. It is
established by a potential difference (or voltage) across a
resistance and is measured in the quantity amperes or amps (A).
The common DC current source is shown below:
(Independent Current Sources)
The arrow of the independent current source represents the
direction of current flow.
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Current
Current is not across two points as is voltage, but flows through a
circuit element.
Note: Multimeter is set to
Let’s consider the following circuit:
measure current here. It
essentially acts as a short circuit
to take this measurement.
1 kΩ
I
9 mA
-9
9 mA
Digital Multimeter
+
1 kΩ
9 mA
+ I
Current is
Current is
denoted positive
denoted negative
when entering
when entering
the red
the black
(positive) lead.
(negative) lead.
+
Click to see what happens when leads are switched.
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Current
Continued…
Current can only flow through a closed loop. It must travel
where there is a defined path. This concept is pictured below
with current depicted in red.
R1
R3
R2
Notice, there is zero
current flow through R3,
since there is no closed
path for current to flow.
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Nodes
A node is a connection between one or more elements in a
circuit. Here, the nodes of each circuit are circled in red. Notice
that the wires composing each node have no resistance, thus
there is no voltage drop within the red areas.
Note: When taking measurements with a digital multimeter the
negative lead is connected to ground (node 1).
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Nodes
Continued…
Notice
that
this
voltage
reading
is also
For
same
reason
also
measure
Next,
From
the
we
will
previous
use
the
slides,
digital
you
might
The the
voltage
across
Rwe
and
R4multimeter
(taken
at 9 V
However,
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might
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3not
and
that
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voltage
dropped
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V
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2 is
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5.9across
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. these resistors share the
same two nodes.
R1
R2
R3
9V
Digital Multimeter
+
R5
9V
R4
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
-
Nodes
Continued…
Now
Previously,
What
we
result
are
when
would
going
measuring
to
wewhat
take
getbe
when
athe
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webut
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The
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athe
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9node
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4,V
Now,measurement
Can
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because
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and
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4?to
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the
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of
the
(red)
node
with
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measuring
the top (red) node in reference to ground (the purple
itself?voltage.
node).
If your answer was 0 V, then you were correct!
R1
R2
R3
9V
R5
<09???
9VV
Digital Multimeter
+
R4
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
-
Branches
A branch is a part of a circuit that contains one or more circuit
elements in series with a separate node at each end. Notice that,
the current flowing through a branch is equal for every element
contained in the branch network.
For example,
Note: The current
I1 flows through
R1, R2, and R3.
The value of I1
does not change
through the
branch.
R1 I1
R5
IS
V
I3
I2
R2
R4
R6
R3
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Resistance (R)
Resistance is a hindrance/opposition to the passage of an
electrical current. Resistance in a circuit is represented by a
resistor. The unit of resistance is the ohm (Ω).
The symbol used to represent a resistor is
schematic capture
actual representation
Materials such as metal (conductors) have a small resistance,
where materials such as rubber (insulators) have a large
resistance.
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Capacitance (C)
Capacitance is the ratio of charge to voltage across two
conductive elements (or plates). Capacitance is represented by a
capacitor in circuits and measured in farads (F). A farad is a
very large value of capacitance. A more likely value of
capacitance would be 0.01 µF (1x10-8 F).
The symbol used to represent a capacitor is
+
-
schematic capture
actual representation
Some capacitors, especially electrolytic capacitors, are polarized.
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Capacitance Continued
Continued…
When analyzing a steady-state DC circuit, capacitors act as
open circuits—meaning there is no steady-state DC current
flowing through them (infinite resistance).
C
R1
V
R=∞Ω
C
R2
R1
R3
V
R2
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
R3
Inductance
Continued…
When analyzing a steady-state DC circuit, inductors act as
short circuits— meaning that steady-state current is passed
directly through them (zero resistance).
L
R=0Ω
R1
V
L
R2
R1
R3
V
R2
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
R3
Material Properties
Resistivity (ρ)– Resistivity is the intrinsic property that accounts
for the nature of a material. It is defined as the ability of a material
to resist electrical conduction, with units ohm-meter (Ωm).
The resistance of a material is related to its resistivity such that:
R = ρ (L/A) where,
L
ρ = resistivity of material
Some
Material
I
h
w
L = length of conductor which
current flows along
A = cross-sectional area of
conductor that current flows
through
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
 Increasing the length of the resistor increases
the resistance
 Increasing the cross sectional area of the
resistor decreases the resistance
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Material Properties
Continued…
Conductivity (σ) – Conductivity is the inverse of resistivity. It is
defined as the ability of a material to conduct electricity, with
units inversed ohm-meter (Ωm-1).
The conductance of a material is related to its conductivity by:
G = σ (A/L) = 1/R, where
L
σ = conductivity of material
Some
Material
I
h
w
L = length of conductor which
current flows through
A = cross-sectional area of
conductor that current
flows through
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
 Increasing the cross sectional area of the
conductor increases the conductance
 Increasing the length of a conductor decreases
the conductance
© 2002 University of North Carolina at Charlotte, ALL RIGHTS RESERVED
Ohm’s Law
Voltage  Current  Resistance
V  I R
V = Voltage
I = Current
R = Resistance
(Volts = V)
(Amperes = A)
(Ohms = Ω)
Ohm’s Law continued
The total resistance of a circuit is dependant on
the number of resistors in the circuit and their
configuration
Series Circuit
Rtotal  R  R1  R2  ...
Parallel Circuit
1
1 1
1
  
 ...
Rtotal R R1 R2
Kirchhoff’s Current Law
Current into junction = Current leaving junction
I in  I out
The amount of current that enters a junction is
equivalent to the amount of current that leaves the
junction
Iin
I1
I1
I2
I2
Iout
I in  I1  I 2  I out
I in  I out  0
Kirchhoff’s Voltage Law
Sum of all voltage rises and voltage drops
in a circuit (a closed loop) equals zero
Vin  VoltageAcrossEachResistor
Vin  V1  V2  ...
Net Voltage for a circuit = 0
V1
V2
V  V1  V2
V  V1  V2  0
V
Series Circuit
Current is constant
 Why?
– Only one path for the
current to take
V  I R
V  V1  V2  V3
I  I1  I 2  I 3
R  R1  R2  R3
Parallel Circuit
V  I R
V  V1  V2  V3
I  I1  I 2  I 3  I1  I 23
Voltage is constant
where I 23  I 2  I 3
 Why?
1 1
1
1
 

R R1 R2 R3
– There are 3 closed
loops in the circuit
The Light Bulb and its
Components
 Has two metal contacts at
the base which connect
to the ends of an
electrical circuit
 The metal contacts are
attached to two stiff wires,
which are attached to a
thin metal filament.
 The filament is in the
middle of the bulb, held
up by a glass mount.
 The wires and the
filament are housed in a
glass bulb, which is filled
with an inert gas, such as
argon.
Light bulbs and Power
Power dissipated by a bulb relates to the
brightness of the bulb.
The higher the power, the brighter the bulb.
Power is measured in Watts [W]
2
V
P  I2  R  V  I 
R
For example, think of the bulbs you use at home.
The 100W bulbs are brighter than the 50W
bulbs.
Bulbs in series experiment
One bulb connected to the batteries. Add another
bulb to the circuit in series.
Q: When the second bulb is added, will the bulbs
become brighter, dimmer, or not change?
 We can use Ohm’s Law to approximate what will
happen in the circuit in theory:
V  IR
P V I
Bulbs in series experiment
continued…
V
Recall:V  I  R  I 
R
When we add the second lightbulb:
V supplied doesn't change, but R increases
 I for the circuit decreases (but I1  I2 )
P  V  I  decreases
 The bulbs get dimmer
because the power dissipated decreases
Bulbs in parallel experiment
One bulb connected to the batteries. Add a
second bulb to the circuit in parallel.
Q: What happens when the second bulb is
added?
 We can use Ohm’s Law to approximate what will
happen in the circuit:
V  IR
P V I
1
1
1


R R1 R2
Bulbs in parallel experiment
continued…
V
V  IR  I 
R
P V I
1
1
1
1


R
1
1
R R1 R2

R1 R2
V constant for the circuit, R decreases  I increases
 P increases as R decreases
The bulbs do not change in brightness,
but the total power of the circuit is increased
How to use a voltmeter:
Voltmeter:
- connect either end of the meter to each side of
the resistor
If you are reading a negative value, you have the
probes switched.
There should be no continuity beeping. If you hear
beeping, STOP what you are doing and ask
someone for help!
Voltmeter
Measuring Voltage
Voltage:
Probes connect
to either side of
the resistor
Breadboards
 You encountered breadboards early in the year.
Let’s review them:
The breadboard
How the holes
on the top of the
board are
connected:
Series
Resistors are connected
such that the current can
only take one path
Parallel
Resistors are connected
such that the current can
take multiple paths