DC Circuits Lab

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DC Circuits Lab

• ECE 002
• Professor Ahmadi

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Outline

• Basic Components of a Circuit
• Series Circuit
• Parallel Circuit
• Ohms Law
• Lab Overview

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Basic Circuit Components
We represent real electrical components with
symbols
A Battery
can be represented with this symbol
called a DC voltage source

• A DC Voltage Source
• Provides Power for our circuit
• Battery or Lab power supply is an example
• DC voltage is supplied across the two terminals
• Its voltage is VOLTS (V)

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Basic Circuit Components
We represent real electrical components with
symbols
can be represented with this symbol
called a resistor
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Basic Circuit Components
We represent real electrical components with
symbols
The Earth
can be represented with this symbol
called the ground symbol
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Basic Circuit Components
We represent real electrical components with
symbols
A Tollboothor any barrier
can be represented with this symbol
called a diode
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Basic Circuit Components
The Diode is Like a Switch That Takes 0.7V To
Close

• The Diode Has Two Modes of Operation
• Negative DC Voltage Source
• When the Anode is at least 0.7V. Replace the
  diode by a -0.7V DC Source.
• Open Circuit
• When the Anode is less than 0.7V, the diode is
  an open circuit. This means no current can flow
  through it!

0.7V

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Building a Circuit

• We wish to power our flashlights light bulb

• We need a battery

• We need to attach the light bulb to the battery

• We use wires to connect the light bulb to the
  battery

Insteadlet's represent the real components with
their symbols
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Building a Circuit
creating a schematic

• Replace the battery with a DC Voltage Source
  symbol

• Replace the light bulb with a Resistor symbol

.5 O

• Mark the symbols values (V, R, etc.)

Since this node is at GND (OV) this node
must be 1.5Volts higher

• Add the Ground reference

0V
Insteadlet's represent the real components with
their symbols
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Analyzing the Circuitusing Ohms Law

• When we attach the resistor to the DC voltage
  source, current begins to flow

• How much current will flow?

.5K O

• Ohms Law (VIR)
• -gtDescribes the relationship between the voltage
  (V), current (I), and resistance (R) in a circuit

• Using Ohms Law, we can determine how much
  current is flowing through our circuit

0V
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Analyzing the Circuitusing Ohms Law

• How much current will flow?

• Use Ohms Law
• V I x R
• 1.5V I x .5K O
• Solve for I
• I 1.5V / .5 KO 3 mA

.5K O
0V
So, 3 mA will flow through the .5kO resistor,
when 1.5 Volts are across it
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Resistors in Series

• Resistors connected by only 1 terminal,
  back-to-back, are considered to be in series

R1 .5K O

• We can replace the two series resistors with 1
  single resistor, we call Req
• The value of Req is the SUM of R1 R2
• ReqR1R2.5K O .5K O 1KO

R2 .5K O
Req 1K O
0V
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Resistors in Series

• Now we can find the current through the circuit
  using Ohms Law

• Use Ohms Law
• V I x Req
• 1.5V I x 1K O
• Solve for I
• I 1.5V / 1K O 1.5 mA

Req 1K O
0V
The bigger the resistance in the circuit, the
harder it is for current to flow
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Resistors in Series

• Back to our original series circuit, with R1 and
  R2

• The current is the SAME through each resistor

R1 .5K O

• Current flows like water through the circuit,
  notice how the 1.5 mA stream of current flows
  through both resistors equally

R2 .5K O

• Ohms Law shows us voltage across each resistor
• V(R1) 1.5mA x .5K O .75V
• V(R2) 1.5mA x .5K O .75V

0V
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Resistors in Parallel

• Resistors connected at 2 terminals, sharing the
  same node on each side, are considered to be in
  parallel

• Unlike before, we cannot just add them. We must
  add their inverses to find Req

R1 .5K O
R2 .5K O
0V
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Resistors in Parallel

• This is the equivalent circuit

• Use Ohms Law, we find the current through Req
• V I x Req
• 1.5V I x .25K O
• Solve for I
• I 1.5V / .25KO 6 mA

Req .25K O
0V
The smaller the resistance in the circuit, the
easier it is for current to flow
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Resistors in Parallel

• Back to our original series circuit, with R1 and
  R2

• The current is NOT the SAME through all parts of
  the circuit

• Current flows like water through the circuit,
  notice how the 6 mA stream of current splits to
  flow into the two resistors

R1 .5K O
R2 .5KO

• The Voltage across each resistor is equal when
  they are in parallel

0V
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Resistors in Parallel

• The voltage is 1.5 V across each resistor

• Ohms Law tells us the current through each
• I(R1)V/R 1.5V /.5KO 3mA
• I(R2)V/R 1.5V /.5KO 3mA

• The 6mA of current has split down the two legs of
  our circuit
• It split equally between the two legs, because
  the resistors have the same value

R1 .5K O
R2 .5K O
0V
The current will split differently if the
resistors are not equal
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Resistors in Parallel

• This is the equivalent circuit

• Use Ohms Law, we find the current through Req
• V I x Req
• 1.5V I x .25K O
• Solve for I
• I 1.5V / .25K O 6 mA

Req .25K O
0V
The smaller the resistance in the circuit, the
easier it is for current to flow
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Including a Diode

• Steps to Analyze the Circuit

Anode 1.5V

• First, is the anode potential at least 0.7V?

• Yes, it is at 1.5V. So, replace the diode with a
  -0.7V DC Source.

R .5K O
0V
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Including a Diode

• Steps to Analyze the Circuit

• Voltage sources in series can be combined.

0.7V

• 1.5V (-0.7)V 0.8V
• Use that 0.8V value as the V in Ohms Law!

R .5K O
0V
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Including a Diode

• Steps to Analyze the Circuit

• Now, how much current will flow through R?

• Use Ohms Law
• V I x R
• 0.8V I x .5K O
• Solve for I
• I 0.8V / .5 O 1.6 mA

R .5K O
0V
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Including a Diode

• Check Your Answer

• The Voltage on the Left (From the DC Source)
  Should equal the Voltage Drops on the Right.

0.7V

• Use Ohms Law For the Resistor
• VR I x R
• 0.8V 1.6mA x .5K O
• For the Diode
• VD 0.7V
• Add the Voltage Drops
• VR VD 0.8V0.7V 1.5V
• This matches our voltage sourceYAY!

R .5K O
0V
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Including a Diode

• Steps to Analyze the Circuit

Anode 0.5V

• First, is the anode potential at least 0.7V?

R .5K O

• No, it is at 0.5V. Therefore, no current can
  flow through the resistor.

0V
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In Summary

• Ohms Law VIR
• Describes the relationship between the voltage
  (V), current (I), and resistance (R) in a circuit
• Current is equal through two resistors in series
• Voltage drops across each resistor
• Req R1 R2 . . .
• Voltage is equal across two resistors in parallel
• Current splits through branches of parallel
  circuits
• 1/Req 1/R1 1/R2

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In Summary

• Diodes
• There is a voltage cost associated with every
  diode.
• Current will only flow through the diode if the
  voltage at the anode is to that cost.

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In Lab Today

• You will build series circuits
• Build parallel circuits
• Work with a breadboard
• Verify Ohms Law by measuring voltage using a
  multimeter
• And yes, there is HW!