Investigating Basic Circuits

Post-Activity Discussion

© 2014 Project Lead The Way, Inc.Digital Electronics

Answers the following questions:• What are some of the basic components that make up

simple circuits and what do they do?• What are the important characteristics of a circuit and

how do I measure different parts of a circuit?• How do I work safely with circuits?• How do I measure voltage in a circuit? • How does the arrangement of components affect the

characteristics of the circuit?• How can I use calculations to design circuits before I

start creating one?

This Presentation Will…

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Light Emitting Diode (LED)

3

• In Activity 1.1.2 Investigating Basic Circuits you created a simple circuit similar to the one shown below.

• With the circuit active, what happened when you flipped the LED in the opposite direction?

The LED will not light up.

• What does that tell you about LEDs (a type of diode)?

They are semiconductors that only work in one direction.

Resistors

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• What do you think the role of the resistor is in the circuit?

The resistor protects the LED by limiting the flow of current through it.

Resistor - Component made of material that opposes flow of current and therefore has some value of resistance.

How to Properly Use a DMM

• What happened when you switched the leads?

• Everyone read slightly different values. Why?

5

5 V - 5 V

The DMM still reads the voltage, it is just negative.

Tolerances of components.The voltage sources are slightly different.

How to Properly Use a DMM

• How do you ensure the best precision in reading voltage with the DMM? (most significant figures)

The DMM reading becomes more precise by a factor of ten each time the voltage range is decreased.

6

Range Reading

600V-0V 005V (1 s.f.)20V-0V 4.7V (2 s.f.)2V-0V 1 or +Over

• Why was there no reading at 2V-0V?

The range is too small.

2V-0V range cannot measure 4.7V.

What is Voltage?

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• Now that you can measure it, let’s explore what voltage is in more detail.

• Voltage is the electrical force that causes current to flow in a circuit. It is measured in VOLTS.

• This force can be created by separating charges.

• Voltage has been described many different ways as the science around electricity has evolved.

• We will describe voltage by looking at another common component in electronics called a capacitor.

Capacitors

• A capacitor is an electronic component that can be used to store an electrical charge.

• A capacitor can be thought of as a temporary battery.

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++++++++++

----------

- +

What is Voltage?

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Example:Parallel Plate Capacitor • A battery pushes charge onto

opposite plates which generates an electric field.

• Theoretically, a positive test charge placed in the field has the potential to move.

• Can you guess which way the test charge would move in this electric field?

++++++++++

----------

+Test Charge

Good guess! The test charge has the potential to move left.

(opposites attract)

- +

What is Voltage?

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ExampleParallel Plate Capacitor • If a conductor were to touch

both plates, all the charges one would move to the other.

• This can create a lot of current!

• Be careful when dealing with high voltage capacitors.

++++++++++

----------

- +

Voltage Source: Battery

• A battery is a device that converts chemical energy into electrical energy.

• The chemical reaction provides more charges for a longer time than a capacitor does.

• One side of a battery has the potential to do work

(12V) High Potential (right side of battery)

• One side of a battery has no potential to do work

(0V) Low Potential or Ground (left side of battery)

• The battery would make both test charges move to the right.

- + +Test Charge A

+Test Charge B

What is Voltage?

In order for a charge to move, there must be a separation of charge or a potential difference across two points in the circuit.

Voltage is defined mathematically as

ΔV = V final – V initial

A Volt(V) is a Joule(J) of work per Coulomb (C) of charge.

1V = 1J1C

A 12V battery is able to do 12 Joules of work for every 1 Coulomb of charge the battery can provide. 12

What is Voltage?

Both of these situations read zero volts on the DMM. Why?

(6a) (6b)

There is no separation of charge. For each of these arrangements, the potential difference or voltage across the test points is zero. (6a) ΔV = 5V-5V=0(6b) ΔV = 0V-0V=0

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Current: An Analogy

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Force

The flow of water from one tank to another is a good analogy for an electrical circuit and the mathematical relationship between voltage, resistance, and current.

Force: The difference in the water levels ≡ Voltage

Flow: The flow of the water between the tanks ≡ Current

Opposition: The valve that limits the amount of water ≡ Resistance

Flow

Opposition

- +

Anatomy of a Flashlight

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D - Cell

Switch Switch

LightBulb

LightBulb

BatteryBattery

Block Diagram Schematic Diagram

Flashlight Schematic

• Closed circuit (switch closed)

• Current flow

• Lamp is on

• Lamp is resistance, uses energy to produce light (and heat)

• Open circuit (switch open)

• No current flow

• Lamp is off

• Lamp is resistance, but is not using any energy

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- +- +

Current

Voltage

Resistance

Current Flow

• Conventional Current assumes that current flows out of the positive side of the battery, through the circuit, and back to the negative side of the battery. This was the convention established when electricity was first discovered, but it is incorrect!

• Electron Flow is what actually happens. The electrons flow out of the negative side of the battery, through the circuit, and back to the positive side of the battery.

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ElectronFlow

Conventional Current

Engineering vs. Science

• The direction that the current flows does not affect what the current is doing; thus, it doesn’t make any difference which convention is used as long as you are consistent.

• Both Conventional Current and Electron Flow are used. In general, the science disciplines use Electron Flow, whereas the engineering disciplines use Conventional Current.

• Since this is an engineering course, we will use Conventional Current.

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ElectronFlow

Conventional Current

Ohm’s Law• Defines the relationship between voltage, current, and

resistance in an electric circuit

• Ohm’s Law:

Current in a resistor varies in direct proportion to the voltage applied to it and is inversely proportional to the resistor’s value.

• Stated mathematically:

R

VI

Where: I is the current (amperes)

V is the potential difference (volts)

R is the resistance (ohms)

V

I R

+ -

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Ohm’s Law Triangle

V

I R)A,amperes(

R

VI

),ohms( I

VR

)V,volts( R I V

V

I R

V

I R

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Example: Ohm’s Law

Example:

The flashlight shown uses a 6 volt battery and has a bulb with a resistance of 150 . When the flashlight is on, how much current will be drawn from the battery?

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VT =+

-VR

IR

Schematic Diagram

Example: Ohm’s Law

Example:

The flashlight shown uses a 6 volt battery and has a bulb with a resistance of 150 . When the flashlight is on, how much current will be drawn from the battery?

Solution:

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VT =+

-VR

IR

Schematic Diagram

mA 40 A 0.04 150

6

R

V I R

R

V

V

I R

Circuit Configuration

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What happened when you removed an LED from each of these circuits?

The other LED went out. The other LED remained lite.

Circuit Configuration

Series Circuits• Components are connected

end-to-end.• There is only a single path for

current to flow.

Parallel Circuits• Both ends of the components are

connected together.• There are multiple paths for

current to flow.

24Components (i.e., resistors, batteries, capacitors, etc.)

Components in a circuit can be connected in one of two ways.

Kirchoff’s Voltage Law (KVL)

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• In this circuit we used a 5V power source. • The resistor you measured had roughly 3V across it.• What did you guess would be the voltage across the LED?

VTotal=VR1 + VLED

5V = 3V + 2V

Power Source (a)Voltage across LED and Resistor (b) Voltage across Resistor only 5V 5V 3V

Series Circuits

Characteristics of a series circuit• The current flowing through every series component is equal.• The total resistance (RT) is equal to the sum of all of the resistances

(i.e., R1 + R2 + R3).

• The sum of all of the voltage drops (VR1 + VR2 + VR2) is equal to the total applied voltage (VT). This is called Kirchhoff’s Voltage Law.

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VT

+

-

VR2

+

-

VR1

+ -

VR3

+-RT

IT

Example: Series Circuit

Example:

For the series circuit shown, use the laws of circuit theory to calculate the following:

• The total resistance (RT)

• The current flowing through each component (IT, IR1, IR2, and IR3)

• The voltage across each component (VT, VR1, VR2, and VR3)

• Use the results to verify Kirchhoff’s Voltage Law.

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VT

+

-

VR2

+

-

VR1+ -

VR3

+-RT

IT

IR1

IR3

IR2

Example: Series Circuit

Solution:

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V

I R

k 1.89 1890 R

k 1.2 470 220 R

R3 R2 R1 R

T

T

T

Total Resistance:

mAmp 6.349I I I I

:circuit series a is this Since

mAmp 6.349k 1.89

v 12 I

Law) s(Ohm' R

V I

R3R2R1T

T

T

TT

Current Through Each Component:

Example: Series Circuit

Solution:

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volts 7.619ΩK 1.2mA 6.349 V

Law) s(Ohm' R3I V

volts 2.984Ω 470mA 6.349 V

Law) s(Ohm' R2I V

volts 1.397Ω 220mA 6.349 V

Law) s(Ohm' R1I V

R3

R3R3

R2

R2R2

R1

R1R1

Voltage Across Each Component:

V

I R

Example: Series Circuit

Solution:

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v 12 v 12

v 619.7v 984.2v 397.1 v 12

VV V VR3R2R1T

Verify Kirchhoff’s Voltage Law:

Kirchoff’s Current Law (KCL)

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Note:

• LEDs can be viewed as resistors in this circuit to simplify the discussion.

• The 330Ω resistor was also removed to make the relationship easier to see.

• Why do you think the 330Ω resistor placed in the actual circuit when the components are arranged this way?

• For components that are in series, the current is the same in each component regardless of the resistance values.

• In this circuit configuration, if R1 and R2 have different resistances the current is not the same.

• What would R1 and R2 have in common? Voltage

Parallel Circuits

Characteristics of a Parallel Circuit• The voltage across every parallel component is equal.• The total resistance (RT) is equal to the reciprocal of the sum of the

reciprocal:

• The sum of all of the currents in each branch (IR1 + IR2 + IR3) is equal to the total current (IT). This is called Kirchhoff’s Current Law.

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321

T

321T

R1

R1

R1

1 R

R

1

R

1

R

1

R

1

+

-

+

-

VR1

+

-

VR2 VR3

RT

VT

IT

+

-

Example: Parallel Circuit

Example:

For the parallel circuit shown, use the laws of circuit theory to calculate the following:

• The total resistance (RT)

• The voltage across each component (VT, VR1, VR2, and VR3)

• The current flowing through each component (IT, IR1, IR2, and IR3)

• Use the results to verify Kirchhoff’s Current Law.

3333

+

-

+

-

VR1

+

-

VR2 VR3

RT

VT

IT

+

-

IR1 IR2 IR3

Example: Parallel Circuit

Solution:

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Total Resistance:

volts 15V V V V

:circuit parallel a is this Since

R3R2R1T

59.346R

k 3.31

k 2.2

1

4701

1 R

R1

R1

R1

1 R

T

T

321

T

Voltage Across Each Component:

Example: Parallel Circuit

Solution:

35 mAmp 43.278 346.59

v 15

R

VI

mAmp 545.4 k 3.3

v 15

R3

VI

mAmps 6.818 k 2.2

v 15

R2

VI

mAmps 31.915 470

v 15

R1

VI

Law) s(Ohm' R1

VI

T

T

T

R3

R3

R2

R2

R1

R1

R1

R1

V

I R

Current Through Each Component:

Example: Parallel Circuit

Solution:

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Verify Kirchhoff’s Current Law:

mAmps 43.278 mAmps 43.278

mA 545.4mA 818.6mA 31.915 mAmps 43.278

III IR3R2R1T

Summary of Kirchhoff’s Laws

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Kirchhoff’s Voltage Law (KVL):The sum of all of the voltage drops in a series circuit equals the total applied voltage.

Gustav Kirchhoff1824-1887

German Physicist

Kirchhoff’s Current Law (KCL):The total current in a parallel circuit equals the sum of the individual branch currents.

Up Next

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Now that you have been introduced to some of the basic characteristics, components, and measurement tools used in electronics, we will build on that knowledge in the upcoming activities.

• Scientific & Engineering Notation• Component Identification: Analog Devices• Circuit Theory Laws

Hand Calculations

Simulation

Breadboarding

Analog Versus Digital

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• The circuits we have explored to this point have included only analog components.

• Later we will be learning what some of the digital components are and how they can be used to create desired outputs to a circuit given specific inputs.

The Random Number Generator

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• The Random Number Generator (RNG) is an example circuit that we will use to illustrate all the parts of a complete circuit design.

• It includes an analog section and two digital sections.

Push Button

0 0 0 1 1 1

0 1 1 0 0 1

1 0 1 0 1 0

1 2 3 4 5 6

AnalogSection

SequentialLogic

Section(Digital)

CombinationalLogic

Section(Digital)

Random NumberOutput

PushButtonImput