IEEE’sHands on Practical Electronics (HOPE)

Lesson 2: Voltage, Current, Resistance

Last Week

• Course Introduction

• Safety

• Soldering

• Terminology

Last Week’s Circuit

• Consisted of a 9V battery, 1 resistor, and 1 or more LEDs

9V Battery

Resistor

LED

This Week

• History Lesson

• Voltage

• Current

• Resistance

• Digital Multimeter (DMM)

History Lesson

• The common units: Volts, Amps, Ohms, Coulombs are all named after people

Volta Ampere Ohm Coulomb

Voltage

Definition: Difference of electrical potential between two points of an electrical circuit

Units: Volts (V) 1 V = 1 Joules per Coulomb (J/C)

Example: The electrical potential difference between the + and – ends of a battery is

9 V

Voltage Sources

9V

• Two ways to represent a voltage source

Current flows from + to -

Voltage

• There is no absolute number or quantifiable value for voltage.

• Remember integrals? The integral of f(x) is equal to F(x) + C.

• The + C allows us to choose whatever number is most convenient for our calculations.

Voltage

• We will assign a point on our circuit to have 0 volts.

• We will call this ground.

• We will use this symbol to represent ground.

Ground

• We will not spend too much time on this.

• Just know that it is the place on our circuit that we choose to be 0V.

• For more reading see

http://www.ese.upenn.edu/rca/instruments/misctutorials/Ground/grd.html

http://en.wikipedia.org/wiki/Electrical_ground

Current

Definition: Flow (movement) of positive electric charge

Units: Amperes (A) 1 A = 1 Coulomb per second (C/s)

Example: The rate that a stream of water flows is analogous to the amount of current flowing through a circuit

Coulombs

• Symbol: Q• Unit: Coulomb

• 1 coulomb is the amount of electrical charge in 6.241×1018

electrons

• Amps = C/s, current is the amount of electrical charge flowing per second

We will revisit Coulombs when we study Capacitors

p+

• Electron Flow– However, in reality, electrons

move in the opposite direction!

Current Convention

• Conventional Current– Current is conventionally defined

as the movement of positive charge

• It doesn’t matter which way define current flow- Current behaves the same regardless of convention.- But, it is important to use the same convention consistently

e-

Resistance

Definition: Measure of the degree to which an object opposes the passage of an electric current

Units: Ohms (Ω) 1 Ω = 1 Volt per Ampere (V/A)

Example: Hurdles serve as obstacles to a runner, so it requires more energy to overcome them

Circuit Symbols

Battery LEDResistor

Resistors• Resistors are manufactured and labeled with

another convention.

• There are bands of colors used to indicate the resistance of the particular resistor.

• See: http://en.wikipedia.org/wiki/Resistor

Calculating Resistance

• It’s possible to calculate resistance of a resistor using the color bands on it– AB represent a 2 digit number

– C represents the magnitude

– Resistance = AB * 10C + D

• However, we will mainly be measuring resistances with a multimeter

Example: Calculating Resistance

• The first two bands correspond to 4 and 7. The third band tells you the number of zeros following.

47*103 = 47,000 Ω + 10%

Example Resistor Usage

• LEDs are designed to work for approximately 1-2 Volts of power.

• Too much voltage across the LED will cause it to burn out from overheating

• Always put a resistor before (or after) an LED to limit the current.

• You do not want to burn out your LEDs

LED Introduction

• LED = Light Emitting Diode

• Lights up when current flows through it

• LEDs only allow current to go through it in one direction

Current Flows LED’s have 1 lead that is longer than the other. The longer lead is thepositive side. Current flows from the longer lead to the shorter lead.

Putting it all Together

• Battery provides energy to the charges so that they can travel through the circuit

• Resistor opposes the movement of these charges, thus slowing them down.

• Current through the LED provides energy to the LED, which transforms into light.

Example 1: Last Week’s Circuit

i

i

9V

9V

1V

1V

0V

1V drop

8V drop

Example 2

DC

9V

0V

0V

0V

i

i2

9V

9V 1V

9V

8V 1V

i3

Using the Multimeter

To measure current:

-Turn dial to “20mA”

-OPEN the current circuit

-Complete the circuit with the two wires of the multimeter

To measure resistance:

-Turn dial to “2K”

- Touch the 2 wires of the multimeter to the two ends of the resistor

To measure voltage:

-Turn on multimeter by turning dial to “20V”

-Touch one of the wires to the first point in the circuit to measure

-Touch the other wire to a point across the circuit element

Everyday Use

• Multimeters are used to measure voltages and currents at different points on the circuit.

• They are used to diagnose a circuit to see if current is flowing or not (potentially an open circuit or short draining the current)

• A soldering iron can then be used to fix the damage (Week 1)

Digital Multimeter (DMM)

• Combination of– Ammeter: measures current

– Voltmeter: measures voltage

– Ohmmeter: measures resistance

• We will go into more detail on how to use multimeters next week

DMM Usage

• A Digital Multimeter is a measurement device commonly used as a diagnostic tool.

• Fancier multimeters can measure more quantities such as frequency, temperature, conductance, inductance, capacitance and so on.

Today’s Lab

• More practice on soldering

• You can continue building on your last week’s device

• If you feel comfortable soldering, try using the real soldering irons.

Today’s Lab

• Build the following circuit that consists of 1 battery, 2 resistors and 6 LEDs:

9V