IEEE’s Hands on Practical Electronics (HOPE)

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 • 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

Current Convention • Conventional Current • Current is conventionally defined as the movement of positive charge • Electron Flow • However, in reality, electrons move in the opposite direction! p+ e- • 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

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 Resistor LED

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

Current Flows LED’s have 1 lead that is longer than the other. The longer lead is the positive side. Current flows from the longer lead to the shorter lead. LED Introduction • LED = Light Emitting Diode • Lights up when current flows through it • LEDs only allow current to go through it in one direction

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 9V 9V 8V drop 1V drop 1V 0V i 1V

Example 2 9V 0V i2 9V i3 0V 9V 8V 1V 9V 1V 0V i

Using the Multimeter • 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 • 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

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

IEEE’s Hands on Practical Electronics (HOPE)