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IEEE’s Hands 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 9V DC Resistor LED This Week • • • • • History Lesson Voltage Current Resistance Digital Multimeter (DMM) History Lesson Volta Ampere Ohm Coulomb • The common units: Volts, Amps, Ohms, Coulombs are all named after people 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 9V Voltage Sources • Two ways to represent a voltage source 9V 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/misctut orials/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 p+ • Electron Flow – However, in reality, electrons move in the opposite direction! 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 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 the positive 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 9V 9V 8V drop 9V DC 1V drop 1V 0V i 1V Example 2 9V 0V i2 9V i3 0V 9V 8V 1V DC 9V 1V i 0V 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: To measure resistance: -Turn dial to “20mA” -Turn dial to “2K” -OPEN the current circuit - Touch the 2 wires of the multimeter to the two ends of the resistor -Complete the circuit with the two wires of the multimeter 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. 300 Ohms 300 Ohms Note: Check polarity of LEDs Today’s Lab • Build the following circuit that consists of 1 battery, 2 resistors and 6 LEDs: 300 Ohms 300 Ohms 9V Note: Check polarity of LEDs