Post on 22-Dec-2015
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
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
• 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
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.
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.