IEEE’s
Hands on Practical Electronics (HOPE)
Lesson 2: Voltage, Current, Resistance
Last Week
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Course Introduction
Safety
Soldering
Terminology
This Week
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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
• 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
• Electron Flow
– However, in reality, electrons
move in the opposite direction!
– Current is conventionally
defined as the movement of
positive charge
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
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.
Using the Multimeter
To measure voltage:
-Turn on multimeter!
-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:
-Switch the meter to “A” mode
-Switch the meter to “Ohm”
mode
-OPEN the current circuit
-Complete the circuit with the
two wires of the multimeter
- 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