Transcript E1 Subject 2

E1 – Electrical Fundamentals
# 3 – Meters, Circuits, Loads
and Switches

Meter Types
•
•
•
•
Voltmeter – measures voltage
Ohmmeter – measures resistance (ohms)
Ammeter – measures current (amps)
Multimeter – a combination meter that
measures volts, ohms, & amps
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Voltmeters
• Measure electromotive force of a circuit in volts
• Always set meter at the highest voltage scale
to prevent meter damage
• 1 Volt = 1,000 millivolts (mV)
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Using a Voltmeter
Line Voltage 120V
Load
OHMS
VOLTS
AC
DC
V/
COM
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Ohmmeter
• The meter uses an internal battery to push voltage
through a device
– The resistance encountered by the battery’s
current is measured in ohms.
• Open: Infinite resistance (∞ or OL)
– Example: Switch open, broken wire, etc.
• Closed or Short: No resistance (0)
– Example: Switch closed, wires connected, or
shorted winding
• Measurable resistance: Any value between 0 - ∞
– Example: Resistance of a motor winding or heater
wire
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How to Read an Ohmmeter
No Resistance
(Short or closed circuit)
OHMS
VOLTS
Infinite Resistance
(Broken wire or open switch)
AC
DC
Measurable resistance
Good for loads (coils, heaters, and motors)
V/
COM
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Using a voltmeter to check switch contacts
• Checking switches with power on the
circuit
• The voltmeter can show whether they are
open or closed
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Checking Switches with a Voltmeter
Line Voltage 240V
Open
Switch
? Or ?
Closed
Load
Switch
Closed
Open
OHMS
VOLTS
AC
DC
V/
COM
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Checking for “Continuity”
• Determine if the wiring within a load is
continuous
– Example: Checking a resistance heater
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Checking Continuity
Prove heater wire is broken
Disconnect wires
120 v
Neutral
Power OFF
1200 Watt
Heater
Hot

Disconnect wires
DC
An open circuit has infinite resistance
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VAC
COM
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V/
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Ammeters (Amp Meters)
• Current flow creates a magnetic field
• Ammeters measure the intensity of the field
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Measuring Current in Amperes
Power In
Current produces a magnetic field
Ammeter measures the
intensity (I) of the magnetic field
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Using an Ammeter
• Current intensity is measured in amperes
– 1 Amp = 1,000 milliamps (mA)
• Most common ammeter is a “Clamp-on”
type
– Meter jaws must encircle only one wire
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Measuring Current Flow
120 v
Neutral
Power
Power OFF
ON
Heater energized
Hot

VAC
DC
COM
Current
flow
No current
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V/
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Series Circuit
• Only one path for electrons to flow.
• Current must be able to go through one
device before it can go to the next device.
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Series Circuits
A string of "old-fashioned " Christmas tree
lights is an example of a series circuit.
Simple wiring, but if one blows out – all the lights go out!
120v
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Amperage in series circuits
• The more loads in a series circuit, the
greater the total resistance
• The greater the resistance, the lower the
total amperage (I = E/R or A = V/R)
• The amperage will be the same everywhere
in the circuit
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Calculating Series Circuit Amperage
Ohm’s Law: I = E  R
or Amps = Volts  Ohms
L1
R1 = 4 Ω
R2 = 10 Ω
Rt = 40 Ω
120 Volts
120
V
R4 = 14 Ω
N
Itotal =

R3 = 12 Ω
= 3 Amps
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Measuring Series Circuit Amperage
It = I1 = I2 = I3 = I4 = 3 Amps
L1
R1 = 4 Ω
120 Volts
Rt = 40 Ω
R4 = 14 Ω
N
R2 = 10 Ω
R3 = 12 Ω
It = 120v  40Ω = 3 Amps
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Calculating Voltage in Series Circuits
• All loads share the available voltage
• The total voltage is the sum of all the
voltage drops across each load:
• Etotal = E1 + E2 + E3 + E4 +…
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Calculating Voltage Drop in a Series Circuit
The voltage drop across each load is E = IR,
or volts = amps x ohms
L1
E1 =3A x 4Ω=12v
12v
E2 =3A x 10Ω=30v
30v
120 Volts
E4 =3A x 14Ω=42v
42v
N
Et =
+
+
E3 =3A x 12 Ω=36v
36v
+
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= 120v
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Measuring Voltage in Series Circuits
• All loads share the available voltage
• The voltage of each load drops as more
loads are added
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Bulb dims as more bulbs are added
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VAC

DC
COM
V/
VAC

DC
COM
V/
VAC
DC
COM
V/
L1
120v
N
Why does adding bulbs to the circuit make them all dimmer?
Because there is less voltage available to each bulb.
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What happens when a series circuit is opened?
Why?
All loads are de-energized because
the flow of current is interrupted. 120v
L1
120v
N
120v
No
Circuit L1 N current
is open
flow
That is why switches and controls are in series
with the loads they control.
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Parallel Circuits
• Loads are parallel to each other, not in
series
• There is more than one path for electrons to
flow
• Therefore:
 Each load receives full voltage
 Each load can operate independently
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Measuring voltage in parallel circuits
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VAC
VAC

DC
VAC
DC
DC
COM
COM
V/
COM
V/
V/
L1
R1=4Ω
R2=10Ω
L2
Each load receives the same voltage
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Measuring Amperage in Parallel Circuits
• An ammeter
 Measures each circuit
 Also verifies total amperage
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Measuring Amperage in Parallel Circuits
Each circuit is measured.
Amperage increases with the number of loads.
120 Volts
L1
I1=120v/4Ω
I1= 30A
I3=120v/12Ω
I3= 10A
I2=120v/10Ω
I2= 12A
I4=120v/14Ω
I4 = 8.6A
L2
Itotal = I1 + I2 + I3 + I4 + …
Itotal = 30A + 12A + 10A + 8.6A = 60.6A
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Single Load Resistance in a Parallel Circuit
• One load provides the only path for current
flow
• Its resistance is the total circuit resistance
• The following slide compares resistance to
crossing a river:
– Resistance is the open space between the
shores
– Cars represent electrons
– Bridges represent loads
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Single Load
Go Team!
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Two Loads in a Parallel Circuit
• Two loads provide two paths for electrons
• More total current flow than a single circuit
• The total resistance is less than that for a
single load
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Two Loads
Go Team!
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Three Loads in a Parallel Circuit
• Three loads provide three paths for electrons
• More total current flow than with one or two
circuits
• Because the total resistance is less than
with only one or two loads
– The total resistance decreases as the number of
loads increase
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Three Loads
Go Team!
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Simple Diagram of Parallel Circuits
• The following slide shows how the loads in
an air conditioning unit with electric heat
might be sketched into a simple diagram
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A/C-Heating Unit Parallel Circuits
L2
Load 1
Electric
Heater
Load 2
Load 3
Evap
Mtr
Load 4
Comp
Cond
Mtr
L1
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Diagram Development
• A schematic diagram is also called a
“ladder diagram”
• The rungs of the ladder are parallel circuits
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Schematic Diagram
A/C-Heating
UnitDiagram)
ParallelL2 circuits
(Ladder
L1
Electric
Heater
L2
Load 1
Electric
Heater
Load 2
Load 1
Load 3 Load 4
Load 2
Evap
Mtr
Comp
Cond
Mtr
Load 3
Comp
L1
Cond
Mtr
Load 4
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Diagram Set-Up
• The left side is usually considered the
main power
• The right side is usually considered
common
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Schematic Diagram
L1
The left side (L1)
is the “hot” side
(Ladder Diagram)
Electric
Heater
Evap
Mtr
Load 1
Load 2
L2
The right side (L2) is
the “common” side.
On a 120v circuit this
side would be the
“neutral”.
Load 3
Comp
Cond
Mtr
Load 4
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Series – Parallel Circuits
• Controls and switches are in series with
loads
• An open switch stops current to any load
in that one circuit
• A disconnect switch in the main power line
stops current to all circuits after it
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Series - Parallel Circuits
L2
L1
A disconnect switch
Electric
Heater
A heating thermostat
in series with the
heater
Evap
Mtr
A cooling thermostat
and pressure control
in series with the
compressor
Load 1
Load 2
Load 3
Comp
Cond
Mtr
Load 4
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