E1 – Electrical Fundamentals
# 3 – Meters, Circuits, Loads and Switches
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 2
Meter Types
• Voltmeter – measures voltage• Ohmmeter – measures resistance (ohms)• Ammeter – measures current (amps)• Multimeter – a combination meter that
measures volts, ohms, & amps
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 3
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)
COM V/
OHMS VOLTS
AC
DC
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 4
Using a Voltmeter
Load
Line Voltage 120V
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 5
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
COM V/
OHMS VOLTS
AC
DC
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How to Read an Ohmmeter
No Resistance
Measurable resistance
Infinite Resistance
(Broken wire or open switch)
Good for loads (coils, heaters, and motors)
(Short or closed circuit)
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 7
Using a voltmeter to check switch contacts
• Checking switches with power on the circuit
• The voltmeter can show whether they are open or closed
COM V/
OHMS VOLTS
AC
DC
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Switch
Switch
Checking Switches with a Voltmeter
LoadSwitchOpen? Or ?Closed
SwitchOpenSwitchClosed
Line Voltage 240V
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 9
Checking for “Continuity”
• Determine if the wiring within a load is continuous– Example: Checking a resistance heater
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 10
COM V/
VAC
DC
Checking Continuity
Neutral
Hot
Power OFF 1200 Watt Heater
Prove heater wire is broken
An open circuit has infinite resistance
120
vDisconnect wires
Disconnect wires
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 11
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
OFFV
AMPSΩ
AMPS
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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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Neutral
Hot
Measuring Current Flow
Power OFF
No current
Power ON
Current flow
Heater energized12
0 v
COM V/
VAC
DC
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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.
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 16
120v
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!
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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
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 18
120 V
Ohm’s Law: I = E R or Amps = Volts Ohms
Rt = 40 Ω
R1 = 4 Ω
R4 = 14 Ω
R2 = 10 Ω
R3 = 12 Ω
L1
N
40 Ω 120 Volts
Itotal = = 3 Amps
Calculating Series Circuit Amperage
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 19
It = 120v 40Ω = 3 Amps
Measuring Series Circuit Amperage
It = I1 = I2 = I3 = I4 = 3 Amps
Rt = 40 Ω
R1 = 4 Ω
R4 = 14 Ω
R2 = 10 Ω
R3 = 12 Ω
L1
N
120 Volts
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 20
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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The voltage drop across each load is E = IR, or volts = amps x ohms
120 Volts
E1 =3A x 4Ω=12v
E4 =3A x 14Ω=42v
E2 =3A x 10Ω=30v
E3 =3A x 12 Ω=36v
Calculating Voltage Drop in a Series Circuit
L1
N
= 120v Et =
12v 30v
36v42v
+ + +
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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
L1
N
COM V/
VAC
DC
COM V/
VAC
DC
120v
Why does adding bulbs to the circuit make them all dimmer?Because there is less voltage available to each bulb.
COM V/
VAC
DC
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120v
What happens when a series circuit is opened?
L1
N
120v
L1 N
Why?All loads are de-energized because
the flow of current is interrupted.
That is why switches and controls are in series with the loads they control.
120vCircuit is open
No current
flow
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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
R1=4Ω
R2=10Ω
L1
L2
Each load receives the same voltage
COM V/
VAC
DC
COM V/
VAC
DC
COM V/
VAC
DC
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Measuring Amperage in Parallel Circuits
• An ammeter Þ Measures each circuit Þ Also verifies total amperage
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Measuring Amperage in Parallel Circuits
Each circuit is measured.Amperage increases with the number of loads.
I1=120v/4Ω
I1= 30A
I2=120v/10Ω
I2= 12A
L1
L2
I3=120v/12Ω
I3= 10A
I4=120v/14Ω
I4 = 8.6A
Itotal = I1 + I2 + I3 + I4 + …
120
Vol
ts
Itotal = = 60.6A+ + +12A 10A 8.6A30A
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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
Go Team!
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 30
Single Load
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 31
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
Go Team!
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 32
Two Loads
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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
Go Team!
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 34
Three Loads
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 35
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
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 36
A/C-Heating Unit Parallel Circuits
Load 2 Load 3 Load 4 Evap
MtrComp
Cond
Mtr
L2
L1
Load 1 Electric
Heater
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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(Ladder Diagram) A/C-Heating Unit Parallel circuits
Load 2 Load 3 Load 4 Evap
MtrComp
Cond
Mtr
L2
L1
Load 1 Electric
Heater
Load 2
Load 3
Load 4
Evap
Mtr
Comp
Cond
Mtr
L2L1
Load 1Electric
Heater
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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(Ladder Diagram) L2L1
The left side (L1) is the “hot” side
The right side (L2) is the “common” side.
On a 120v circuit this side would be the “neutral”.
Load 2
Load 3
Load 4
Load 1
Evap
Mtr
Comp
Cond
Mtr
Electric
Heater
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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
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 42
L2L1
Load 2
Load 3
Load 4
Load 1
Evap
Mtr
Comp
Cond
Mtr
Electric
Heater
Series - Parallel Circuits
A cooling thermostat and pressure control in series with the compressor
A heating thermostat in series with the heater
A disconnect switch
© 2005 Refrigeration Training Services - E1#3 Meters, Circuits, Loads & Switches v1.2 43
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