E1 – Electrical Fundamentals # 3 – Meters, Circuits, Loads and Switches

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


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


Using a Voltmeter

Load

Line Voltage 120V


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


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)


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


Switch

Switch

Checking Switches with a Voltmeter

LoadSwitchOpen? Or ?Closed

SwitchOpenSwitchClosed

Line Voltage 240V


Checking for “Continuity”

• Determine if the wiring within a load is continuous– Example: Checking a resistance heater


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


Ammeters (Amp Meters)

• Current flow creates a magnetic field • Ammeters measure the intensity of the field


Measuring Current in Amperes

Power In

Current produces a magnetic field

Ammeter measures the intensity (I) of the magnetic field

OFFV

AMPSΩ

AMPS


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


Neutral

Hot

Measuring Current Flow

Power OFF

No current

Power ON

Current flow

Heater energized12

0 v

COM V/

VAC

DC


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.


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!


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


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


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


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 +…


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

+ + +


Measuring Voltage in Series Circuits

• All loads share the available voltage• The voltage of each load drops as more

loads are added


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


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


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


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


Measuring Amperage in Parallel Circuits

• An ammeter Þ Measures each circuit Þ Also verifies total amperage


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


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!


Single Load


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!


Two Loads


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!


Three Loads


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


A/C-Heating Unit Parallel Circuits

Load 2 Load 3 Load 4 Evap

MtrComp

Cond

Mtr

L2

L1

Load 1 Electric

Heater


Diagram Development

• A schematic diagram is also called a “ladder diagram”

• The rungs of the ladder are parallel circuits


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


Diagram Set-Up

• The left side is usually considered the main power

• The right side is usually considered common


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


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


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


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E1 – Electrical Fundamentals # 3 – Meters, Circuits, Loads and Switches