Transcript A05_100A

Residential Cabling Technologies
Electrical
Measurements
Copyright © 2005 Heathkit Company, Inc. All Rights Reserved
Objectives
At the end of this presentation,
you will be able to:
2
 Define
the following terms: ammeter,
ohmmeter, voltmeter, open, and short.
 Describe
how to properly use an
ammeter.
 Describe
the proper way to use a
voltmeter.
 Describe
how to properly use an
ohmmeter.
3

Describe how shorts and opens affect circuit
operation.

Explain the advantages of an oscilloscope
compared to an AC voltmeter.

Describe why proper safety precautions are
essential when you take measurements.
4
Measurement Basics
 Electrical
measurements serve two
important purposes:
 Determine
if the circuit is operating properly
 Provide
clues as to why the circuit is not
operating
 The
three most basic measurements are
current, voltage, and resistance.
5
Meters
 To
take accurate measurements in a
circuit, you need a meter.
 There
are two major classes of meters;
Analog and Digital.
 Meters
can be specialized or combined.
6
Analog Meters

Analog meters use
a needle that is
deflected across a
scale by the factor
being measured.

Analog meters
introduce a certain
amount of error in
the process.
7

Here you see an example
of parallax error.

Diagram A represents
the correct viewing
position.

Diagram B illustrates the
effect of viewing from
the right.
8
Digital Meters

Digital meters perform
the same basic
functions, but provide a
digital display or
readout.

The digital display
helps to reduce errors in
the measurement.
9
Multimeters

Multimeters combine
several measurement
functions into one
unit.

Typically three meters
are combined into one
unit.

Multimeters are
available in analog and
digital models.
10
Using a Meter

Read the instruction manual before you use the
meter for the first time.

Observe correct negative and positive polarities
when you measure voltages and current.

Always observe the manufacturers recommended
safety guidelines when you use a meter.

When you need to measure an unknown quantity,
select the highest range available on the meter.
11
A Meter Overview

Analog meters are
usually
multimeters.

Models are made
for special
applications that
perform only one
function.
Meter
Range
Select
12
Range
Scales
Meter
Needle
13
Resistance Ranges
Zero
Adjust
DC Volt
Ranges
DC mA
Ranges
AC Volt
Ranges
Inputs
14
Zeroing the Meter

Analog ohmmeters should be zeroed
before you take any measurements.

To do this:
1.
Select the ohm function and range.
2.
Connect the test leads together.
3.
Adjust the control until the needle indicates
zero.
15


Digital multimeters
come in two major
styles: Manual and
Auto-ranging.
The manual meter
requires the user to
select the proper
function and range.
Display
Range &
Function
Select
Inputs
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Voltage
Ranges
Voltage
Select
Capacitance
Ranges
Capacitor
Reader
Current
Ranges
Ohm
Ranges
Transistor
Tester
17

This is an example
of an auto-ranging
meter.

Here, the user
selects the function
with the dial and
the meter selects
the correct range.
18

How you connect the meter’s test leads is as
important as measurement accuracy.

Three inputs are typical : common, current,
and voltage/resistance.
“Common”
Input
Current
Input
Voltage/Resistance
Input
19
Meter Functions
 Three
functions are common to most
DMM models in use today:
 Ammeter
–measures current flow in a circuit.
 Voltmeter
–measures voltage rises and drops in
a circuit.
 Ohmmeter
–measures resistance in a circuit.
20
Ammeter Function

Ammeters measure current in a circuit.

Current measures a quantity of electrons.

A coulomb is the basic unit of electrical
charge.

One ampere of current flows when one
coulomb of electrons passes a point every
second.
21

To measure current, you
must connect the meter
in series with the circuit
under test.

You must observe the
proper polarity.

You must make the
connection without
power applied to the
circuit.
A
22

To measure the current in a circuit:
1.
Remove power.
2.
Select the correct
function and range.
3.
Break the circuit.
4.
Connect the meter.
5.
Re-apply power and
take the measurement.
1
5
2
3
A
4
23
Voltmeter Function

The voltmeter function is used to measure AC and
DC voltages in a circuit.

When you measure a voltage drop, connect the
meter leads on either side of the load.

When you measure a voltage rise, connect the
meter leads across the voltage supply.

Always observe correct polarity when you
measure voltages.
24

To measure a voltage,
you must connect the
meter in parallel with
the circuit under test.

You must observe the
proper polarity.

You can make the
connection with power
applied to the circuit.

You must take care
when you measure live
circuits.
V
25

To measure a voltage in a circuit:
1.
Select the correct
function and range.
2.
Determine the proper
polarity.
3.
Connect the test leads
in parallel.
4.
3
Take the measurement.
V
1
2
4
26
1. Select AC
2. Select Range
3. Connect Leads
27
1. Select AC
2. Select Range
3. Connect Leads
28
Ohmmeter Function

The ohmmeter function is used to measure
resistances in a circuit.

When you measure a resistance, you must isolate
the component you want to measure.

When you make the measurement, connect the
meter leads across the component.

NEVER attempt to measure resistance while
power is applied to a circuit.
29
 To
measure a resistance in a circuit:
1. Select
the correct
function and range.
2. Disconnect
3. Isolate
power.
the component.
4. Connect
5. Take
the test leads.
measurement.
6. Switch
2
4
3

the meter off.
1
5
6
30
Shorts and Opens

Ohmmeters are also used to detect two types of
defective circuit conditions: shorts and opens.

A short condition occurs when a portion of the
circuit that should offer a resistance appears as if
no resistance were present.

An open condition occurs when a portion of the
circuit that should conduct current acts like an
open switch.
31

A switch can be
used to simulate an
open or a short
condition.

An open appears as
an infinite
resistance.

A short appears as
zero resistance.

32
Safety
 Safety
should be your number one
concern in electronics.
 Current
always seeks the path of least
resistance to ground.
 The
effects of current and voltage can
be influenced by the environment.
33
Introduction to Oscilloscopes
 Voltmeters
measure current and
voltage.
 Voltmeters
provide no information
about the waveform.
 To
properly diagnose AC problems, it
helps to view the waveform.
34

The oscilloscope was
developed to solve
these problems.

Oscilloscopes are
available in different
sizes and styles.

Both Analog and
Digital models are
available.
35
 Analog
oscilloscopes are based upon
the CRT or Cathode Ray Tube.
 The
CRT uses essentially the same
technology as a television set.
 The
CRT converts the input signal into
a visual display that replicates the input
signal.
36
Trace and Blank
Circuits
Amplifier
Circuits
CRT
Input signal
37
Time base
Y-Amplifier
Input
38
 We
need a means to measure the signal
that appears on the CRT screen.
 The
Graticule is a grid that is
positioned in front of the CRT screen.
 The
graticule is divided into squares to
provide a reference for measurement of
the signal.
39

This is a typical
CRT graticule.

Usually the grid
consists of 1 cm
squares.

The horizontal axis
is referred to as the
X-axis and the
vertical axis is the
Y-axis.
X-axis
Y-axis
40
controls
controls
41
Using the Oscilloscope
 An
oscilloscope requires a different set
of skills.
 The
basic functions are time and
amplitude measurements.
 Advanced
features may require
specialized training.
42
 Oscilloscope
basics:
 Always
read the user’s manual before you
attempt to operate an oscilloscope.
 Always
be aware of the limitations of the
oscilloscope.
 Always
anticipate the magnitude of the signal
you want to measure.
 Observe
all electrical safety precautions as well
as any additional precautions provided by the
scope manufacturer.
 Input
probes should be grounded to a common
circuit ground.
43
Positioning

The input cable
connects to the scope
with a BNC connector.

Positioning controls
allow you to make
precise alignment.
BNC Connectors

Volts/Div allows you
to set the voltage per
division.
44

The Volts/Div
setting allows us to
measure amplitude.

If Volts/Div =1

Peak-to-Peak
amplitude is 6
divisions or 6 volts.
Voltage Scale
V
0.5
0.2
1
0.1
50
20
2
10
5
10
20
mV
45

The Time/Div
control adjusts the
timing per division.

Positioning controls
allow us to make
precise alignment.

External triggering
is an advanced
feature.
Time Selection
Position
Display Adj.
External Trigger
46

The time/Div
setting allows us to
measure frequency.

If Time/Div = 20S

The wave period is
equal to 84
microseconds
(4.2 x 20).
Time Base
mS
1
0.5 0.2
0.1
2
50
5
20
10
20
10
S
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Alternate Oscilloscopes

Unique testing and flexibility requirements
have resulted in several new types of
oscilloscopes.

Some are designed to meet specific
performance requirements.

Others are designed with flexibility in mind.
48

Hand-held
oscilloscopes are
smaller and more
portable.

They offer a more
limited number of
functions.

They have unique
safety considerations.
49

Digital PC-based
oscilloscopes come in
a variety of styles.

These are softwarebased devices with
hardware interfaces.

The functions and
features depend upon
the cost and quality of
the software.
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Residential Cabling Technologies
End
Copyright © 2005 Heathkit Company, Inc. All Rights Reserved