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Analog and Digital Meters

Devices and Measurements

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Objective

At the conclusion of this presentation the student will describe

and identify:

Safety precautions when using test equipment

Analog Multimeters

Digital Multimeters

How to connect a voltmeter to a circuit to measure voltage

How to connect an ammeter to a circuit to measure current How to connect an ohmmeter to measure resistance

The student will be able to:

Read voltage from an analog voltmeter

Read current from an analog ammeter

Read resistance from an analog ohmmeter

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Basic Circuit Measurements

A voltmeter measures voltage across (in parallel) a

resistance or load

An ammeter is inserted in the current path (in series) to

measure current

Resistance is measured across a resistor, out-of-circuit,

with an ohmmeter

Digital Multimeters (DMMs) measure voltage, current

and resistance

5 February 2004 Meters 4

Electrical Test Equipment - The Multimeter

A multimeter is used to measure voltage, current or

resistance, depending upon the function selected

A voltmeter is used to measure voltage across a

component or circuit

An ammeter is used to measure current through a

circuit An ohmmeter is used to measure resistance

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Typical dArsonval Meter Movement

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Meter Deflection is Proportional to Current

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A Typical Seven Segment Display

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Examples of Meter Displays and Symbols

Each of the above may be used to represent either an

ammeter (A), a voltmeter (V), or an ohmmeter ()

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Typical Portable Multimeters

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Example of a Digital and Analog Multimeter

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Typical Digital Multimeters (DMM)

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A Typical Analog Multimeter

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Selecting the Proper Range on an Analog Meter

Select a range for maximum deflection of the pointer

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Measuring a Resistor

Remove power if

necessary and isolate

the component from

the circuit under test

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Schematic Symbols for Meters

The analog voltmeter and ammeter must be connected to the

circuit with proper polarity to avoid damaging the meter

movement

The ohmmeter may only be used on unenergized components

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Voltage Measurements

To measure voltage, the voltmeter is placed in parallel across

the component; that is, one lead is place on each side of the

component

Set an analog ammeter to the highest range when measuring an

unknown voltage

Then switch to the next lowest rage until maximum deflection

of the pointer is obtained while remaining on the scale

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A Voltmeter Connected to Measure Voltage

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Current Measurements

To measure current, the ammeter must be placed in series with

the component; that is, it must be in line with the current path

Usually, power is removed from the circuit before connecting

or removing the ammeter

Set an analog ammeter to the highest range when measuring an

unknown current

Then switch to the next lowest rage until maximum deflectionof the pointer is obtained while remaining on the scale

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An Ammeter Connected to Measure Current

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Resistance Measurements

To measure resistance

If necessary circuit power must be removed

Usually at least one lead of the component must be

removed from the circuit

the ohmmeter is connected across a component

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Ohmmeter Connection to Measure Resistance

Reading the Analog Meter

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Example 1

Assume that the VOM shown is on the AC/DC 6 V scale. What is

the reading?

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00

0AC

DC

ACD

C000

50

10

2

52

1

100

20

410

4

2

150

30

6

200408

208

4

2

505010

2510

5

30060123012

6

ANALOG MULTIMETER

ACAMPS

dBm

dBm

OHMS

0

1

2345

10

20

20

50

OHMS

1k

AC

AMPS 10

42

02

4 6 8

10

11

Example 1 Solution

Assume that the VOM shown is on the AC/DC 6 V scale. What is

the reading?4.5 V Note that the reading is made on the 60

V scale and the user places the decimal.

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Example 2

Assume that the VOM shown is on the AC AMPS 3 A scale. What

is the reading?

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00

0AC

DC

ACD

C000

50

10

2

52

1

100

20

4

104

2

150

30

6

20040 8

208

4

25050

10

2510

5

30060123012

6

ANALOG MULTIMETER

ACAMPS

dBm

dBm

OHMS

0

1

2345

10

20

20

50

OHMS

1k

AC

AMPS 10

42

02

4 6 8

10

11

Example 2 Solution

Assume that the VOM shown is on the AC AMPS 3 A scale. What

is the reading? 1.25 A Note that the reading is made on the 30 A ACscale and the user places the decimal place.

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Example 3

When you change resistance scales on a VOM, what must first be

done before measuring a resistor?

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Example 3 Solution

When you change resistance scales on a VOM, what must first be

done before measuring a resistor?

Zero the meter by shorting the leads together and adjusting the

Zero Adjust control.

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Example 4

Assume that the VOM shown is on the x100 scale. What is thereading?

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Problem 4 Solution

Assume that the VOM shown is on the x100 scale. What is thereading? 340 Note that the reading is made on the scale

and the user multiplies the reading by 100.