Chapter 13

AC Measurements

2

• Objectives– After completing this chapter, the student

should be able to:• Identify the types of meters available for AC

measurements.

• Identify the types of meter movements used to make AC measurements.

• Explain the function of an oscilloscope.

3

• Identify the basic parts of an oscilloscope and explain their functions.

• Demonstrate the proper setup of an oscilloscope.

• Describe how to use an oscilloscope to make a measurement.

• Explain how a counter works.

• Identify the basic parts of a counter.

4

• AC Meters– Moving-coil meter movement.

• referred to as d’Arsonval meter movement.

• designed to measure DC current.

• AC current must be converted to DC current to be measured.

• The process is called rectification.

• The rectifiers convert the sine wave into a pulsating DC current.

5

6

– Iron-vane meter movement• Does not require the conversion of AC to DC.

• Consists of two iron vanes within a coil.– One stationary vane

– One movable vane

• Used primarily for 60 hertz applications.

7

8

• Clamp-on meter– Uses a split-core transformer.– Used for measuring high values of AC current.

9

10

• Oscilloscopes– Most versatile piece of test equipment available

for working on electronic equipment and circuits.

– Provides a visual display of what is occurring in the circuit.

11

12

• Oscilloscopes provide:– The frequency of a signal.– The duration of a signal.– The phase relationship between signal

waveforms.– The shape of a signal’s waveform.– The amplitude of a signal.

13

• The basic parts of an oscilloscope are:– A cathode ray tube

(CRT).

– A sweep generator.

– Horizontal and vertical deflection amplifiers.

– Power supplies.

14

• Cathode-ray tube (CRT)– A phosphor screen.

– Deflection plates.

– An electron gun.

15

• Faceplate– Marked in centimeters along the vertical and

horizontal axis.– Can be calibrated with a known voltage before

testing an unknown signal.– Called a graticule, and is separate from the

oscilloscope.– Mounted in front of the CRT.

16

• Power switch– Usually on the front panel.– May be a toggle, push-button or rotary switch.– Mounted separately or with another switch.– Used to apply line voltage to operate the

oscilloscope.

17

• Intensity switch– Also called brightness.– Controls the electron beam within the CRT.– It is a rotary control.– Too much intensity for too long can burn a hole

or etch a line in the phosphor screen.

18

• Focus and astigmatism controls– Connected to the electron gun.– Used to adjust the electron beam size and

shape.– Rotary controls.

19

• Horizontal and vertical position controls– Rotary controls.– Allows the electron beam to be positioned

anywhere on the face of the CRT.

20

• Horizontal block– Consists of:

• A vertical input jack.

• An AC/DC switch.

• A volts/cm rotary switch.

– The oscilloscope probe is connected to the input jack.

– The probe is then connected to the circuit to be tested.

21

• Horizontal block– Also called the time base.– Consists of:

• A time/cm rotary switch.

• A trigger-control switch.

• A triggering level control.

22

• Level control– Sets the amplitude that the triggering signal

must exceed before the sweep generator starts.

• Initial oscilloscope control settings:– Intensity: set to the center of range.– Focus: set to the center of range.– Astigmatism: set to the center of range.– Position: set to the center of range.

23

– Triggering: INT +– Level: AUTO– Tine/cm: 1 msec– Volts/cm: 0.02– Power: ON

24

• Frequency counters– Measures frequency by comparing a known

frequency against an input frequency.– Consist of:

• A time base.• An input-signal conditioner.• A gate-control circuit.• A main gate.• A decade counter.• A display.

25

26

• The electronic counter is used in/on:– electronics repair shops.– engineering departments.– ham radio shacks.– industrial production lines.

27

• Wide use of the electronics counter can be attributed to the integrated circuit, which has:– reduced the size and price.– increased its accuracy.– increased its reliability.– increased its stability.– increased its frequency range.

28

• Bode Plots– Named for H.W. Bode.– Used for studying amplifier feedback.– Required semi-log graph paper.– Required two graphs.

• gain in decibels.

• phase shift in degrees.

29

• Bode plotters today– Computer simulations make them easier to use.– Used to measure voltage gain or phase shift of a

signal.– Produces a graph of circuit’s frequency

response.– Useful in analyzing filter circuits.

30

• In Summary– Measuring AC current with:

• A moving coil meter.

• Iron-vane meter.

• A clamp-on meter.

– An oscilloscope provides:• Frequency of the signal.

• Duration of the signal.

31

• Phase relationships between signal waveforms.

• Shape of the signal’s waveform.

• Amplitude of the signal.

– The basic parts of an oscilloscope are:• Cathode-ray tube (CRT).

• Sweep generator.

• Horizontal deflection amplifier.

• Vertical deflection amplifier.

• Power supply.

32

– Frequency counters– Basic parts of a frequency counter are:

• Time base• Input signal conditioner• Gate-control circuit• Main gate• Decade counter• Display

– Bode plotters