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.

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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.

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

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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.

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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.

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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