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AC Measurements

Topics covered in this presentation:

Alternating Current - AC The Oscilloscope

ClassAct SRS enabled.

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Alternating Current - AC

The public electricity supply is usually an alternating voltage.

Direct current (DC) only flows in one direction around a circuit.

Alternating current (AC) flows alternately in each direction around a circuit.

Voltage alternates in the same way as the current.

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1

With an alternating current the current flows in both directions at the same time. Is this true or false?

Question

Answer True or False.

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Alternating Voltage Waveform

The voltage changes from a maximum in one direction to a maximum in the other direction.

Voltage changes over time as shown.

From zero volts, the voltage increases in a positive direction to a maximum before decreasing to zero volts.

The cycle then repeats.

Voltage then changes direction and increases in a negative direction to a maximum before decreasing again to zero volts.

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2

The average voltage value of a single cycle is zero volts. Is this true or false?

Question

Answer True or False.

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Frequency

The number of times the cycle repeats in one second is called the frequency.

Frequency measured in hertz, (Hz).

The cycle repeats many times per second.

1Hz = 1 cycle in one second.

6Hz = 6 cycles in one second.

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3

A) Volts

B) Amps

C) Hertz

D) Seconds

Frequency is measured in which unit?

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If a wave has a frequency of 5 hertz, how many complete cycles will occur in 1 second?

Question 4

Enter your answer and press SEND.

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Frequency and Period

Alternating current of 50Hz, will have 50 cycles in 1 second.

Each cycle takes:

Time taken for one cycle is called the period of the waveform.

.ms20s02.0501

frequency1

period

period1

frequency

If frequency is known, the period can be calculated.

Frequency can be calculated if the period is known.

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5

A) 1

B) 2

C) 5

D) 10

If the period of a wave is 0.5 seconds what is its frequency in hertz?

Question

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Alternating Voltage Values

Alternating voltage continually changes in value over time.

Size of the alternating voltage is described using:

Peak value - maximum instantaneous voltage.

Peak-to-peak value - value between the positive and negative maximum values.

These values are also used to describe the current in the circuit.

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6

The peak to peak voltage will always be larger than the peak voltage for the same wave. Is this true or false?

Question

Answer True or False.

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R.M.S. Values

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1V

Peak value only occurs for a very short time, so cannot be used when calculating power.

An average, called the Root Mean Square (R.M.S.), of all the values in one cycle is taken.

The root mean square value is 70.7% of the peak value.

The r.m.s. voltage is calculated using the formula

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7

A) 6.0V

B) 8.5V

C) 10V

D) 11.5V

If the peak voltage is 12V what will be the approximate RMS voltage for an AC signal?

Question

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AC Measurements with a Multimeter

A multimeter measures an AC waveform in r.m.s. values.

Separate range settings are used for AC measurements.

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Viewing AC Waveforms

An oscilloscope can be used to display an AC waveform.

Measurements can be taken from the display.

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Viewing AC Waveforms

The timebase control sets the scale for the x-axis, which moves the trace at a constant rate across the screen.

Instantaneous voltage is applied to, and displayed on the y-axis, and time represented on the x-axis.

AC waveform is graphically displayed on the screen using conventional x and y axes.

The y-amplifier control sets the scale for the y-axis (voltage).

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8

A) Timebase

B) Focus

C) Y-amplifier

D) Brilliance

Which control of an oscilloscope would you adjust to increase the height of the trace?

Question

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AC Measurements with an Oscilloscope

A grid of horizontal and vertical lines on the screen can be used to take measurements.

The timebase and y-amplifier controls set the scale for this grid of lines.

Y-amplifier set to 0.2V/div. Each square in the vertical direction equals 0.2V.

Timebase set to 10ms/div.Each square in the horizontal direction equals 10ms.

For example,

Each square can be split further. Example shows a waveform with a time period spanning 6.4 squares.

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AC Measurements with an Oscilloscope

Waveform spans 6.4 divisions.Period is 10ms/div x 6.4div = 64ms

Examine the waveform and settings shown below.

Y-amplifier = 0.2V/div

Timebase = 10ms/div

Peak-to-peak measurement spans 6 divisions.Peak-to-peak voltage is 0.2V/div x 6div = 1.2V

X-axis

Y-axis

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Summary

You should now be aware of:

Alternating Current - AC

The Oscilloscope

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