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1. AC Measurements
Electronics - AC Circuits
AC Measurements
Topics covered in this presentation:
 Alternating Current - AC
 The Oscilloscope
ClassAct SRS enabled.
1. AC Measurements
Alternating Current - AC
Electronics - AC Circuits
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.
The public electricity supply is
usually an alternating voltage.
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1. AC Measurements
Question 1
Electronics - AC Circuits
With an alternating current the current flows in both directions
at the same time. Is this true or false?
Answer True or False.
1. AC Measurements
Alternating Voltage Waveform
The voltage changes from a maximum in one direction to a maximum in
the other direction.
Electronics - AC Circuits
Voltage changes over time as shown.
From zero volts, the voltage
increases in a positive direction
to a maximum before
decreasing to zero volts.
Voltage then changes direction
and increases in a negative
direction to a maximum before
decreasing again to zero volts.
The cycle then repeats.
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1. AC Measurements
Question 2
Electronics - AC Circuits
The average voltage value of a single cycle is zero volts.
Is this true or false?
Answer True or False.
1. AC Measurements
Frequency
Electronics - AC Circuits
The cycle repeats many
times per second.
The number of times the cycle
repeats in one second is called
the frequency.
Frequency measured in hertz, (Hz).
1Hz = 1 cycle in one second.
6Hz = 6 cycles in one second.
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1. AC Measurements
Question 3
Electronics - AC Circuits
Frequency is measured in which unit?
A) Volts
B) Amps
C) Hertz
D) Seconds
1. AC Measurements
Question 4
Electronics - AC Circuits
If a wave has a frequency of 5 hertz, how many complete
cycles will occur in 1 second?
Enter your answer and press SEND.
1. AC Measurements
Frequency and Period
Electronics - AC Circuits
Alternating current of 50Hz, will
have 50 cycles in 1 second.
Each cycle takes:
1
 0.02 s  20ms.
50
Time taken for one cycle is called the period of the waveform.
1
frequency
If frequency is known, the period
can be calculated.
period 
Frequency can be calculated if
the period is known.
frequency 
1
period
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1. AC Measurements
Question 5
Electronics - AC Circuits
If the period of a wave is 0.5 seconds what is its
frequency in hertz?
A) 1
B) 2
C) 5
D) 10
1. AC Measurements
Alternating Voltage Values
Electronics - AC Circuits
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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1. AC Measurements
Question 6
Electronics - AC Circuits
The peak to peak voltage will always be larger than the peak
voltage for the same wave. Is this true or false?
Answer True or False.
1. AC Measurements
R.M.S. Values
Electronics - AC Circuits
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 r.m.s. voltage is calculated using the formula
Vrms 
1
 Vpeak  0.707  Vpeak
2
The root mean square value is 70.7% of the peak value.
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1. AC Measurements
Question 7
Electronics - AC Circuits
If the peak voltage is 12V what will be the approximate
RMS voltage for an AC signal?
A) 6.0V
B) 8.5V
C) 10V
D) 11.5V
1. AC Measurements
AC Measurements with a Multimeter
Electronics - AC Circuits
A multimeter measures an AC
waveform in r.m.s. values.
Separate range settings are used
for AC measurements.
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1. AC Measurements
Viewing AC Waveforms
An oscilloscope can be used to display an AC waveform.
Electronics - AC Circuits
Measurements can be taken from the display.
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1. AC Measurements
Viewing AC Waveforms
Electronics - AC Circuits
AC waveform is graphically displayed on the screen
using conventional x and y axes.
Instantaneous voltage is applied to, and displayed on the
y-axis, and time represented on the x-axis.
The timebase control sets the
scale for the x-axis, which moves
the trace at a constant rate across
the screen.
The y-amplifier control sets the
scale for the y-axis (voltage).
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1. AC Measurements
Question 8
Electronics - AC Circuits
Which control of an oscilloscope would you adjust to
increase the height of the trace?
A) Timebase
B) Focus
C) Y-amplifier
D) Brilliance
1. AC Measurements
AC Measurements with an Oscilloscope
Electronics - AC Circuits
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.
For example,
Timebase set to 10ms/div.
Each square in the horizontal
direction equals 10ms.
Y-amplifier set to 0.2V/div.
Each square in the vertical
direction equals 0.2V.
Each square can be split further. Example shows a waveform with a
time period spanning 6.4 squares.
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1. AC Measurements
AC Measurements with an Oscilloscope
Examine the waveform and settings shown below.
Timebase = 10ms/div
Electronics - AC Circuits
Y-amplifier = 0.2V/div
X-axis
Waveform spans 6.4 divisions.
Period is 10ms/div x 6.4div = 64ms
Y-axis
Peak-to-peak measurement spans 6 divisions.
Peak-to-peak voltage is 0.2V/div x 6div = 1.2V
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1. AC Measurements
Summary
You should now be aware of:
Electronics - AC Circuits
 Alternating Current - AC
 The Oscilloscope
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