1. AC Measurements Topics covered in this presentation:
Alternating Current - AC
The Oscilloscope
ClassAct SRS enabled.

2. Alternating Current - AC
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.
Next >

3. Question 1
With an alternating current the current flows in both directions at the same time. Is this true or false?
Answer True or False.
Correct Answer = F

4. 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.
Voltage then changes direction and increases in a negative direction to a maximum before decreasing again to zero volts.
The cycle then repeats.
Next >

5. Question 2
The average voltage value of a single cycle is zero volts. Is this true or false?
Answer True or False.
Correct Answer = T

6. Frequency 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.
Next >

7. Question 3 Frequency is measured in which unit? A) Volts B) Amps
C) Hertz
Correct Answer = C
D) Seconds

8. Question 4
If a wave has a frequency of 5 hertz, how many complete cycles will occur in 1 second?
Enter your answer and press SEND.
Correct Answer = 5

9. 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.
If frequency is known, the period can be calculated.
Frequency can be calculated if the period is known.
Next >

10. Question 5
If the period of a wave is 0.5 seconds what is its frequency in hertz?
A) 1
B) 2
C) 5
Correct Answer = B
D) 10

11. 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.
Next >

12. Question 6
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.
Correct Answer = T

13. R.M.S. Values
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
The root mean square value is 70.7% of the peak value.
Next >

14. Question 7
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
Correct Answer = B
D) 11.5V

15. AC Measurements with a Multimeter
A multimeter measures an AC waveform in r.m.s. values.
Separate range settings are used for AC measurements.
Next >

16. Viewing AC Waveforms
An oscilloscope can be used to display an AC waveform.
Measurements can be taken from the display.
Next >

17. Viewing AC Waveforms
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).
Next >

18. Question 8
Which control of an oscilloscope would you adjust to increase the height of the trace?
A) Timebase
B) Focus
C) Y-amplifier
Correct Answer = C
D) Brilliance

19. 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.
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.
Next >

20. AC Measurements with an Oscilloscope
Examine the waveform and settings shown below.
Timebase = 10ms/div
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
Next >

21. Summary You should now be aware of: Alternating Current - AC
The Oscilloscope
End >