AC Waveform Values
Figure 6 shows the Peak, Root Mean Square and Average values for a 1 Volt AC supply. These values also apply to the negative half cycle.
Peak Value of a Waveform
The Peak value is simply the highest value on the waveform. It is also known as the Maximum value. The peak value of the waveform in Figure 6 is 1 Volt. This may be written as VP = 1 Volt.
Root Mean Square Value of a Waveform
If you measure the mains supply you will find it to be 230 Volts.
This is the Root-Mean- Square ( RMS ) value of the alternating voltage. It is also known as the Effective value. The RMS value of the waveform in Figure 6 is 0.707 Volt.
This may be written as VRMS = 0.707 Volt.
The RMS value of an AC voltage or current is defined as the equivalent DC value, which would have the same heating effect. The RMS or Effective value is the value normally used. All multimeters are designed to read RMS values.
Average Value of a Waveform
The Average value of a waveform is calculated over one half of a cycle. It is also known as the Mean value. The average value of the waveform in Figure 6 is 0.637 Volt. This may be written as VAVE = 0.637 Volt. It is simply the mathematical average value of the positive or negative half cycle. If an attempt is made to average an alternating waveform over a complete cycle, the negative half of the waveform will cancel the positive half, and so the result is zero.
Relationship Between Waveform Values
If you measure the mains supply with a multimeter you will find it to be about 230 Volts. Remember, this is the RMS value. From this, the peak value can be calculated as follows:
The Peak or Maximum Value of the 230 Volt mains supply is about 325 Volts. Please note that 325 Volts will be across your body if you receive an electric shock from the 230 Volt mains.
Now that we know the peak value of the supply, the average value can be calculated as follows:
Example
The peak value of a sine wave is 12 Volts and it has a periodic time of 16 mS. Calculate the following:
(1) RMS value
(2) Average value of full wave
(3) Average value of half wave
(4) Frequency of supply.
Solution
