voltages can be produced.
An example is given in Figure 9. The pulse amplitudes
are constant, but the pulse width or duration varies. As
the pulse durations increase, the low pass filter produces a
higher average output voltage. As the pulses get narrower,
the average output voltage decreases. The load averages
the pulses into a near sine. Using more pulses results in a
smoother output sine wave. The pulses increase gradually
and then decrease gradually, and their average is a sine
wave. Additional filtering can be added as needed.
This technique is used in some variable motor drive
systems to change the frequency of the sine wave applied
to an AC induction motor to vary its speed (as with solar
power inverters and uninterruptible power supplies).
The sequence of variable width pulses is usually
generated by a microcontroller. Most of these
processors have pulse-width modulation (PWM)
instructions and one or more PWM outputs. The
key to creating a low distortion sine wave is the
selection of number, sequence, and pattern of pulses.
Prolific engineer and writer, Don Lancaster developed a
mathematical technique to determine the number of pulses
and their durations to create a sine wave with minimum
harmonic distortion. It’s called magic sine waves. Take a
look at www.tinaja.com.
The circuits covered here do work if you care to play
with them. I used a TL081 op-amp but almost any other
works (741, etc.). It’s also a good idea to make the gain
of the op-amp variable with a pot in the feedback path to
adjust the gain to initiate or sustain oscillation. NV
THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
Figure 9. A PWM scheme to generate a pulse equivalent
sine wave. Using multiple pulses reduces the harmonic
distortion and averages into a smoother sine wave.
88 May/June 2018
