Fourier Series SCR Switching Waveform.

Cuthbert Nyack
In some power control applications, SCR's are used to switch the power on at a certain point during the positive half of the waveform. Because of the sharp edge introduced in the waveform, it has a high harmonic content. The result is illustrated by the applet below. The power signal is shown in yellow and the switched current waveform in red. On the left, the even coefficients of the Fourier series are shown in magenta, and to the right, the odd coefficients are shown in green. As 'a' changes from 0 to 0.9 the switching point changes from 0 to 0.9T/2. Waveform has unit amplitude and with a gain of 1, scale for spectrum is from +1(top) to -1. Ripples on switched waveform arise because it is reconstructed from its Fourier Series.
The applet shows the value of the spectral component at ndb as a ratio of the component at n = 1. If the Fundamental is at 50Hz then ndb from 10,000 to 30,000 corresponds roughly to the AM band. The attenuation in this range is ~ -83 to 93dB. If the fundamental is ~ 100V then the component at ndb is still ~ 8mV. Depending on the radiation characteristics of the circuit and the current in the circuit, this is still large enough to affect an AM receiver.

When activated the following gif image show how the applet should appear.

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COPYRIGHT © 1996 Cuthbert Nyack.