Feature: Power
Figure 2: Circuit A and B output signals
Figure 3: The relation between pulse amplitude and pulse width in Circuit B
Figure 4: The relation between pulse amplitude and full pulse wave width in Circuit B
Figure 5: The changes in full pulse width when Vac
frequency is changing in Circuit B
both circuits went through their startup oscillations, lasting about 5ms.
• T e Vac frequency was assumed to be
60Hz, powered from the mains through a step-down transformer.
Monitored impacts We then observed the following diff erences between the two circuits:
Pulse amplitude: In Circuit A, the output pulse signals were clear and well-defi ned, whereas in Circuit B they suff ered an amplitude drop of 290.58mV.
Pulse width: In Circuit A, the pulse widths were unchanged, whereas in Circuit B they were changing with the amplitude. Figure 3 shows the pulse width changes, and Figure 4
Figure 6: The changes in output amplitude when Vac frequency changes in Circuit B
shows the changes in pulse lengths.
Recommendations It is clear that the 555 timer is unsuitable for powering by an AC supply because the impact it has on the pulses’ amplitudes and wave widths. If the 555 timer must be powered by
an AC supply, then the operating voltage should be regulated very precisely to avoid distortion at the circuit’s output. However, an AC power source can be used to power this timer in applications where a pulse amplitude swing of 290mV is not a problem and neither are irregular wave widths.
www.electronicsworld.co.uk March 2021 21
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