Investigating the Smoothing Effect of a Capacitor on a Resistive Load

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Investigating the Smoothing Effect of a Capacitor on a Resistive Load

Introduction

Capacitive smoothing is a process in which the voltage from a rectified D.C. supply, which is not stable, is smoothed so that more reliable readings can be obtained of the voltage trace shown on a CRO. Smoothing the D.C supply involves placing a capacitor in parallel across the circuit being supplied by the power pack as shown in the diagram below. However there still may be some ripple in the trace shown on the oscilloscope and the aim of my experiment is to investigate the degree of smoothing that can be achieved with a capacitor.

The circuit was set up as shown in the diagram above and readings of the voltage were recorded from the oscilloscope for different values of capacitance. We used a various capacitors ranging from 10 μF to 1000 μF in order to observe how the degree of smoothing was affected by increasing capacitance. The apparatus used to carry out this experiment were as follows:

  • An oscilloscope
  • An ammeter
  • A D.c. power supply
  • A range of capacitors of different values of capacitance
  • A resistor load
  • Leads
  • Voltmeter

Results

The experiment was repeated and more readings recorded in order to diminish the possibility of anomalies and errors occurring and to obtain more reliable results.

Problems encountered

Using the oscilloscope required us to ensure that the trace that appeared on the screen would be appropriate to read from and to calculate such values as the frequency.  In order to do this we had to take into consideration the scales of the axis and how the sensitivity of the CRO would affect our results. One recurring problem that I came across was the flashing of the trace. In order to overcome this problem I had to adjust the CRO settings using the trigger and the hold-off controls so that the trace would remain stable long enough for me to record precise values for the ripple voltage and the frequency. The flashing trace would occur almost every time the capacitance was changed and required holding the trace each time the readings were made.

The focus of the line is important as the thickness of the line can affect the readings when looking at each division. Though the thickness of the line is quite finite, this can affect the ripple voltage by ± 2mm on the scale (the squares are 1cm apart divided into 5 divisions.). The thickness of the line of the trace can be adjusted using the focus control as well as the intensity where we can obtain a trace of minimal thickness and high intensity so that we can see it clearly.

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Errors

Errors of great significance and errors that can be considered negligible could have occurred during this experiment, giving rise to anomalous results. There are two categories under which errors can be regarded as; systematic errors and random errors. Systematic errors occur often when there are faults in the apparatus i.e calibration faults and zeroing of the apparatus. Random errors are also called human errors and are a result of the experimenter’s technique. Random errors such as taking readings from the oscilloscope and the ammeter are common errors. Parallax error, where the line of vision did not meet ...

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