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Showing Simple Patterns with the Oscilloscope

Author: N.H. Crowhurst

Oscilloscope pattern for a resistor-capacitor combination

When a resistor and capacitor are connected in series, the same current must flow through both. The voltage across the resistor is always proportional to the current through it. The voltage across the capacitor, however, depends on the instantaneous charge on it, which is constantly changed by the fluctuating current. Of course the current does not flow through the capacitor, although that expression is often used. What really happens is that current reaching the capacitor changes the charge on it and, with it, the voltage across the capacitor. This voltage changes in proportion to the instantaneous current reaching it. A bigger current will change the voltage more rapidly.

When a sinusoidal voltage fluctuation is applied to the resistor-capacitor combination, there will be two sine waves, one across the resistor and one across the capacitor. The waves will have a time difference, such that the steepest slope on one coincides with the peak on the other. By applying the voltage from the resistor to one pair of oscilloscope plates, and that across the capacitor to the others, the spot will be made to travel up and down and from side to side, but because the two movements are not synchronized, the spot traces an ellipse. If both pairs of plates are connected across resistors, so the voltages always vary in proportion, the spot will travel along a sloping line, because the up-and-down and the sideways movements always occur at the same time and in proportion.

Oscilloscope pattern for resistors only.

Last Update: 2010-11-03