Remember our circuit for a smoothed 12V 2A dc power supply. We found these waveforms in our supply. The capacitor acts to store what is capacitor ripple current when the voltage rises, and releases it when it falls. During the time when the voltage from the transformer is low all the load current is supplied by the capacitor.

This means there is an AC current through the capacitor – the “ripple current”. This ripple current causes heating in the capacitor, and over time can be destructive. For high power PSU’s we may need to determine the ripple current. Here the RMS ripple current is 3.

If the internal resistance of the transformer is low the shape of the ripple wave becomes more like a sawtooth. Here the transformer internal resistance is set as 0. However the ripple current also changes. Even though the load has remained the same, the behaviour of current through the capacitor has changed dramatically. The current pulses are shorter and more triangular in shape. The RMS value of ripple current is now 4. 4A and the initial current pulse has doubled in size.