DC Power Supply

In this power supply, a 7809 is used to produce that variable output voltage. As in the earlier design, it is recommended by the manufacturer that a 0.33 μF capacitor be connected from the input terminal to ground and a 0.1 μF capacitor be connected from the output terminal to ground, as shown in Figure, to prevent high frequency oscillations and improve the performance. The reason for a small-value capacitor in parallel with a large one is that the large filter capacitor has an internal equivalent series resistance, which affects the high frequency response of the system. The effect is cancelled with the small capacitor.

The Transformer