power supply A power supply is used to reduce the voltage and convert ac to dc. The voltage is reduced using transformers. Changing ac to dc is called rectification. It is done using diodes.

The circuit shown in fig. p12 converts ac to dc. The diode lets the forward parts of the alternating current through but stops the backward parts. This causes the current to flow in one way through the resistor. Half the ac waveform is missing in the output. This is half wave rectification. The current is smoothed out using a capacitor connected across the load. The capacitor collects charge during surges and releases it when current from the diode stops. This gives a much smoother flow of current through the resistor.

The circuit of a simple power supply is shown in fig. p13. The transformer steps down the mains voltage. For safety a switch and fuse are fitted in the input circuit, and the core of the transformer is earthed. The four diodes together make a bridge rectifier. The bridge allows the current to pass in the forward as well as backward cycle. It reverses the backward parts of the alternating current so that whole flow is forwards. The result is full wave rectification. When the ac flows forwards, X is positive with respect to Y. In this situation diodes AA will conduct and diodes BB will block. In the backward cycle, diodes BB will conduct and diodes AA will block. The capacitor smoothes the current flow. The load can be a simple resistor, a radio, or a cassette recorder, or any other equipment needing a low dc voltage.