Most of the direct current (DC) power used in electronic circuits is derived by
converting 50 Hz, 220 VRMS alternating current (AC) power to direct current power. This AC
to DC conversion usually involves a step-down transformer, rectifier, filter, and a regulator.
The step-down transformer is used to decrease the AC line voltage from 220 VRMS to a lower
value near the dc voltage needed. The output of the step-down transformer is then fed into a
diode rectifier circuit that only outputs positive halves of the input sinusoid. A filter is then
used to smooth the rectifier output to achieve a nearly constant dc voltage level. Figure 1
shows an example of an unregulated DC power supply. The step-down transformer decreases
the AC voltage to 12 VRMS ( = approximately 17 V peak).