Mostly equipment require a DC power supply to bias the transistors or IC components of several electronic circuits contained in them. Usually a DC power supply is constructed using a step down transformer to get a reduced amplitude ac voltage which is then rectified and filtered to get a DC signal of needed amplitude. A good DC power supply, additionally, also has a regulator circuit to provide a constant DC voltage, reasonably insensitive to changes in the input voltages or load currents. Study of rectifiers, which are built from PN junction diodes, is significant in this context and is the subject matter of this section.
An ideal diode can be considered to be 'on', acting as a 'short circuit' and passing all current when it is forward-biased. while reverse-biased, it may be considered to be 'off', thereby acting as an 'open circuit' and passing no current. In other terms, if the polarity of an applied voltage matches that of the diode (referred to as forward bias), the diode turns 'on' and functions as a 'short circuit' and while the applied voltage polarity is opposite (reverse biased), diode turns 'off' and behaves as an 'open circuit'.