Half-Wave Rectifier

• A junction diode allows current to pass only when it is forward biased.
• This property is used for rectification of alternating voltages.
• The circuit used for this purpose is called a rectifier.

Half-Wave Rectifier

• A half-wave rectifier circuit includes a diode in series with a load.
• A pulsating voltage appears across the load during the half cycles when the diode is forward biased.
• The secondary of a transformer supplies the desired AC voltage across terminals A and B.
• When the voltage at A is positive, the diode is forward biased and conducts.
• When A is negative, the diode is reverse-biased and does not conduct.
• The reverse saturation current of a diode is negligible and can be considered equal to zero for practical purposes.
• The reverse breakdown voltage of the diode must be sufficiently higher than the peak AC voltage at the secondary of the transformer to protect the diode from reverse breakdown.
• During the positive half-cycle of AC, there is a current through the load resistor RL, and an output voltage.
• There is no current in the negative half-cycle.
• The output voltage is restricted to one direction after being rectified.
• As the rectified output is only for half the input AC wave, it is called a half-wave rectifier.

Full-Wave Rectifier

• A circuit using two diodes gives output rectified voltage corresponding to both the positive and negative half of the AC cycle and it is known as a full-wave rectifier.
• The P-side of the two diodes connect to the ends of the transformer's secondary coil.
• The N-side of the diodes are connected together, and the output is taken between the diode's common point and the transformer's secondary coil midpoint.
• The secondary of the transformer is provided with a center tap, becoming a center-tap transformer.
• The voltage rectified by each diode is half the total secondary voltage.
• Each diode rectifies only for half the cycle, doing so for alternate cycles.
• The output between the diode's common terminals and the transformer's center-tap is a full-wave rectifier output.
• Input voltage to A with respect to the center tap at any instant would be positive.
• Voltage at B is out of phase and will be negative.
• Diode D₁ gets forward biased and conducts, while D₂ is reverse biased and not conducting.
• During the positive half-cycle, an output current and output voltage is generated across load resistor RL.
• When the voltage at A becomes negative with respect to the center tap, the voltage at B becomes positive.
• Diode D₁ does not conduct, but diode D₂ does, providing an output current and voltage across RL during the negative half of the AC input.
• A full-wave rectifier is a more efficient circuit for getting rectified voltage or current than a half-wave rectifier.

Filters

• Rectified voltage is in the form of pulses of the shape of half sinusoids.
• The voltage is unidirectional but does not have a steady value.
• A capacitor is connected across the output terminals (parallel to the load RL) to get steady DC output from the pulsating voltage and is called filters.
• One can also use an inductor in series with RL for the same purpose.
• When the voltage across the capacitor rises, the capacitor gets charged.
• If there is no external load, it remains charged to the peak voltage of the rectified output.
• When there is a load, it gets discharged through the load, and the voltage across it begins to fall.
• In the next half-cycle of rectified output, it again gets charged to the peak value.
• The rate of fall of the voltage across the capacitor depends inversely upon the product of capacitance C and the effective resistance RL used in the circuit and is called the time constant.
• To make the time constant large, the value of C should be large.
• Capacitor input filters use large capacitors.
• The output voltage obtained by using a capacitor input filter is nearer to the peak voltage of the rectified voltage.
• This type of filter is most widely used in power supplies.