Half-Wave Rectifier

• Rectification converts AC signals (voltage, current, and power) into DC signals.
• A rectifier is the circuit that performs rectification.
• P-N junction diodes allow current flow in one direction (forward bias) but block it in the opposite direction (reverse bias). A rectifier uses the property of low resistance during forward bias and high resistance during reverse bias.
• Half wave and full wave are the 2 types of rectifiers

Half Wave Rectifier

• The circuit includes a transformer, a junction diode, and a load resistance (RL).
• The transformer's primary coil connects to the AC mains voltage.
• The secondary coil connects in series with the junction diode and load resistance. This entire circuit is a half wave rectifier.
• The required AC voltage is obtained between the ends of the transformer's secondary coil.
• During the positive half cycle, the p-n junction diode is in forward bias, allowing current to flow through the load resistance RL in one direction.
• During the negative half cycle, the p-n junction diode is in reverse bias, blocking current flow in the load resistance RL.
• Current flows through RL only during half of the incoming AC cycles.
• The diode's reverse breakdown voltage must be higher than the transformer's peak AC voltage to prevent damage.
• Only half of each full AC cycle gets voltage, and the rest is not received, hence the name half wave rectifier.
• Direct current will flow through RL, and the voltage developed will be DC voltage.
• Only one junction diode is used in half wave rectification.

Full Wave Rectifier

• The circuit includes a transformer, two junction diodes (D1 and D2), and a load resistance (RL).
• The primary coil connects to the AC supply.
• The p-sides of the diodes connect to the transformer's secondary coil ends, and the n-sides connect to the load resistance RL.
• Each diode works during half of the AC cycle.
• During the full AC cycle, current flows in one direction between the load resistance RL. This is called rectified (unidirectional) current.
• When the voltage becomes positive for the duration of AC cycles the D2 will become forward bias with current flow, but current will not flow as D1 is in reverse bias.
• The current from both circuit flows and hence current flows in one direction, so DC voltage is obtained.

Comparison of Rectifiers

• Half Wave Rectifiers
  • Uses one p-n junction diode.
  • Employs a simple transformer.
  • Has an output frequency equal to the input frequency.
• Full Wave Rectifiers
  • Uses two p-n junction diodes.
  • Uses a center tap transformer.
  • Has an output frequency double the input frequency.

Filter Circuits

• Filters are used to obtain smooth rectified voltage from junction diode rectifiers

• The output waveforms is that of half sinusoids. Even though unidirectional, it's without a steady value.

• A capacitor is used parallel to the output terminals of the load RL, and an inductor can be used in series with RL, to get steady DC output from the pulsating voltage.

• These added circuits filter out AC ripple and give pure DC voltage.

• The capacitor gets charged as voltage rises.

• The capacitor remains at peak voltage if there is no external load

• If there is a load, the capacitor discharges through it, causing voltage to fall.

• The capacitor recharges in the next half cycle.

• The rate of voltage fall across the capacitor is inversely proportional to the product of capacitance(C) and effective resistance(RL), and is called the time constant.

• To be larger the time constant should have a larger capacitance value

• Capacitor input filters use large capacitors.

• Capacitor input filters give the peak voltage of rectifier.

• Filters of this type are mainly used in power supplies.

Output Frequency

• In half-wave rectification, the output frequency is equal to the input frequency (e.g., 50 Hz input yields 50 Hz output).
• In full-wave rectification, the output frequency is double the input frequency (e.g., 50 Hz input yields 100 Hz output).