Diode as a Rectifier

Application of junction diode as a Rectifier

• Rectifier is a device used to convert alternating current (AC) or alternating voltage into direct current (DC) or direct voltage.
• A p-n junction diode can be used as a half-wave or full-wave rectifier.
• The working principle: the p-n junction has low resistance when forward biased and high resistance when reverse biased; a diode therefore conducts during one polarity of the input AC and blocks during the opposite polarity.

Half-wave Rectifier

Circuit diagram

• Main components: transformer (primary and secondary), single diode, and load resistance R_L.
• The AC voltage to be rectified is applied to the primary of the transformer. The secondary output is connected to the diode and the load.

Input and output waveforms

Working

• Case 1 - positive half-cycle: During the positive half-cycle of the primary AC, the corresponding secondary terminal makes the diode forward biased. The diode conducts, offering low resistance, and current flows through the load R_L. The load sees the positive half of the input (pulses of one polarity).
• Case 2 - negative half-cycle: During the negative half-cycle the diode becomes reverse biased, its resistance becomes very high, and essentially no current flows in the circuit. The load sees no output during this half-cycle.

The conduction repeats each cycle so the output consists of a series of positive pulses separated by intervals of zero output. This is called half-wave rectification. The pulsating output contains a DC component plus an AC (ripple) component and is not suitable for many applications without further filtering.

Important points for half-wave rectifier

• The output pulsation frequency (ripple frequency) is equal to the input frequency f.
• The average (DC) value of the output (ideal diode, no voltage drop) is V_DC = V_m/π, where V_m is the peak value of the secondary output voltage.
• In practice, diode forward voltage drop (≈ 0.7 V for silicon) reduces the peak of the output pulses by that amount.
• Half-wave rectifier is simple but inefficient in energy utilisation and produces large ripple; it is therefore rarely used where a steady DC is required.

Full-wave Rectifier (centre-tap type)

Circuit diagram

• The centre-tap full-wave rectifier uses two diodes (D1 and D2) and a centre-tapped secondary winding of the transformer.
• The main difference from the half-wave rectifier is that current flows through the load during both halves of the input cycle, although via alternate diodes, so the output pulses appear twice per cycle and always in the same polarity across the load.

Input and output waveforms

Working

• Case 1 - positive half-cycle: In the positive half-cycle one end of the secondary becomes positive relative to the centre tap and the corresponding diode (say D1) is forward biased. D1 conducts and current flows through the load in one direction. The other diode (D2) is reverse biased and does not conduct.
• Case 2 - negative half-cycle: In the negative half-cycle the polarity of the secondary reverses. The other end of the secondary becomes positive relative to the centre tap and the other diode (D2) forward conducts. D1 is reverse biased. The load current again flows in the same direction as before.

Because one diode conducts during one half-cycle and the other during the alternate half-cycle, the load current is unidirectional and pulses occur twice per input cycle. The output therefore has fewer gaps and a higher average value than the half-wave rectifier.

Important points for centre-tap full-wave rectifier

• The ripple frequency at the output is twice the input frequency: f_out = 2 f_in.
• The average (DC) output value (ideal diodes) is V_DC = 2 V_m/π, where V_m is the peak voltage of one half of the secondary winding (peak across each half-winding).
• The output still contains ripple (AC component) superposed on DC; therefore filtering is normally used to obtain a smoother DC.

Filtering the rectified output - Capacitor filter

Simple RC or C filter circuit

• A capacitor of suitably large capacitance C is connected across the load (parallel to R_L) to act as a reservoir or smoothing capacitor.
• The capacitor charges quickly to the peak of each output pulse and discharges slowly through R_L between peaks, reducing the amplitude of ripple.
• The capacitor offers low impedance to the AC (ripple) component and a very high impedance to the DC component, so the AC component is shunted to ground while the DC appears across the load.

Working (qualitative)

• When a pulse appears at the rectifier output, the capacitor charges to near the pulse peak (minus diode drop).
• Between pulses the capacitor discharges through the load, supplying current so that the load voltage falls slowly rather than to zero; this reduces ripple.
• A larger C or larger R_L (longer time constant R_LC) gives better smoothing (less ripple), while a smaller time constant gives more ripple.

Comparative features, practical notes and applications

• Half-wave rectifier is simple and uses one diode but wastes half the input power, has lower average output, and larger ripple. Typical output frequency equals input frequency.
• Full-wave rectifier (centre-tap) uses two diodes and a centre-tapped transformer; it delivers pulses twice per cycle and gives higher average DC and lower ripple compared to half-wave.
• Bridge full-wave rectifier (not shown in the diagrams above) uses four diodes and does not need a centre-tapped transformer; it also provides full-wave rectification with output frequency twice the input frequency.
• In practical rectifier design include diode forward voltage drop (≈ 0.7 V for silicon) when estimating DC output and valley voltages. Also consider the diodes' reverse voltage rating (peak inverse voltage-PIV) and current rating.
• Common application: DC power supplies for electronic circuits, battery chargers, DC motors and any equipment requiring DC from AC mains; filtering and regulation stages follow rectification to obtain a stable DC supply.

Example problem

Problem: In half-wave rectification, what is the output frequency if the input frequency is 50 Hz. What is the output frequency of full-wave rectifier for the same input frequency?

Solution:

For half-wave rectifier the output pulses appear once in each cycle of the input AC. Therefore the output frequency equals the input frequency.

Input frequency = 50 Hz

Output frequency for half-wave = 50 Hz

For full-wave rectifier the output pulses appear twice in each cycle of the input AC. Therefore the output frequency is twice the input frequency.

Output frequency for full-wave = 2 × 50 Hz = 100 Hz

Summary

• A diode used as a rectifier converts AC into pulsating DC by conducting in one polarity and blocking in the opposite polarity.
• Half-wave rectification produces one pulse per input cycle; full-wave rectification produces two pulses per input cycle and therefore yields higher average DC and lower ripple.
• Filtering (commonly with a capacitor) is necessary to reduce ripple and obtain a smoother DC output for practical applications.