Electric Technology

Half-Wave Rectifier: Working principles and Applications

Half-Wave Rectifier: Working principles and Applications

A Half-Wave Rectifier is a single PN junction diode connected in series to the load resistor. It allows the electric current to flow in only one direction. This property of the diode is useful in creating simple rectifiers which are further used to convert AC to DC.

Half-Wave Rectifier Operation

A half-wave rectifier removes the negative half cycle of an AC input and allows only the positive cycles to pass, hence creating a DC flow. To understand the operation of a half-wave rectifier, you must know the basics well.

The operation of a half-wave rectifier is simple. A PN junction diode conducts the current only in 1 direction. A PN junction diode conducts current only when it is forward biased. The principle is made use of in a half-wave rectifier to convert it from AC to DC. The input we give here is alternating current. This input voltage is stepped down by using a transformer. The reduced voltage is fed to the diode ‘D’ and load resistance RL. During the positive half cycles of the input wave, the diode ‘D’ will be forward biased and during the negative half, the diode ‘D’ will be reverse biased. We take the output across the load resistor. The diode passes current only during a one-half cycle. The output is positive and significant during the positive half cycles of a wave. At the same time output is zero or insignificant during negative half cycles of the wave. This is called half-wave rectification.

Half-Wave Rectifier: Working principles and Applications

Working of a Half-wave rectifier

The half-wave rectifier circuit using a semiconductor diode with a resistance. The diode is connected in series with the secondary of the transformer and the resistance.  The voltage across the secondary winding changes polarities after every half cycle. During the positive half-cycles of the input ac voltage i.e. when the secondary winding is positive, the diode is forward biased and therefore conducts current. If the forward resistance of the diode is assumed to be the input voltage during the positive half-cycles is directly applied to the load resistance, making its upper-end positive. The waveforms of the output current and output voltage are of the same shape as that of the input voltage.

During the negative half cycles of the input voltage i.e. when the lower end of the secondary winding is positive, the diode is reverse biased and so does not conduct. Thus during the negative half cycles of the input ac voltage, the current through and the voltage remains zero. The output voltage developed across load resistance is a series of positive half cycles, with intervening very small constant negative voltage levels. Only half-cycles of the input wave are used, it is called a half-wave rectifier.  

Advantages and Disadvantages of Half-wave rectifier:

A half-wave rectifier is very rarely used in practice. It is never preferred as the power supply of an audio circuit because of the high ripple factor. The high ripple factor results in noises in the input signal, which in turn will affect the quality.

The advantage of a half-wave rectifier is only that it is cheap, simple, and easy to construct. It is cheap because of the low number of components. Simple because of the straightforwardness.

Half-Wave Rectifier with Capacitor Filter  

The output of the half-wave rectifier is not a DC voltage that is constant. We need a power supply with smooth waveforms. We need a DC power supply with constant output. A constant output voltage from the DC power supply is important as it directly impacts the reliability of the electronic device we connect to the supply. We can make the output of the half-wave rectifier smooth by using a filter across the diode.  In some cases, a resistor-capacitor coupled filter is also used.

Half-Wave Rectifier: Working principles and Applications

The Efficiency of Half-Wave Rectifier

 Rectifier efficiency (ɳ) is the ratio of output DC power to the input AC power. It is calculated as:

 ɳ = (Pdc/Pac)

The efficiency of HWR is 40.6% (ɳmax = 40.6%)

Applications of Half-Wave Rectifier

In day-to-day life, the half-wave rectifier is mostly used in low power applications because of its major disadvantage being the output amplitude which is less than the input amplitude. Some of the uses and applications of rectifiers are in :

Appliances

Used with transformers

Soldering

AM radio

Pulse generated circuits

Single demodulation

Voltage multiplier

The Full-Wave Rectifier

To rectify both half cycles of a sine wave, the full-wave rectifier uses two diodes, one for each half of the cycle. It also uses a transformer with a center-tapped secondary winding. The full-wave rectifier is like two simultaneous half-wave rectifiers.

Consider the first half-cycle, when point A is positive with respect to C. At this time, D1 is forward biased and D2 is reverse biased. Therefore, only the top half of the transformer’s secondary winding carries current during this half-cycle. This produces a positive load voltage across the load resistor.

During the next half-cycle, the source voltage polarity reverses. Now, point B is positive with respect to C. This time, D2 is forward biased and D1 is reverse biased. As you can see, only the other half of the transformer’s secondary winding carries current. This also produces a positive load voltage across the load resistor as before.

Filtering the Output of a Rectifier

The output we get from a full-wave rectifier is a pulsating DC voltage that increases to a maximum and then decreases to zero. We do not need this kind of DC voltage.