half wave rectifier

What is a rectifier ?

A rectifier ( half wave rectifier ) is a device that converts alternating current ( AC ) to fluctuating / pulsating direct current ( DC ). It is done by making special arrangement of diodes. Based on arrangement of diodes rectifiers are categorized into three base categories.

1. Half wave rectifier
2. Full wave rectifier
3. Bridge rectifier

What is a half wave rectifier ?

It is a device that converts alternating current to fluctuating direct current using only one diode. It converts only one half cycle of AC signal to DC signal and blocks the other half cycle.

Whereas a full wave rectifier or a bridge rectifier uses multiple diodes and converts full cycle of AC signal to DC signal.

The working of half-wave rectifier is based on the function of diode. A diode allow electrical current to flow in only one direction.

Working

Among rectifiers, half wave is the simplest version.

The diagram above illustrates the working of half-wave rectifier with respect to input and output signal. The signal graph shows – when the diode conducts we get a signal ( current ) in the output and during reverse biased condition there is a gap ( no current ) in the output.

When a standard AC signal is passed through this rectifier. Only one half if AC wave remains in the output and the other half gets blocked. It allows either positive half or the negative half according to the biasing of diode.

Only one diode is required for the construction of half-wave rectifier.

DC circuits are designed to accept that flow in a single direction. AC signals ( current ) can easily destroy them. In this case we use rectifiers ( Half wave ) to convert Alternating current to Direct current.

To build a half wave rectifier, three major components are required –

1. A step-down transformer
2. Load resistor
3. A diode

Half Wave Rectifier Circuit

Now let’s see how the above circuit works.

The AC voltage is applied to the primary side of the transformer and a low voltage AC is obtained from the secondary side. A series combination of diode and load resistor is connected to the secondary winding.

Now, during the positive half cycle the diode gets forward biased and current flows through the diode and load resistor. During the negative half cycle the diode gets reverse biased and acts as open circuit. Hence, no current flows in this case.

Filter Capacitor for Rectifier

We have obtained a fluctuating DC from the above circuit. So now we add a capacitor / inductor as a filter that converts fluctuating DC to pure DC ( DC with minimum or no ripples ).

Filters are generally used to convert pulsating DC signals ( current ) to pure DC signal. The charging and discharging of capacitor / inductor overcomes the gap created in the half wave output signal.

The half-wave rectifier is theoretically possible but it cannot be applied to practical applications without filter. DC circuits require pure DC waveform. This is why we always use filter with rectifiers.

The diagram below shows how capacitor smoothens DC.

Ripple Factor of Half Wave Rectifier

The smoothness of DC current in the output is measured by a factor known as ripple factor. The output of half wave rectifier provides pulsating DC, it means it has some frequency. In actual Direct Current has no frequency.

Ripple factor can be calculated by formula –

Ripple factor ( γ ) = [ ( I²_{rms –} I²_dc ) / I_dc ]

The value of Ripple factor for half wave rectifier is 1.21

For a good rectifier, ripple factor should be as low as possible. Capacitors and inductors reduce the ripples in the circuit.

Efficiency

Efficiency of a rectifier is the ratio between the output DC power and the input AC power.

The formula is –

Efficiency ( η ) = P_dc / P_ac

Efficiency of half wave rectifier  is equal to 40.6 %.

RMS Value of Half Wave Rectifier

I_rms = I_m / 2

Where, I_m = I_max which is equal to the peak current across the load.

Peak Inverse Voltage

Peak inverse voltage ( PIV ) is a maximum voltage that a diode can handle in reverse biased condition.

Advantages

Advantages of half wave rectifier are –

1. Simple circuit. It uses only one diode.
2. Cheap

Disadvantages

Disadvantages of half wave rectifier are –

1. It allows only one half cycle of AC wave to pass through it. That decreases efficiency.
2. Low efficiency when compared to Full wave or bridge rectifier.
3. The output DC contains very high ripples. Full wave and bridge rectifier has comparatively low ripples.
4. Low peak inverse voltage when compared to bridge rectifier.

Author

Akash Sharma

Discover more – 

1. Difference between AC and DC 
2. Bridge rectifier

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