Half Wave Rectifiers
A rectifier is a device that converts alternating current (AC) to direct current (DC). It is done by using a diode or a group of diodes. We know that the diode permits current only in one direction and blocks the current in the other. We will use this principle to construct various rectifiers.
By the no. of diodes used, we can classify rectifiers into two types:
- Half-Wave Rectifier
- Full-Wave Rectifier
Half Wave RectifierHalf wave rectifier is the simplest form of rectifiers available. Half-wave rectifier will allow only one half-cycle (positive or negative half-cycle) of the AC voltage through and will block the other half-cycle. We only need one diode to construct a half-wave rectifier.
Construction of Half Wave RectifierA half-wave rectifier system consists of:
- Transformer (Step-down)
- Resistive Load
Working Principle of Half Wave RectifierWe will apply a high AC voltage to the primary side of the step-down transformer and we will get a low voltage at the secondary winding which will be applied to the diode. During the positive half cycle of the AC voltage, the diode will be forward biased and the current flows through the diode. During the negative half cycle of the AC voltage, the diode will be reverse biased and the flow of current will be blocked. The output voltage waveform will be as above.
We can classify the half-wave rectifier into two types based on the waveforms allowed:
Positive Half Wave RectifierPositive half-wave rectifier will allow positive half-cycles through the diode and will block the negative half-cycle.
Negative Half Wave RectifierNegative half-wave rectifier will allow negative half-cycles through the diode and will block positive half-cycle
Filtering of Half Wave RectifierThe output waveform we have obtained is a pulsating DC waveform and it can’t be used for any practical application. To use for any practical application, we need a pure or constant DC waveform. To obtain that, we use a Filter. Filters are components used to convert (smoothen) pulsating DC to constant DC. They do it by suppressing the DC ripples in the waveform. They can be of capacitive or inductive type. Here we will be using a capacitive filter to smoothen the pulsating DC to constant DC as shown in the figure below.
Characteristics of Half Wave RectifierRipple Factor (γ) The output we will get from the rectifier will consist of both AC and DC components. The AC components are undesirable to us and will cause pulsations in the output. This unwanted AC component is called Ripple. Ripple factor is the ratio between the RMS value of the AC component and the DC component in the rectifier. For half-wave rectifier, we obtain γ = 1.21 Note: For us to construct a good rectifier, we need to keep the ripple factor as minimum as possible. We can use capacitors or inductors to reduce the ripples in the circuit.
Rectifier Efficiency (η) Rectifier efficiency is the ratio between the output DC power and the input AC power. For half-wave rectifier, ηmax = 40.6%
Form Factor (F.F) Form factor is the ratio between RMS value and average value. For half-wave rectifier, F.F = 1.57
Peak Inverse Voltage (PIV) Peak Inverse Voltage (PIV) is the maximum voltage that the diode should withstand during reverse bias condition. If a voltage is applied more than the PIV, the diode will be destroyed.
Output DC Current (IDC) The DC current obtained across the load is given by the equation Output DC Voltage (VDC) The voltage appeared across the load resistor is denoted by
Applications of Half Wave RectifierCompared to full-wave rectifier, we won’t use half-wave rectifier much but for some special purpose we use them:
- For rectification purpose
- For signal demodulation purpose
- For signal peak detection