A Rectifier is a Power electronics device that helps to convert AC signal to DC signal. An AC signal has two polarities changing continuously. Every load cannot take an AC signal. So we need a DC supply for them. A half-wave and full-wave rectifier convert AC to DC signal. Half wave rectifier gives discontinuous rippled DC output. But a full wave rectifier gives continuous rippled DC output.
A rectifier is classified as controlled and uncontrolled depending upon the switch used to rectify the signal.
Uncontrolled Rectifier:
An uncontrolled Rectifier uses a diode to convert from AC to DC signal. A diode is an uncontrolled switch blocking always reverse polarity signals.
An uncontrolled Rectifier can be a half wave and a full wave. Half wave rectifier (HWR) uses a single diode whereas full wave rectifier (FWR) uses two parallel diodes.
Controlled Rectifier
A controlled Rectifier uses a thyristor or a controlled switch to convert AC to DC signal. The output of this rectifier is controllable.
Similarly, a controlled Rectifier also uses a single switch in HWR, and a double switch in FWR.
There may be any type of load for these converters.
Half wave rectifier (HWR)
A half wave rectifier is the AC to DC signal converter, eliminating one of the polarities either positive or negative.
The output of the HWR is a DC signal. But it is rippled in nature. The HWR rectifies either the positive or the negative cycle of the signal depending upon the connection of the switch.
As compared to the pure DC signal, the output of the HWR differs. Only because of the discontinuity and ripples.
Hence form factor of the output of the half wave rectifier is 1.57. Whereas pure DC signal has a form factor equal to one (FF=1).
Ripple factor: It defines how much closer to a DC signal a converter can perform. The more ripple factor means poor AC to DC converter. The ripple factor of a half wave rectifier (HWR) is 1.21. It is unitless.
Since an HWR has a discontinuous output waveform, it acts as a switch to the AC source. If a switch is operated at a very high frequency it creates an imbalance in the power supply. This means an HWR injects harmonics into the AC source, which is not a good thing.
Efficiency: This is the measure that defines the closeness of the desired output to the converter output. More efficiency is better to use. The half wave rectifier has an efficiency of approximately 40.5%, which is pretty much lower than FWR.
Due to the lower efficiency of HWR, we don’t use it in practical applications. It has more harmonics in the output waveform. Let’s see a better converter for the AC to DC signal.
Full wave rectifier (FWR)
Firstly, a Full wave rectifier (FWR) is a converter in electronics to convert from AC to DC signal. It has a continuous output DC signal. Resulting in lesser harmonics in the output signal.
An FWR converts both the positive and negative cycles of the AC signal to a single positive cycle in the output. It means converting a negative cycle to a positive cycle too. Hence resulting output is rippled in nature but continuous.
A Full wave rectifier FWR has a form factor closer to one, which is FF=1.11. Whereas the Ripple factor is less than HWR, which is 0.482.
Rectifier efficiency is the ratio of output DC power to input AC power. The efficiency of the full wave rectifier FWR is 81.2%. which is almost double the HWR. That’s why in practice we use FWR instead of half wave rectifier HWR.
Differences between half wave rectifiers (HWR) and full wave rectifiers
| Specification | Half wave rectifier | Full wave rectifier |
|---|---|---|
| Definition | The half wave rectifier converts AC to DC signal by eliminating either of the positive or negative cycles. | Full wave rectifier also converts AC to DC signal but without elimination of any cycles. It converts all the negative cycles to positive cycles. |
| Output type | Discontinuous | Continuous |
| Harmonics | More | Less |
| Converter efficiency | 40.5% | 81.2% |
| Ripple Factor | 1.21 | 0.482 |
| Form Factor | 1.57 | 1.11 |
| Practical application | NOT preferred. | Preferred over HWR. |
Advantages of the Full wave rectifier FWR over half wave rectifier HWR
Why an FWR is used over HWR:
- More efficiency.
- Continuous DC output.
- Power loss is low due to the conversion of negative to positive cycles.
- Lesser Ripple factor.
Disadvantages of full wave rectifier
Having better specifications doesn’t mean it doesn’t have disadvantages.
- Expensive: The use of a center-tapped transformer, which is costlier than a simpler transformer makes the rectifier expensive.
- An extra switch (diode, thyristor, etc) gives rise to the cost.
- Again the extra protection system is needed to protect the switch.
