Full Wave Rectifiers
The circuits which convert alternating current (AC) into direct current (DC) are known as rectifiers. If such rectifiers rectify both the positive and negative half cycles of an input alternating waveform, the rectifiers are referred as full wave rectifiers. Alternatively, we can say, a rectifier is a device that converts alternating current (AC) to direct current (DC). It does it by using a diode or a group of diodes. We know that a diode permits current only in one direction and blocks the current in the other. We use this principle to construct various rectifiers.
We can classify rectifiers into two types:
- Half Wave Rectifier
- Full Wave Rectifier
We will discuss here Full Wave Rectifier. When we use a half-wave rectifier, a significant amount of power gets wasted as the only one half of each cycle passes through and the other the cycle gets blocked. Moreover, the half-wave rectifier is not efficient (40.6%) and we can not use it for applications which need a smooth and steady DC output. For more efficient and steady DC, we will use a full wave rectifier.
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We can further classify full wave rectifiers into
- Centre-tapped Full Wave Rectifier
- Full Wave Bridge Rectifier
Centre-tapped Full Wave Rectifier
Construction of Centre-tapped Full Wave RectifierA centre-tapped full wave rectifier system consists of:
- Centre-tapped Transformer
- Two Diodes
- Resistive Load
Working of Centre-tapped Full Wave RectifierWe apply an AC voltage to the input transformer. During the positive half-cycle of the AC voltage, terminal 1 will be positive, centre-tap will be at zero potential and terminal 2 will be negative potential. This will lead to forward bias in diode D1 and cause current to flow through it. During this time, diode D2 is in reverse bias and will block current through it. During the negative half-cycle of the input AC voltage, terminal 2 will become positive with relative to terminal 2 and centre-tap. This will lead to forward bias in diode D2 and cause current to flow through it. During this time, diode D1 is in reverse bias and will block current through it. During the positive cycle, diode D1 conducts and during negative cycle diode D2 conducts and during positive cycle. As a result, both half-cycles are allowed to pass through. The average output DC voltage here is almost twice of the DC output voltage of a half-wave rectifier.
Filter CircuitWe get a pulsating DC voltage with a lot of ripples as the output of the centre-tapped full wave rectifier. We cannot use this pulsating for practical applications. So, to convert the pulsating DC voltage to pure DC voltage, we use a filter circuit as shown above. Here we place a capacitor across the load. The working of the capacitive filter circuit is to short the ripples and block the DC component so that it flows through another path and is available across the load. During the positive half-wave, the diode D1 starts conducting. The capacitor is uncharged, and when we apply an input AC voltage which happens to be more than the capacitor voltage, it charges the capacitor immediately to the maximum value of the input voltage. At this point, the supply voltage is equal to capacitor voltage. When the applied AC voltage starts decreasing and less than the capacitor, the capacitor starts discharging slowly but this is slower when compared to the charging of the capacitor and it does not get enough time to discharge entirely and the charging starts again. So around half of the charge present in the capacitor gets discharged. During the negative cycle, the diode D2 starts conducting, and the above process happens again. This will cause the current to flow through the same direction across the load.
Full Wave Bridge Rectifier
Construction of Full Wave Bridge RectifierA full wave bridge rectifier is a type of rectifier which will use four diodes or more than that in a bridge formation. A full wave bridge rectifier system consists of 1) Four Diodes 2) Resistive Load We use the diodes namely A, B, C and D which form a bridge circuit. The circuit diagram is as follows
Principle of Full Wave Bridge RectifierWe apply an AC across the bridge. During the positive half-cycle, the terminal 1 becomes positive, and terminal 2 becomes negative. This will cause the diodes A and C to become forward-biased, and the current will flow through it. Meanwhile diodes B and D will become reverse-biased and block current through them. The current will flow from 1 to 4 to 3 to 2. During the negative half-cycle, the terminal 1 will become negative, and terminal 2 will become positive. This will cause the diodes B and D to become forward-biased and will allow current through them. At the same time, diodes A and C will be reverse-biased and will block the current through them. The current will flow from 2 to 4 to 3 to 1.
Filter CircuitWe get a pulsating DC voltage with a lot of ripples as the output of the full wave bridge rectifier. We can not use this voltage for practical applications. So, to convert the pulsating DC voltage to pure DC voltage, we use a filter circuit as shown above. Here we place a capacitor across the load. The working of the capacitive filter circuit is to short the ripples and block the DC component so that it flows through another path and that is through the load. During the half-wave, the diodes A and C conduct. It charges the capacitor immediately to the maximum value of the input voltage. When the rectified pulsating voltage starts decreasing and less than the capacitor voltage, the capacitor starts discharging and supplies current to the load. This discharging is slower when compared to the charging of the capacitor, and it does not get enough time to discharge entirely and the charging starts again in next pulse of the rectified voltage waveform. So around half of the charge present in the capacitor gets discharged. During the negative cycle, the diodes B and D start conducting, and the above process happens again. This causes, the current continues to flow through the same direction across the load.
Characteristics of a Full-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 components are called Ripple. The expression ripple factor is given above where Vrms is the RMS value of the AC component and Vdc is the DC component in the rectifier. For centre-tapped full-wave rectifier, we obtain γ = 0.48 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 centre-tapped full-wave rectifier, ηmax = 81.2% Form Factor (F.F) The form factor is the ratio between RMS value and average value. For centre-tapped full wave rectifier, FF = 1.11
Advantages of Full Wave Rectifiers
- Full wave rectifiers have higher rectifying efficiency than half-wave rectifiers. This means that they convert AC to DC more efficiently.
- They have low power loss because no voltage signal is wasted in the rectification process.
- The output voltage of centre-tapped full wave rectifier has lower ripples than a halfwave rectifiers.
Disadvantages of Full Wave Rectifiers
- The centre-tapped rectifier is more expensive than half-wave rectifier and tends to occupy a lot of space.