What is a rectifier ?
Majority of electronics circuit require DC power and our household supply gets AC power. Here come the role of rectifier. Rectifiers ( bridge rectifier ) convert Alternating current to fluctuating Direct current. Fluctuating in the sense it has some ripples ( minor frequency in the output ). Whereas, pure DC has zero frequency. To fix the minor frequency to zero frequency a filter capacity is added in the output.
Among rectifiers, bridge rectifier is the most efficient rectifier with 81.2 % efficiency. It is a enhanced version of full wave rectifier with a combination of four diode whereas full wave rectifier consist of two diode.
Construction
The bridge form of rectifier is constructed as shown in above figure. It is made using four diodes D1 , D2 , D3 and D4. A load resistance is also added. The arrangement of four diodes shown efficiently convert Alternating Current to Direct Current. The advantage of this kind of rectifier is it does not require centre-tapped transformer for AC input. Therefore the cost and size gets minimised.
The alternating current input is applied to terminals named C and D and the output is obtained from the terminals named A and B ( across RL ). The diodes are arranged in such a way that only two diodes conduct during each half cycle. The diode D2 And D4 conducts current in the positive half cycle and D1 and D3 will conduct current in the negative half cycle.
Working of Bridge Rectifier
When an AC input is applied to terminals CD, then during the positive half cycle the terminal C becomes positive and terminal D becomes negative. In this case the diodes D2 and D4 becomes forward biased and conducts electricity while diode D1 and D3 become reverse biased and acts as open switch.
The figure shows current flow during positive half cycle –
During the negative half cycle terminal D becomes positive and terminal C becomes negative. In this case, the diodes D2 and D4 becomes reverse biased and act as open switch. Diodes D1 and D3 becomes forward biased and conducts current.
The figure shows current flow during negative half cycle –
In the above figures we see that the direction of current flow in the load resistor RL remains the same during both positive and negative half cycle. The DC output polarity is positive at terminal A and negative at terminal B.
Thus the bridge arrangement of diodes allows electric current to flow during both positive and negative half cycle of input Alternating current.
The waveforms of the input and output signals of the bridge / full wave rectifier is shown below
Advantages of Bridge Rectifier
The advantages are –
- DC output signal of bridge and full wave rectifier is smoother than output signal half wave rectifier.
- In half wave rectifier only one half of AC signal is used while bridge rectifier allows both positive and negative half cycle of the input AC signal.
- The efficiency of bridge rectifier is higher than half-wave rectifier. Bridge and full wave rectifier has same efficiency. Efficiency of half wave rectifier is 40.6 % and the efficiency of bridge and full wave rectifier is 81.2 %.
- Bridge rectifier is widely used in industrial applications.
Disadvantages of Bridge Rectifier
The disadvantages are –
- It uses four diodes and full wave rectifier uses two diodes. Bridge rectifier uses two diodes in series during each half cycle. Two diodes result in double voltage drop.
- It has more complex circuit when compared to half wave or centre-tapped full wave rectifier.
Efficiency of Bridge Rectifier
The efficiency of rectifier is the comparison between input power and output power. In other way, it is the measure of output DC power when a fixed input AC power is supplied.
Efficiency ( η ) = [ ( DC output power ) / ( AC input power ) ]
The efficiency of bridge and full wave rectifier is 81.2 %
Peak Inverse Voltage
The voltage across diode when it is in reverse biased condition is known as inverse voltage.
Peak inverse voltage is a maximum voltage that a diode can handle in reverse biased condition. The non conducting diodes in their respective cycles tolerate the inverse voltage. Bridge rectifier has two diodes in series during each cycle that helps it to tolerate double inverse voltage than full wave or half wave rectifier.
Ripple Factor
The smoothness of DC signal in the output is measured by a factor known as ripple factor. The output of rectifier provides ripple DC, it means it has some frequency. In actual Direct Current has no frequency.
The DC output with very less frequency can be considered as smooth DC. The Dc output with high ripples is known as ripple DC or pulsating DC. These ripples are further fixed by filter capacitor.
Mathematically, the ripple factor is known as the ratio of ripple voltage to the pure Dc voltage.
Ripple factor ( RF ) = γ = √[ ( Vrms2 ) / ( VDC ) ] – 1
Ripple factor for bridge rectifier is 0.48
Author
Akash Sharma
Discover more –