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Full wave bridge rectifier

2016-05-17 15:51  
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In the bridge circuit four diodes are connected in the form of a Wheatstone bridge, two diametrically opposite junctions of the bridge are connected to the? secondary of a transformer and the other two are connected to the load.



When the upper end of the transformer second-ary winding is positive, say during first half-cycles of the input supply, diodes D1 and D3are forward biased and current flows through arm AB, enters the load at positive terminal, leaves the load at negative termi-nal, and returns back flowing through arm DC. Dur-ing this half of each input cycle, the diodes D2and D4are reverse biased and so the current is not allowed to flow in arms AD and BC. The flow of current is indicated by solid arrows in the figure. In the second half of the input cycle the lower end of ac supply becomes positive, diodes D,2and D4become forward biased and current flows through arm CB, enters the load at the positive terminal, leaves the load at nega-tive terminal and returns back flowing through arm DA. Flow of current has been shown by dotted ar-rows in the figure. Thus the direction of flow of cur-rent through the load resistance RLremains the same during both half^eycles of the input supply voltage.

Peak Inverse Voltage of a Full wave bridge rectifier:

Figure shows a bridge-rectifier circuit. Let us consider the instant the secondary voltage attains its positive peak value Vgmax. Now diodes D1, and D3are conducting whereas diodes D2and D4are non-conducting being reverse biased. The conducting diodes D1 and D3have almost zero resistance(i.e.zero voltage drop across them). Point B has the same potential as point A and simi-larly point D has the same potential as point C. The entire voltage of the transformer secondary winding,Vgmaxis developed across the load resistance RL. The same voltagei.e.Vgmaxacts across each of the non-conducting diodes D2and D3.


Merits and Demerits of Full-wave Rectifiers Over Half-Wave Rectifiers.


l.The rectification efficiency of full-wave rectifier is double of that of ahalf-wave rectifier

2.The ripple voltage is low and of higher frequency in case of a full-wave rectifier so simple filtering circuit is required.

3.Higher output voltage higher output power and higher TUF in case of a full-wave rectifier.

4.In a full-wave rectifier, there is no problem due to dc saturation of the core because the dc currents in the two halves of the transformer secondary flow in opposite directions.

Demerits.Full-wave rectifier needs more circuit elements and is costlier.

Merits and Demerits of Bridge Rectifiers Over Centre-Tap Rectifiers.

With the availability of low-cost, highly reliable and small-sized silicon diodes bridge rectifier is becoming more and more popular in comparison to centre-tap rectifier. It has manyadvantages over a centre-tap rectifier, as given below.

1.No centre tap is required in the transformer secondary so in case of a bridge rectifier the transformer required is simpler. If stepping up or stepping down of voltage is not required, transformer can be eliminated even.

2.The PIV is one half that of centre-tap rectifier. Hence bridge rectifier is highly suited for high voltage applications.

3.Transformer utilisation factor, in case of a bridge rectifier, is higher than that of a centre-tap rectifier.

4.For a given power output, power transformer of smaller size can be used in case of the bridge rectifier because current in both (primary and secondary) windings of the supply transformer flow for the entire ac cycle.

The main drawback of a bridge rectifier is that it needs four diodes, two of which conduct in alternate Half-cycles. Because of this the total voltage drop in diodes becomes double of that in case of a centre-tap rectifier. An-other drawback of bridge rectifier is that the load resistor RLand the supply source have no common point which may be earthed.

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