Bridge Rectifier

The bridge rectifier circuits are mainly used as,

a) A power rectifier circuit for converting a.c. power to d.c. power, and

b) A rectifying system in rectifier type a.c. meters, such as a.c. voltmeter, in which the a.c. voltage under measurement is first converted into d.c. and measured with conventional meter. In this system, the rectifying elements are either copper oxide type or selenium type.

The basic bridge rectifier circuit is shown in Fig. 1.

Fig. 1 Bridge rectifier circuit

The bridge rectifier circuit is essentially a full-wave rectifier circuit, using four diodes, forming the four arms of an electrical bridge. To one diagonal of the bridge, the a.c. voltage is applied through a transformer if necessary, and the rectified d.c. voltage is taken from the other diagonal of the bridge. The main advantage of this circuit is that it does not require a centre tap on the secondary winding of the transformer. Hence wherever possible, a.c. voltage can be directly applied to the bridge.

1.1 Operation of the Circuit

Consider the positive half of ac input voltage. The point A of secondary becomes positive. The diodes D₁ and D₂ will be forward biased, while D₃and D₄reverse biased. The two diodes D₁ and D₂ conduct in series with the load and the current flows as shown in Fig. 2.

Fig. 2 Current flow during positive half cycle

In the next half cycle, when the polarity of ac voltage reverses hence point B becomes positive diodes D₃ and D₄ are forward biased, while D₁ and D₂ reverse biased. Now the diodes D₃ and D₄ conduct is series with the load and the current flows as shown in Fig. 3.

Fig 3 Current flow during negative half cycle

Note : It is seen that in both cycles of a.c., the load current is flowing in the same direction hence, we get a full-wave rectified output.

The waveforms of load current and voltage remain exactly same as shown before for full-wave rectifier.

1.2 Expressions for Various Parameters

The bridge rectifier circuit, being basically a full wave rectifier circuit; all the characteristics discussed previously for a full-wave circuit using two diodes, are the characteristic of a bridge rectifier circuit.

The relation between I_m the maximum value of load current and I_DC, I_RMS remains same as derived earlier for the full wave rectifier circuit.

The expression for I_m will change slightly. This will be clear from the equivalent circuit shown in the Fig. 4.

Fig. 4

In each of half cycle two diodes conduct simultaneously. Hence maximum value of load current is,

Note : So the only modification is that instead of R_f, which is forward resistance of each diode, the term 2R_f appears in the denominator.

The remaining expression are identical to those derived for two diode full wave rectifier and reproduced for the convenience of the reader.

Note : The E_sm is the maximum value of a.c. voltage across full secondary winding of the transformer used.

As the current flows through the entire secondary of the transformer for all the time, the transformer utilization factor is 0.812. This is more then the T.U.F. for full wave rectifier circuit.

1.3 PIV Rating of Diodes

The reverse voltage appearing across the reverse biased diodes is but two diodes are sharing it. Hence PIV rating of the diode is E_sm and not 2E_sm as in case of full wave rectifier.

1.4 What Happens if Input and Output Terminals are Reversed ?

The bridge rectifier can be represented in a simplified way as shown in the Fig. 5.

Fig. 5

For positive half cycle of input D₁, D₂ conduct while for negative half cycle D₃, D₄conduct.

It can be noted that if input and output terminals in bridge rectifier are reversed without any change in diodes then it will not work. For one cycle, supply will get shorted through the forward biased diodes across the supply while for other cycle the circuit will be open. The output will be zero.

1.5 Advantages of Bridge Rectifier Circuit

a) The current in both the primary and secondary of the power transformer flows for the entire cycle and hence for a given power output, power transformer of a small size and less cost may be used.

b) No centre tap is required in the transformer secondary. Hence, wherever possible, ac voltage can directly be applied to the bridge.

c) The current in the secondary of the transformer is in opposite direction in two half cycles. Hence net d.c. component flowing is zero which reduces the losses and danger of saturation.

d) Due to pure alternating current in secondary of transformer, the transformer gets utilized effectively and hence the circuit is suitable for applications where large powers are required.

e) As two diodes conduct in series in each half cycle, inverse voltage appearing across diodes get shared. Hence the circuit can be used for high voltage applications. Such a peak reverse voltage appearing across diode is called peak inverse voltage rating (PIV) of diode.

1.6 Disadvantages of Bridge Rectifier

The only disadvantages of bridge rectifier is the use of four diodes as compared to two diodes in normal full wave rectifier. This causes additional voltage drop as indicated by term present in expression of I_m instead of R_f. This reduces the output voltage.

1.7 Applications

1. Used as rectifier in power circuits convert a.c. to d.c.
2. In rectifier type meters, to convert a.c. voltage to be measured to d.c.
3. In power supply circuits.