Bridge Rectifier

Bridge Rectifier

AU : May-10,14,17,Dec.-ll,14,17,18

• 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. 1.17.1.

• 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. 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 D₃ and D₄ reverse biased. The two diodes D₄ and D₂ conduct in series with the load and the current flows as shown in Fig. 1.17.2.

• 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 D4 and D2 reverse biased. Now the diodes D3 and D4 conduct is series with the load and the current flows as shown in Fig. 1.17.3.

Key Point : 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.

2. Expressions for Various Parameters

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

• The relation between Im 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 will change slightly. This will be clear from the equivalent circuit shown in the Fig. 1.17.4.

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

Key Point : So the only modification is that instead of Rp which is forward resistance of each diode, the term 2Rf appears in the denominator.

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

Key Point : 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 than the T.U.F for full wave rectifier circuit. 

3. PIV Rating of Diodes

• The reverse voltage appearing across the reverse biased diodes is 2E_sm but two diodes are sharing it.

Hence PIV rating of the is E_am and not 2E_am as in case of full wave rectifier.

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. 1.17.5.

• For positive half cycle of input D₁, D₂ conduct while for negative half cycle D₃, D₄ conduct.

• 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.

5. Advantages of Bridge Rectifier Circuit

1) 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.

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

3) 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.

4) 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.

5) 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.

6. Disadvantages of Bridge Rectifier

• The only disadvantage 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 2Rf present in expression of I_m instead of R_f . This reduces the output voltage.

7. Applications

1. Used as rectifier in power circuits to convert a.c. to

2. In rectifier type meters, to convert a.c. voltage to be measured to d.c.

3. In power supply circuits.

8. Comparison with Two Diode Rectifier

Ex. 1.17.1 The four semiconductor diodes used in a bridge rectifier circuit each having a forward resistance of 0.1 Q and infinite reverse resistance, feed a d.c. current of 10 A to a resistive load from a sinusoidally varying alternating supply of 30 V (r.m.s). Determine the resistance of the load and the efficiency of the circuit.

Sol. : The given values are,

Ex. 1.17.2 In a bridge rectifier circuit, input supply is 230 V, 50 Hz. Primary to secondary turns ratio is 4:1, load resistance is 200 Ω. The diodes are ideal. Find dc output voltage, PIV and output signal frequency.

AU : Dec-18, Marks 7

Sol. :

Review Questions

1. Draw the circuit diagram and explain of full wave bridge rectifier and expression for average output current, efficiency, peak inverse voltage, TUF factor. the working derive the rectification and ripple

AU : May-10,14,17, Dec.-ll,14,18, Marks 8

2. Draw the circuit diagram and explain of full wave bridge rectifier and expression of average output current factor. the working derive the and ripple

AU : May-17, Marks 13

3. Explain the working of a bridge rectifier circuit. Discuss its advantages over centre-tap full wave rectifier circuit.

4. The four semiconductor diodes used in a bridge rectifier circuit, each having a forward resistance of 0.05 Ω and infinite reverse resistance, feed a mean current of 10 A to a resistive load from a sinusoidally varying alternating supply of 30 V (r.m.s.). Determine the resistance of the load and the efficiency of the circuit.

(Ans.: RL = 2 Q, % Efficiency = 78 %)

5. If the required output d.c. voltage is 9 volts and the voltage drop across each diode is 0.8, calculate the a.c. r.m.s. input voltage required in the following cases :

i) Bridge rectifier ii) Centre tap full-wave rectifier.

(V _rms = 11.7736 V, V_rms = 10.88 V)