The Bridge Rectifier

When four diodes are connected as shown in figure 4-8, the circuit is called a BRIDGE
RECTIFIER. The input to the circuit is applied to the diagonally opposite corners of the
network, and the output is taken from the remaining two corners.

Figure 4-8. - Bridge rectifier.

One complete cycle of operation will be discussed to help you understand how this
circuit works. We have discussed transformers in previous modules in the NEETS series and
will not go into their characteristics at this time. Let us assume the transformer is
working properly and there is a positive potential at point A and a negative potential at
point B. The positive potential at point A will forward bias D3 and reverse bias D4. The
negative potential at point B will forward bias D1 and reverse bias D2. At this time D3
and D1 are forward biased and will allow current flow to pass through them; D4 and D2 are
reverse biased and will block current flow. The path for current flow is from point B
through D1, up through R_{L}, through D3, through the secondary of the transformer
back to point B. This path is indicated by the solid arrows. Waveforms (1) and (2) can be
observed across D1 and D3.

One-half cycle later the polarity across the secondary of the transformer reverses,
forward biasing D2 and D4 and reverse biasing D1 and D3. Current flow will now be from
point A through D4, up through R_{L}, through D2, through the secondary of T1, and
back to point A. This path is indicated by the broken arrows. Waveforms (3) and (4) can be
observed across D2 and D4. You should have noted that the current flow through R_{L}
is always in the same direction. In flowing through R_{L} this current develops a
voltage corresponding to that shown in waveform (5). Since current flows through the load
(R_{L}) during both half cycles of the applied voltage, this bridge rectifier is a
full-wave rectifier.

One advantage of a bridge rectifier over a conventional full-wave rectifier is that
with a given transformer the bridge rectifier produces a voltage output that is nearly
twice that of the conventional full-wave circuit. This may be shown by assigning values to
some of the components shown in views A and B of figure 4-9. Assume that the same
transformer is used in both circuits. The peak voltage developed between points X and Y is
1000 volts in both circuits. In the conventional full-wave circuit shown in view A, the
peak voltage from the center tap to either X or Y is 500 volts. Since only one diode can
conduct at any instant, the maximum voltage that can be rectified at any instant is 500
volts. Therefore, the maximum voltage that appears across the load resistor is nearly -
but never exceeds - 500 volts, as a result of the small voltage drop across the diode. In
the bridge rectifier shown in view B, the maximum voltage that can be rectified is the
full secondary voltage, which is 1000 volts. Therefore, the peak output voltage across the
load resistor is nearly 1000 volts. With both circuits using the same transformer, the
bridge rectifier circuit produces a higher output voltage than the conventional full-wave
rectifier circuit.

Figure 4-9A. - Comparison of a conventional and bridge full-wave rectifier.
CONVENTIONAL FULL-WAVE RECTIFIER

Figure 4-9B. - Comparison of a conventional and bridge full-wave rectifier. FULL-WAVE
BRIDGE RECTIFIER

Q.11 What is the main disadvantage of a conventional full-wave rectifier?

Q.12 What main advantage does a bridge rectifier have over a conventional full-wave
rectifier?