Activity Full Wave Rectifier Operation

Before building a full wave rectifier, let's first attempt to understand what it is and how it operates. While our half wave rectifier used only the positive portion of the AC input, a full wave rectifier uses both the positive and negative portions, thus doubling the output power. The way it does this is also quite interesting. In order to capture the full positive and negative portions of the input AC wave, a full wave rectifier uses a combination of four diodes arranged in circuit like that in Figure 5-21. This diode arrangement is generally called a "bridge rectifier" and here's how it works.

The AC Input cycle is illustrated with numbers ranging from 1 to 6. The AC Output is similarly numbered to show its correspondence with the input wave.

At point 1 on the AC input wave, point A is more positive as compared with point F on the bridge rectifier. Furthermore, the voltage is just high enough to forward bias diode AB allowing it to conduct and deliver current to resistor CD. Current cannot flow into diode BF since it is reversed biased at this point. At the same time diode AE cannot conduct current since the voltage at point A, the cathode, is essentially the same as it is at point E, the anode. Therefore, current only flows from diode AB through resistor CD and then through diode EF thus completing the return path to the AC input source.

When the AC input wave reverses and goes negative at point 4, point F becomes more positive as compared with point A; this is important to note since it is key to the operation of our full wave rectifier.

To capture the negative cycle of the AC input wave, diode FB becomes forward biased at point 4 of the AC input wave, and current begins to flow through resistor CD then through diode EA, which is also forward biased at this point. The current flow then returns to the other side of the input AC wave thus completing the circuit in the negative direction. The other two diodes are reversed biased during the negative portion of the AC input.

Notice that we are labeling the components in the direction of current flow to make the description clearer.

Therefore, the four-diode bridge rectifier uses two diodes, AB and EF, to conduct current through the resistor during the positive half of the AC input cycle, while diodes FB and EA conduct current through the resistor during the negative half of the AC input cycle. The result is that the AC Output voltage contains twice as many positive peaks as compared with the half wave rectifier, which is significant in terms of capturing nearly all of the AC input's energy.

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