Although the half wave rectifier is used in some low power applications such as signal and peak detector, it is seldom used in power rectification. The most used rectifier in the power rectification field is the full wave rectifier.
The full wave rectifier is more complex than the half wave rectifier, but it offers some significant benefits. It uses both half cycles of the sine wave resulting in a DC output voltage that is higher than that of the half wave rectifier. Another advantage is that the output has much less ripples, which makes it easier to produce a smooth output waveform.
The Full-Wave Rectifier
To rectify both half cycles of a sine wave, the full-wave rectifier uses two diodes, one for each half of the cycle. It also uses a transformer with a center-tapped secondary winding.
The full-wave rectifier is like two back-to-back half-wave rectifiers. Following image shows a Full-wave rectifier circuit.
This circuit’s operation is easily understood one half-cycle at a time.
Consider the first half-cycle, when point A is positive with respect to C. At this time, D1 is forward biased and D2 is reverse biased. Therefore, only the top half of the transformer’s secondary winding carries current during this half-cycle. This produces a positive load voltage across the load resistor.
During the next half-cycle, the source voltage polarity reverses. Now, point B is positive with respect to C. This time, D2 is forward biased and D1 is reverse biased. As you can see, only the other half of the transformer’s secondary winding carry current. This also produces a positive load voltage across the load resistor as before.
As a result, the rectified load current flows during both half-cycles due to which we get Full-wave signal across the load.
DC Value of a Full-Wave Signal
Since the full-wave rectifier produces an output during both half-cycles, it has twice as many positive cycles as the half-wave signal. As a result the DC or average value is also twice as much:
The average value of the signal over one cycle is calculated with the below formula:
This equation tells us that the DC value of a full-wave signal is about 63.6% of the peak value. For example, if the peak voltage of the full-wave signal is 10V, the DC voltage will be 6.36V
When you measure the full-wave signal with a DC voltmeter, the reading will equal the average DC value.
A Second-order Approximation
In reality, we do not get a perfect full-wave voltage across the load resistor.
Because of the barrier potential, the diode does not turn on until the source voltage reaches about 0.7V. So, the output voltage is 0.7V lower than the ideal peak output voltage.
The full-wave rectifier inverts each negative half cycle, doubling the number of positive half cycles. Because of this, full-wave output has twice as many cycles as the input.
Therefore the frequency of the full-wave signal is double the input frequency.
For example, if the line frequency is 60Hz, the output frequency will be 120Hz.
Filtering the Output of a Rectifier
The output we get from a full-wave rectifier is a pulsating DC voltage that increases to a maximum and then decreases to zero.
We do not need this kind of DC voltage. What we need is a steady and constant DC voltage, free of any voltage variation or ripple, as we get from the battery.
To obtain such a voltage, we need to filter the full-wave signal. One way to do this is to connect a capacitor, known as a smoothing capacitor, across the load resistor as shown below.
Initially, the capacitor is uncharged. During the first quarter-cycle, the diode D1 is forward biased, so the capacitor starts charging. The charging continues until the input reaches its peak value. At this point, the capacitor voltage equals Vp.
After the input voltage reaches its peak, it begins to decrease. As soon as the input voltage is less than Vp, the voltage across the capacitor exceeds the input voltage which turns off the diode.
As the diode is off, the capacitor discharges through the load resistor and supplies the load current, until the next peak is arrived.
When the next peak arrives, the diode D2 conducts briefly and recharges the capacitor
to the peak value.
One of the disadvantages of this center-tapped full-wave rectifier design is the necessity of a transformer with a center-tapped secondary winding. In high-power rectification, however, the cost and size of such transformers increase substantially. That’s why, the center-tap rectifier design is only seen in low-power applications.
Another disadvantage is that because of the center tap, only half of the secondary voltage is used for rectification.
To overcome these disadvantages four diodes are connected together in a “bridge” configuration to produce a Full Wave Bridge Rectifier as discussed in the next tutorial.