The Conventional Full-Wave Rectifier

A full-wave rectifier is a device that has two or more diodes arranged so the load current flows in the same direction during each half cycle of the ac supply.

A schematic diagram of a simple full-wave rectifier is shown in figure 3-8. The transformer supplies the source voltage for two rectifier tubes (V1 and V2). This power transformer has a CENTER-TAPPED high-voltage secondary winding that is divided into two equal parts (W1 and W2). W1 provides the source voltage for V1 and the other winding (W2) provides the source voltage for V2. The connections to the diodes are arranged so that the diodes conduct on alternate half cycles.

Figure 3-8. - Simple full-wave rectifier (first alternation).

During one alternation of the secondary voltage, the polarities will be as shown in figure 3-8. The source for diode V2 is the voltage induced into the lower half of the transformer secondary winding (W2). At the specific instant of time shown in the figure, the plate voltage on V2 is negative, and V2 cannot conduct.

Throughout the period of time during which the plate of V2 is negative, the plate of V1 is positive. This is illustrated by the polarity signs across W1, which indicate the source for V1. Since the plate of V1 is positive, it conducts, causing current to flow through the load resistor in the direction shown by the arrow.

Figure 3-9 shows the next half cycle of secondary voltage. As you can see, the polarities across W1 and W2 are reversed. During this alternation the plate of V1 is driven negative and V1 cannot conduct.

Figure 3-9. - Simple full-wave recliner (second alternation).

For the period of time that V1 is negative, the plate of V2 is positive, permitting V2 to conduct. Notice that the plate current of V2 passes through the load resistor in the same direction as did the plate current of V1. In this circuit arrangement, a pulse of load current flows during each alternation of the input cycle. Since both alternations of the input voltage cycle are used, the circuit is called a FULL-WAVE RECTIFIER.

Now that you have a basic understanding of how a full-wave rectifier works, let's cover in detail a practical full-wave rectifier and its waveforms.

A Practical Full-Wave Rectifier

A practical full-wave rectifier circuit is shown in figure 3-10. It uses two diodes (V1 and V2) and a center-tapped transformer (T1). When the center tap is grounded, the voltages at the opposite ends of the secondary windings are 180 degrees out of phase with each other. Thus, when the voltage at point A is positive with respect to ground, the voltage at point B is negative with respect to ground. Let's examine the operation of the circuit during one complete cycle.

Figure 3-10. - Complete full-wave rectifier.

During the first half-cycle (as indicated by solid arrows) the plate of V1 is positive with respect to ground and the plate of V2 is negative. As shown, current flows from ground (center tap), up through the load resistor (R_L), through diode V1 to point A. In the transformer, current flows from point A, through the upper winding and back to ground (center tap). When V1 conducts, it acts like a closed switch so that the positive half-cycle is felt across the load.

During the second half-cycle (broken lines), the polarity of the applied voltage has reversed Now the plate of V2 is positive with respect to ground and the plate of V1 is negative. Only V2 can conduct. Current now flows, as shown, from ground (center tap), up through the load resistor (R_L), through diode V2 to point B of T1. In the transformer, current flows from point B up through the lower windings and back to ground (center tap). Notice that the current flows across the load resistor (R_L) in the SAME DIRECTION for both halves of the input cycles.

The output waveform from the full-wave rectifier consists of two pulses of current (or voltage) for each cycle of input voltage. The ripple frequency at the output of the full-wave rectifier is therefore TWICE THE LINE FREQUENCY.

The higher ripple frequency at the output of a full-wave rectifier has a distinct advantage: Because of the higher pulse frequency, the output is closely approximate to pure dc. This higher frequency also makes filtering much easier than the output of the half-wave rectifier.

In terms of peak value, the average value of current and voltage at the output of the full-wave rectifier is twice as great as the average current or voltage at the output of the half-wave rectifier. The relationship between peak and average values is illustrated in figure 3-11.

Figure 3-11. - Peak and average values for a full-wave rectifier.

Since the output waveform is essentially a sine wave with both alternations at the same polarity, the average current or voltage is 63.7 percent (or .637) of the peak current or voltage.

As an equation:

E_avg (the average load voltage) = .637 X E_max

Where

E_max = The peak value of the load voltage pulse

And

I_avg (the average load current) = .637 X I_max

Where:

I_max = The peak value of the load current pulse

Example: The total voltage across the high-voltage secondary of a transformer used to supply a full-wave rectifier is 600 volts. Find the average load voltage. (Ignore the drop across the rectifier tube.)

Solution: Since the total secondary voltage is 600 volts, each diode is supplied one-half of this value, or 300 volts. As the secondary voltage is an rms value, the peak load voltage is:

E_max = 1.414 X E_s

E_max = 1.414 X 300

E_max = 424 volts

The average load voltage is:

E_avg = .637 X E_max

E_avg = .637 X 424

E_avg = 270 volts

NOTE: If you have problems with this equation, review <emphasis type="b">NEETS</emphasis>, module 2, pertaining to this area.

As you may recall from your past studies in electricity, there are advantages and disadvantages in every circuit. The full-wave rectifier is no exception. In studying the full-wave rectifier, you have found that when the output frequency is doubled, the average voltage is also doubled, and the resulting signal is much easier to filter because of the high-ripple frequency. The only disadvantage is that the peak voltage in a full-wave rectifier is only half the peak voltage in a half-wave rectifier. This is because the secondary of the power transformer in a full-wave rectifier is center tapped; therefore only half the source voltage goes to each diode.

Fortunately, there is a rectifier that produces the same peak voltage as a half-wave rectifier and the same ripple frequency as a full-wave rectifier. This circuit, called the BRIDGE RECTIFIER, will be the chapter of our next discussion.

Q.12 What is the ripple frequency of a full-wave rectifier with an input frequency of 60 Hz?
Q.13 What is the average voltage (E_avg) output of a full-wave rectifier that has an output of 100 volts peak?