A rectifier circuit is a circuit that converts an alternating supply current into a direct output current, a process called rectification. Rectification is often accomplished by use of rectifier diodes. Such rectifiers are termed uncontrolled rectifiers. Controlled rectifiers, in contrast, generally rely on the use of thyristors, where the triggering of the thyristor is used to control the output. An alternating current source is converted into a pulsating direct current. Depending on the design of the circuit, rectifiers vary in the purity of the DC output that is produced in the rectification process.
Uncontrolled Rectifiers (AC/DC Converters)
Figure 1. Half-wave rectifier
In half-wave rectifier, a single rectifier diode is used to convert an AC source into a DC output. This is a result of the fact that the diode allows current to flow in only on direction, so only half of the input waveform passes current to the load. Half-wave rectifiers are therefore said to be unidirectional for every second half wave. The voltage alternates between the load and the diode.
A full-wave rectifier converts the entire AC input waveform into a DC output. In a simple full-wave rectifier, use of two diodes and a center tapped transformer provides two output voltages with the same phase. These two voltages are passed alternately through the two diodes connected to its cathode (or anode). The full-wave rectified current flows through the load.
The advantage of the center tap is that-full wave rectification can be achieved using only two diodes. However, two voltage sources are required.
Figure 2. Center tapped transformer and two diodes forming a full wave rectifier
With full-wave rectification, both half waves are utilized hence double performance is achieved when compared to half-wave rectification.
Another device used for full-wave rectification is the bridge rectifier. A bridge rectifier provides full power using a single voltage source but requires 4 rectifier diodes.
Single-Phase and Three-Phase Bridge Rectifier
Figure 3. Bridge Rectifiers
Bridge rectifiers are components which have every branch of a rectifier circuit in a single compact case. Bridge rectifiers can be found that operate from a few amps to several hundred amps.
The amount of AC voltage mixed with the rectifier's DC output is called ripple voltage. In most cases, pure DC output is preferable, so minimizing ripple voltage is of importance. If the power levels are not too great, filtering networks may be used to reduce the amount of ripple in the output voltage.
Sometimes the method of rectification is referred to in terms of the number of DC pulses output for every cycle of AC input. A single-phase, half-wave rectifier circuit, then, would be called a 1-pulse rectifier because it produces a single pulse during the time of one complete cycle of the AC waveform. A single-phase, full-wave rectifier (regardless of design, center-tap or bridge) would be called a 2-pulse rectifier because it outputs two pulses of DC during one cycle of AC input. A three-phase full-wave rectifier would be called a 6-pulse unit. Three-phase rectification leads to less fluctuation of current and voltage as shown in figure 3.
Figure 4. Thyristor Power Controller
In controlled rectifiers, thyristors are used to control the output. Current flows through the load, which must be free of all induction, in only one direction. Thyristor power controllers are used, for example, in heaters and lighting equipment. The mean current can exceed 50% of the maximum.
The process of controllong the current and voltage across the load by adjusting the trigger timing is refered to as phase control. Phase control is the most common form of thyristor power control. The thyristor is initially held in the off state - that is, all current flow in the circuit is blocked by the hyristor except a minute leakage current. Then the Thyristor is triggered into the on state by the control circuitry. For full-wave AC control, a single Triac or two SCRs (Silicon Controlled Rectifiers) connected in inverse parallel may be used.
Single-Phase and Three-Phase half controlled Bridge Rectifiers
Figure 5. Single phase and 3-phase half controlled bridge rectifiers
The mean voltage can be changed by moving the trigger point. This can be used to control and regulate the charging of capacitors. A large or small part of the sinusoidal half wave current may flow through the load depending on the location of the trigger point.
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