Bridge rectifier is an electronic component which converts an input AC current into a DC current as an output. Electronic devices, and particularly portable electronic devices such as portable computers, cellular phones, and personal digital assistants (PDAs) typically make use of alternating current to direct current adapters (AC adapters) either as a direct source of power, or as a source of power to charge on-board batteries. Bridge rectifiers are used to rectify current output from alternative current sources, such as an alternating current generator. Three-phase bridge rectifiers have been provided for rectifying a three-phase alternating current to convert it to a corresponding direct current. The rectifiers generally comprise a three-phase rectifier circuit including six three-phase bridge-connected diodes and a smoothing capacitor connected between DC output terminals of the rectifier circuit. The six pulse bridge phase controlled rectifier is a widely used type of solid state power converter which is used in industry for converting a three phase ac input voltage to a variable dc voltage. The six pulse bridge phase controlled rectifier uses six thyristors as controllable power devices. Bridge rectifiers for motor vehicle alternators generally include two metal parts used as heat sinks that are electrically insulated from each other.
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.
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 RL, 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 RL, 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 RL is always in the same direction. In flowing through RL this current develops a voltage corresponding to that shown in waveform (5). Since current flows through the load (RL) during both half cycles of the applied voltage, this bridge rectifier is a full-wave rectifier.