Diode rectifier circuit
Diode rectifier circuits obviously rely on diodes for their operation. Diodes are devices that only pass current in one direction. When Ambrose Fleming invented the first diode – a thermionic device / vacuum tube – he called it a valve because of its one way action. Semiconductor devices are now used and these provide exactly the same function.
The diode has a characteristic something like that shown below. In the forward direction, a small voltage is required across the diode before it conducts – the turn on voltage. The actual voltage depends on the type of diode rectifier and the material used. For a standard silicon diode rectifier this turn on voltage is around 0.6 volts.
Diode rectifier characteristic
There are many different types of rectifier diode that can be used – each one with its own properties, advantages and disadvantages.
Note on diode types:
There are many different diode types that can be used. Each diode type has its own characteristics and is suited to particular applications. In order to obtain the required performance for a particular application, it is necessary to choose the required type.
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For power rectification applications, power diodes of Schottky diodes are normally used. For signal rectification small point contact diodes, signal diodes, or Schottky diodes may be used. The Schottky diode has the advantage that it only requires a forward voltage of around 0.2 – 0.3volts for forward conduction. This is particularly useful when detecting small radio signals, and when used as a power rectifier the power losses are reduced. However the reverse leakage characteristics are not as good as normal silicon diodes.
Diode symbol and packages
The diode circuit symbol is widely known. Diodes also come in a variety of packages, although some of the more usual formats are shown in the diagram below.
Diode circuit symbol and common package formats
Diode rectifier action
The action of the diode is to allow current to flow in only one direction. Therefor is an alternating waveform is applied to a diode, then it will only allow conduction over half the waveform. The remaining half is blocked.
Diode rectifier action
Diode rectifier circuit configurations
There are a number of different configurations of diode rectifier circuit that can be used. These different configurations each have their own advantages and disadvantages, and are therefore applicable to different applications.
- Half wave rectifier circuit: This is the simplest form of rectifier. Often using only a single diode is blocks half the cycle and allows through the other. As such only half of the waveform is used.
While the advantage of this circuit is its simplicity, the drawback is the fact that there is longer between successive peaks of the rectified signal. This makes smoothing less effective and more difficult to achieve high levels ripple rejection. Read more about the half wave rectifier .
- Full wave rectifier circuit: This form of rectifier circuit uses both halves of the waveform. This makes this form of rectifier more effective, and as there is conduction over both halves of the cycle, smoothing becomes much easier and more effective.
The two diode version of the full wave rectifier circuit requires a centre tap in the transformer. When vacuum tubes / thermionic valves were used, this option was widely used in view of the cost of the valves. However with semiconductors, a four diode bridge circuit saves on the cost of the centre tapped transformer and is equally effective. Read more about the full wave two diode rectifier .
- Bridge rectifier circuit: This is a specific form of full wave rectifier that utilises four diodes in a bridge topology. Bridge rectifiers are widely used, especially for power rectification, and they can be obtained as a single component contain the four diodes connected in the bridge format. Read more about bridge rectifiers.
- Synchronous rectifier circuit: Synchronous or active rectifiers use active elements instead of diodes to provide the switching. This overcomes the diode losses and significantly improves the efficiency levels. Read more about synchronous rectifiers.
The choice of diode rectifier circuit depends on the application. While the full wave rectifier circuits, and in particular the bridge rectifier circuits are possible the most widely used, half wave rectifier circuits may offer a better option in some circumstances.