Rectifier

Rectifier converts an alternating current into a pulsating direct current. Depending on the physical and technical conditions, all those electrical components can be used for this purpose that only allows electricity to pass in a certain direction. Such components include semiconductor diodes, but also electron tubes, gas tubes or electrolyte rectifiers.

Rectifiers convert an alternating current into a pulsating direct current. Depending on the physical and technical conditions, all those electrical components can be used for this purpose that only allows electricity to pass in a certain direction. Such components include semiconductor diodes, but also electron tubes, gas tubes or electrolyte rectifiers.

Diode rectifier

Semiconductor diodes usually referred to as diodes for short, only let the electrical current through if they are polarized in the forward direction. In the reverse direction, an almost charge carrier-free boundary layer forms in them, which prevents the flow of current. Therefore, a single diode installed in an AC circuit converts the alternating current into a pulsating direct current (Fig. 1). This so-called one–way rectification creates current curve over time in which half oscillation periods are prevented. During this time, no electrical current flows in the circuit and therefore the average current strength is relatively low.

Half-wave rectification
Half-wave rectification

This disadvantage of one-way rectification can be remedied by two-way rectification. A circuit made up of four diodes directs both half-oscillations of the electric current in the same direction. A pulsating direct current is obtained (Fig. 2).

Tube rectifier

Rectification can also be done with the help of electron tubes. One then speaks of a tube rectifier. In an electron, tube the electrons only ever come from one electrode, the so-called cathode. Incandescent cathodes have a heating coil that accelerates or even enables the release of electrons by heating the cathode material. Furthermore, cathodes are made of metals that release electrons particularly easily. They also have a special shape. The electrons emitted at the cathode migrate to the anode and are absorbed there. This process cannot be carried out in the opposite direction because the tube anode, even if the negative pole of a voltage source is connected to it, no electrons are emitted. If you connect a simple electron tube to an alternating current circuit, just like a semiconductor diode, it only allows the current to pass in one direction.
Different gas pipes can only let the flow-through in one specific direction.
In principle, the one-way and two-way rectification described for semiconductor diodes can also be carried out with tubes.

Electrolyte rectifier

Another form of rectifier is electrolyte rectifier. When charging, the chemical properties of the electrodes of a battery change in different ways. The pole of a battery to which the positive voltage connection was during the charging process then itself works as a positive pole when the battery is discharged. The same happens with the negative pole of the battery. In an alternating current circuit, an accumulator only allows a current to pass if the external polarity corresponds to its impressed self-polarity. It works similar to a diode and carries out a half-wave rectification in the circuit.

Full wave rectification
Full-wave rectification

DC Smoothing

An exact DC voltage is only supplied by accumulators. Direct current generators or rectifier systems only provide a pulsating direct current that has to be smoothed for many applications. For this purpose, capacitors or special choke coils are used, often a combination of both components.
An exact DC voltage is only supplied by accumulators. Direct current generators or rectifier systems only provide a pulsating direct current that has to be smoothed for many applications. For this purpose, capacitors or special choke coils are used, often a combination of both components.

Pulsating direct current can have different current profiles. Sometimes the current strength drops almost to the value 0 A (Fig. 1) or has a jagged curve in the current strength-time diagram, which is why it is also referred to as a saw-tooth curve. Such temporal current fluctuations can be smoothed.

pulsating direct current

Since it always takes a certain amount of time until the plates of a capacitor have filled with charge carriers or emptied of them, ie a certain current always flows, there are no extremely brief changes in the current intensity in a pulsating DC circuit with a built-in capacitor. It smooths out any changes in current intensity introduced from the outside (Fig. 2).
A choke coil acts in a similar way

 According to the law of induction, short-term fluctuations in the intensity of the current always cause a counter-voltage in the coil, which counteracts the change in the intensity of the current. This also results in a smoothing of the current curve.

pulsating direct current smoothed by capacitor
pulsating direct current smoothed by capacitor

For many purposes, the alternating current smoothing caused by a capacitor is not sufficient. Then special filter circuits are used, which usually consist of a matched combination of ohmic resistance and capacitor ( RC elements ) or a combination of coil and capacitor ( LC elements ) (Fig. 3).

RC element

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