Electrical engineering - Basics Page 50

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9.2 Rectifiers

Rectifiers convert alternating current or three-phase alternating current into direct current. Rectifier circuits consist of a rectifier transformer and a rectifier unit (rectifier diode).

Diagram 64: Structure of a rectifier circuit

The function of a rectifier diode is comparable with the function of a semiconductor diode (see discussion under point 9.1). The only difference is that the rectifier diode is working with direct current. For this reason, the function of the rectifier diode will not be explained here in more detail.

The use of rectifier diodes depends on the number and on the arrangement of the diodes. Normally, in regard to their use in the sanitary, heating and air-conditioning sector one distinguishes between the single-pulse midpoint circuit (one-way/half-wave rectifying) and the two-pulse bridge circuit (two-way rectifying).

The single-pulse midpoint connection only uses one half oscillation of the alternating current. In this way, a pulsating alternating current is created. This characteristic is e.g. used for the ionisation flame control of gas burners. The gas flame of these devices is electrically conducting. The effect of the flame is only directed in one way and so a rectifier effect is resulting. The pulsating direct current the flame is producing can be used to control the flame: If the flame is burning, a pulsating direct current is applied to the control unit. If the flame is not burning, no direct current is applied and the device switches off.

Diagram 65: Single-pulse midpoint circuit

The double-pulse bridge circuit uses both/two half oscillations for regulation and control purposes. Since devices like water heaters, pumps, sanitary installations and also welding sets can not use any pulsating direct current (on-out effect), double-pulse bridge circuits with four rectifier diodes are used to achieve a continuing supply with direct current.

Diagram 66: Double-pulse bridge circuit

9.3 Exercises

1. Conduct different measurements using training installations and devices.


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