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.With the aid of solid-state devices, the automotive industry has quite successfully overcome an irritating reliability and maintenance problem that had plagued the family car for several decades. Although the dc generator formerly used to keep the battery charged incorporated some clever design principles, it was often subject to trouble. This stemmed primarily from the need for the commutator and brush system to carry the full charging current. The overall effects of arcing, wear, and dirt rendered this vital engine accessory a candidate for failure—and all too often, when least anticipated. Also, the unreliability of the automotive electrical system was further reinforced by the electromechanical voltage regulator associated with the generator.
Modern cars use three-phase alternators with self-contained silicon-diode rectifiers. These machines employ slip rings to conduct current to the rotating field. However, a slip ring is a much simpler device than a commutator. Moreover, the field current is a small fraction of the charging current often supplied to the battery. Therefore, the slip rings and brushes require infrequent maintenance compared with the commutator and brush assembly of the old dc generators. The electro mechanical voltage regulator did not immediately become obsolete, but the trend is now clearly toward the electronic voltage regulator.
A typical electronic voltage regulator is shown in FIG. 23. This circuit senses the battery voltage and causes the alternator to deliver either full charge or none at all. This operating mode simulates that of the electromechanical regulator. Whereas the electromechanical regulator might turn the alternator field current off and on at a 100-Hz rate when the charge state of the battery was marginal, the electronic regulator switches on and off at a rate in the vicinity of 1000 Hz. This in itself is of no particular advantage. The important feature is that there is no mechanical wear in the solid-state type voltage regulator.
In the circuit of FIG. 23, the input transistor conducts only when battery volt age becomes high enough to break down the zener diode in its base lead. When this happens, the Darlington output stage is turned off and the field winding of the alternator is deprived of current. Depending on the battery and its load, the field circuit will be energized and opened at widely varying duty cycles. This type of regulator is more closely related to the switching-type power supply rather than to the “linear” voltage-regulated supply.
The circuit of FIG. 23 is not critical—many types of power transistors will be found suitable. It’s particularly adapted for use in mobile radio and in standby power systems that use the automotive alternator.
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