Fig. 2,136.—Diagram of parallel circuit. It is a constant pressure circuit and is very widely used for lighting and power. If each lamp takes say ½ ampere, the current flowing in the circuit will vary with the number of lamps in operation; in the above circuit with all lamps on, the current is ½ × 5 = 2½ amperes.
Ques. State a disadvantage.
Ans. Some source of direct current for field excitation is required.
Ques. In a constant current series system, upon what does the voltage at the alternator depend?
Ans. The number of devices connected in the circuit, the volts required for each, and the line drop.
Parallel Circuits.—These are used for constant pressure operation. Such arrangement provides a separate circuit for each unit making them independent so that they may vary in size and each one can be started or stopped without interfering with the others. Parallel circuits are largely used for incandescent lighting, and since low pressure current is commonly used on such circuits they are usually connected to step down transformers, instead of direct to the alternators.
Fig. 2,137.—Diagram of parallel series circuit, showing fall of pressure between units. This system is very rarely used; it has the disadvantage that if a lamp filament breaks, the resistance of the circuit is altered and the strength of the current changed. The voltmeter shows the fall of pressure along the line. It should be noted that, although the meter across AB is shown as registering zero pressure, there is, strictly speaking, a slight pressure across AB, in amount, being that required to overcome the resistance of the conductor between A and B.
Parallel Series Circuits.—Fig. 2,137 shows the arrangement of a parallel series circuit and the pressure conditions in same. Such a circuit consists of groups of two or more lamps or other devices connected in parallel and these groups connected in series.