Page 23 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 8 Fig 9

Fig 10 Fig 11

Transistor : Figure 12a shows the physical appearance of transistors. There are two symbols to represent a
transistor. (Fig 12b). The selection of a symbol is based on either the NPN or the PNP type of transistor.
SCR (Silicon controlled rectifier) : Figure 14a shows the physical appearance of one type of SCR and the
symbol is shown in Fig 13b. SCRs are also called thyristors and used as switching devices.

Fig 12 Fig 13

Diac : A diac (Fig 14a) is a two-lead device like a diode. It is a bidirectional switching device. Its symbol is shown
in Fig 14b.
Triac : A triac is also a semiconductor device with three leads like two SCRs in parallel. The triac can control the
circuit in either direction. (Fig 15)
Bridge rectifier or diode bridge : It is a single package of four semiconductor diodes connected in bridge circuit.
The input AC and the output DC leads are marked and terminated as shown in the Figure 16.

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CITS : Power - Electrician & Wireman - Lesson 60-69