Page 1005 - Basic Electrical Engineering

From a fixed ac voltage, a variable ac output voltage can be made available
using SCR–diode combinations as shown in Fig. 14.42.
In the positive half cycle of the input voltage when terminal A is positive,

SCR T and diode D will conduct. The conduction angle is decided by the
1
2
time at which the SCR, T will be triggered. In the negative half cycle,
1
terminal B is positive, and hence SCR T and diode D will conduct. The
2
1
time of triggering of the gate of SCR, T will decide the conduction angle in
2
the negative half cycle. By changing the firing angle of the SCRs, a variable
ac voltage will be available across the load, and hence load current will

change.

14.11 DIAC
Like an SCR, DIAC is also a member of the thyristor family. A DIAC is a

bidirectional diode which conducts in both directions whereas a diode
conducts in one direction. Unlike an ordinary diode, a DIAC conducts only

when the applied voltage reaches the breakover voltage. DIAC is a two-
terminal device without a gate terminal. When the applied voltage between its

two terminals, which are termed A and A , is raised to the breakover
2
1
voltage, the DIAC starts conducting. The symbolic representation and V–I
characteristics of a DIAC have been shown in Fig. 14.43. Terminal A is
1
called anode 1 and terminal A is called anode 2.
2

14.12 TRIAC
A TRIAC combines in itself two SCRs connected in anti-parallel or inverse-

parallel with one gate terminal as has been shown in Fig.14.44 (a). The four
layers n p n p form one SCR and the other four layers p n p n form the
4
2
1
1
3
2
2
3
other SCR. Layer p is common to both SCRs and is used as the gate as
2
shown.