5.1 INTRODUCTION
The invention of this high-power solid state switching device, known as thyristor, in the year
1957 suggested a complete change in the concepts of control engineering. It was so named as its
characteristics resembled those of its predecessor, i.e. gas-tube thyratron. With the development
of numerous similar devices of alike characteristics, the whole family of such high-power
switching devices has come to be known as thyristors. The whole thyristor family, therefore,
includes the following devices:
• SCR (Silicon controlled rectifier, as the basic semiconductor material is silicon)
• Triac
• SCS (Silicon controlled switch)
• LASCR (Light activated silicon controlled rectifier)
• LASCS (Light activated silicon controlled switch)
• PUT (Programmable unijunction transistor)
• LAPUT (Light activated programmable unijunction transistor)
• DIAC
• SUS (Silicon unilateral switch) or CSCR (Complementary SCR)
• SBS (Silicon bilateral switch)
Power-switching devices are used for power control in the following applications:
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Speed controllers for ac and dc motors
Temperature and illumination controllers
AC and DC circuit breakers
Variable frequency dc–ac inverters
Variable voltage dc–dc converters
Variable frequency ac–ac converters
Variable voltage ac–dc rectifiers
HVDC transmission lines
Figure 5.1 shows the symbolic representations of the devices belonging to the thyristor family.
Fig. 5.1 Symbolic representations of devices of thyristor family.