454 Electrical Power Systems Technology
IMPORTANT TERMS
Chapter 17 describes control devices that are commonly used in elec-
trical power control systems. After studying this chapter, you should have
an understanding of the following terms:
Silicon Controlled Rectifier (SCR)
Circuit Switching
I – V Characteristics
DC Power Control
AC Power Control
Triac
Static Switching
Start-Stop Control
Diac
SILICON-CONTROLLED RECTIFIERS
A silicon-controlled rectifier or SCR is probably the most popular
electronic power control device today. The SCR is used primarily as a
switching device. Power control is achieved by switching the SCR on and
off during one alter-nation of the ac source voltage. For 60 Hz ac the SCR
would be switched on and off 60 times per second. Control of electrical
power is achieved by altering or delaying the turn-on time of an alterna-
tion.
An SCR, as the name implies, is a solid-state rectifier device. It con-
ducts current in only one direction. It is similar in size to a comparable
silicon power diode. SCRs are usually small, rather inexpensive, waste
little power, and require practically no maintenance. The SCR is available
today in a full range of types and sizes to meet nearly any power control
application. Presently, they are available in current ratings from less than
1 A to over 1400 A. Voltage values range from 15 to 2600 V.
SCR Construction
An SCR is a solid-state device made of four alternate layers of P- and
N-type silicon. Three P-N junctions are formed by the structure. Each SCR
has three leads or terminals. The anode and cathode terminals are similar
to those of a regular silicon diode. The third lead is called the gate. This