Page 1008 - Basic Electrical Engineering

and as such, control is achieved on both positive and negative half cycles of
the input voltage.
A TRIAC control circuit for the speed control of a single-phase induction

motor has been shown in Fig. 14.45. A DIAC has been used to provide the
gate voltage for triggering the TRIAC. These days a DIAC–TRIAC

combination is available in the market. The input and variable output voltage
wave forms have been shown in Fig. 14.45 (b). Let us consider the time when

the TRIAC is off and the input voltage has been applied. During the positive
half cycle of the input voltage, the capacitor is charged as current will flow

through R , R , and C. When the voltage across the capacitor, VC becomes
1
2
equal to the sum of the DIAC switching voltage and the gate triggering
voltage of the TRIAC, the DIAC, D conducts and passes a gate current to
trigger the TRIAC, Q. Now the capacitor will start discharging. The capacitor

will continue to discharge until the capacitor current falls below the holding
current level of D. The TRIAC will get switched off (commutated) at the end
of the positive half cycle of the input voltage. The same process of the

capacitor being charged in the negative half cycle of the supply will take
place and help make the TRIAC getting switched on through the DIAC

supplying gate voltage. The firing angle, a of the TRIAC can be controlled by
adjusting the value of the variable resistance R . The time constant of the R–
1
C circuit (R + R and C) get altered when R is changed, and hence the rate
1
1
2
of charge of C changes.

14.13 OPTOELECTRONIC DEVICES
Photo-electronic devices are made of semiconductor material whose

conduction is affected by the amount of light falling on them. They also
produce light; their resistance change with light falling on them. Some photo-

electronic devices emit light and modify light. For example, Light emitting
diodes (LEDs) produce light. Liquid crystal displays (LCDs) modify light.

Both LEDs and LCDs are used for electronic display of numerals and
alphabets. Light is converted into electricity by solar cells. Resistance of
certain material is affected by the amount of light falling on them. They are