CHAPTER
5
Additional Problems
Solved Problems
5.1
In the phase control circuit shown in Fig. E 5.1, obtain the maximum permissible value of
resistor R1 and the corresponding firing angle. Diac and triac ratings are:
Diac (ST2 type): VBo = 30 V, IBO = 100 mA.
Triac (SCR 245): V = 400 V, VGTmax = 2.5 V
I = 10 A, IGTmax = 50 mA.
For reliable triggering of triac, RG = 1 k W and C1 = 0.1 mF.
Assume that capacitor C1 reset to zero at the end of every half-cycle.
Sol.
A
Load
R1
Diac
230 V
50 Hz
Tricac
R
RG
B
Fig. E 5.1
The minimum firing angle at which triac will trigger is given by
Ê 30 ˆ
ÊV ˆ
amin = sin–1 Á BO ˜ = sin–1 Á
Ë 2.V ¯
Ë 230 2 ˜¯
= 5.29°.
\ Maximum firing angle possible
amax = 180° – 5.29° = 174.71°.
Value of resistance
R1 =
=
2 2V
w ◊ C1 . VBO .
2 2 ¥ 230
2p ¥ 50 ¥ 0.1 ¥ 10-6 ¥ 30
= 690.25 kW.
C1