l11 rc triggering circuit
TRANSCRIPT
EE-321 N
Lecture-11
RC Triggering Circuit
RC Half Wave Circuit
LOAD
vO
R
C
VT
v =V sin tS m
D2
VC
+
-D1
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Waveforms
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vs
0
V sin tm
0 t
t
avc
- /2
a
vc
Vgt
vo
vT
Vm
-Vm
t
High Value of R
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Low Value of R
t
vs
0
V sin tm
0t
avc
- /2
a
vc
Vgt
0
0
vo
vT
Vm Vm
-Vm
(2 + )
t
Working
• The limited firing angle range of R triggering ckt is overcome in RC ckt where the range is 0°-180° (controlled by the variable resistance)
• In the above ckt, capacitor gets charged (through D2) to −Vm in every –ve half cycle
• Upto the zero crossing of the AC supply wave, voltage may decrease to a lower value but can be assumed constant for simplicity
• In +ve half cycle C begins to charge through R
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Working...contd
• As soon as the capacitor voltage rises to VGT, SCR triggers
• Capacitor holds a small constant voltage after this
• D1 serves to prevent breakdown of the G-K junction during –ve half cycle
• Observing the waveform it can be said that firing angle can never be exactly 0° & 180°
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Empirical Relationships
• In the range of power frequencies, it can be shown that the value of RC for zero o/p voltage is given by
f = 1/T is the frequency of the AC line frequency
• At the triggering instant, vC = VGT + VD1
• Max value of R can be found as follows
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1.3 4, 2
2
TRC f
1s GT D
GT
v V VR
I
RC Full Wave Circuit vO
R
C
VT
v =V sin tS m
+
-
LOAD+
-
D1 D3
D4 D2
vd
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Waveforms
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vs
vd
vo
vT
t
t
t
V sin tm
vd
vc vc vcvgt
High Value of R
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V sin tm vs
vd
vo
vT
t
t
t
vgt
Low Value of R
Working
• The above trigger circuit gives full wave o/p
• Diodes form a full-wave bridge
• Initial voltage from which the C begins to charge is almost zero
• Again SCR triggers at vC = VGT
• At the triggering instant vC resets to zero by the clamping action of SCR gate
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Empirical Relationships
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50 157
2
TRC
s GT
GT
v VR
I
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