Part of Lecture series on EE321N, Power Electronics-I delivered by me during Fifth Semester of B.Tech. Electrical Engg., 2012 Z H College of Engg. & Technology, Aligarh Muslim University, Aligarh Please comment and feel free to ask anything related. Thanks!

1. EE-321 N
Lecture-11
RC Triggering Circuit

2. RC Half Wave Circuit
LOAD
vO
R
C
VT
v =V sin tS m 
D2
VC
+
-
D1
31-Oct-12 2EE-321N, Lec- 11

3. Waveforms
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4. 31-Oct-12 4
vs
0
V sin tm 
0 t
t
a
vc
- /2
a
vc
Vgt
vo
vT
 
  
 
Vm
-Vm
t
High Value
of R

5. 31-Oct-12 5
Low Value
of R
t
vs
0
V sin tm 
0 t
a
vc
- /2
a
vc
Vgt
0
0
vo
vT
Vm Vm


  
-Vm
(2 + ) 
t

6. 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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7. 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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8. 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
T
RC f 

  
1s GT D
GT
v V V
R
I
 


9. RC Full Wave Circuit
vO
R
C
VT
v =V sin tS m 
+
-
LOAD
+
-
D1 D3
D4 D2
vd
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10. Waveforms
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11. 31-Oct-12 11
vs
vd
vo
vT
t
t
t
V sin tm 
vd
vc vc vcvgt
  
High Value
of R

12. 31-Oct-12 12
V sin tm vs
vd
vo
vT
t
t
t
vgt

Low Value
of R

13. 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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14. Empirical Relationships
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50 157
2
T
RC

 
s GT
GT
v V
R
I



15. 31-Oct-12 EE-321N, Lec- 11 15