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-10
Different Triggering Circuits
Resistance Triggering Circuit

2. Different Triggering Circuits
1. Resistance (R) Triggering Circuit
2. Resistance-Capacitance (RC) Triggering Circuit
3. UJT Based Triggering Circuit
4. IC Based Triggering Circuit
Many of these circuits are not sophisticated and
not used practically but they provide a basic
understanding of the SCR triggering
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3. R Triggering Circuit
LOAD
vO
a b
i R1
R2
D
R Vg
VT
v =V sin tS m 
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4. R Triggering Ckt...contd
• Simplest triggering circuit
• Limited triggering angle range (0° to 90°)
• Performance depends upon temperature &
SCR characteristics
• In the above fig. R1 is the current limiting
resistor, R2 is the variable resistor which
controls the firing angle and R is the stabilizing
resistor
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5. R Triggering Ckt...contd
• If R2 = 0, then the current is limited by R1.
• This current should not > max. permissible
gate current Igm. Therefore, R1 can be found as
follows
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m m
gm 1
1 gm
V V
I R
R I
  

6. R Triggering Ckt...contd
• R is chosen s. t. max. voltage across it doesn’t
exceed max. forward gate voltage Vgm.
Therefore,
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 gm 1m
2
1 m gm
, 0gm
V RV
R V R R
R R V V
   
 

7. R Triggering Ckt...contd
• Gate trigger ckt draws a small current due to
large values of R1 & R2.
• Gate voltage vg is a half wave pulse because
diode D allows the flow of current only in +ve
half cycle. Its amplitude is governed by R2
• Next, we discuss cases for different values of
R2
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8. Case 1: R2 is large, No triggering
• When R2 is large, current i is small and voltage
vg = iR is also small
• If peak value of gate voltage Vgp < VGT , SCR will
not turn ON and accordingly there will be no
O/P voltage or current and the supply voltage
will appear ax the SCR
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9. Case 2: α = 90°
• When R2 is decreased s. t. Vgp = VGT , α = 90° is
obtained which can’t increase beyond this
value
• This is because the thyristor latches into
conduction as soon as Vgp becomes equal to
VGT for the first time
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10. Case 3: α < 90°
• When Vgp > VGT , α < 90°
• Also α can’t be zero however large vg may be.
• Min. value of α is about 2° − 4° (which is
obtained when R2 = 0)
• Relationship between Vgp & VGT is
Vgp sinα = VGT
or, α = sin−1 (VGT/Vgp )
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11. Contd...
All other quantities except R2 are fixed. Hence
firing angle is proportional to R2
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 
m
gp
1 2
GT 1 21
m
Since,
sin
V R
V
R R R
V R R R
V R
 
 
  
  
   
  
 

12. Waveforms
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13. Tip for
Drawing Waveforms
• Use a graph paper or math notebook (with
squares) to draw waveforms
• This will help you to remember (& draw fast in
exam!) the waveforms easily
• Take 1 cm = 30° (on x-axis) & 2 cm = max. amp.
of sine wave (on y-axis). Use different colors for
showing different waves on same axis
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14. VS
 2
3 4
t
V sin tm 
Vg Vgt
t
t
t
t
Vo
io
VT
Vgp
VgtVgp
R Large2
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Case 1:
No Triggering

15. 16-Oct-12 15
t
t
t
t
t
 2
3 4
VS
Vg
Vo
io
VT
V =Vgp gt

270
0
 2
3 4

90
0 =90
0
Case 2:
α = 90°

16. t
t
t
t
t
 2
3 4
VS
Vg
Vo
io
VT
<90
0
V >Vgp gt
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Case 3:
α < 90°