EE-321 N

Lecture-10

Different Triggering Circuits

Resistance Triggering Circuit

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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R Triggering Circuit

LOAD

vO

a b

i R1

R2

D

R Vg

VT

v =V sin tS m

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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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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 mgm 1

1 gm

V VI R

R I

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 1m2

1 m gm

, 0gm

V RVR V R R

R R V V

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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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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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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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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Contd...

All other quantities except R2 are fixed. Hence firing angle is proportional to R2

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mgp

1 2

GT 1 21

m

Since,

sin

V RV

R R R

V R R R

V R

Waveforms

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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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VS

2

3 4

t

V sin tm

Vg Vgt

t

t

t

t

Vo

io

VT

VgpVgtVgp

R Large216-Oct-12 14

Case 1: No Triggering

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t

t

t

t

t

2

3 4

VS

Vg

Vo

io

VT

V =Vgp gt

2700

2

3 4

900 =90

0

Case 2: α = 90°

t

t

t

t

t

2

3 4

VS

Vg

Vo

io

VT

<900

V >Vgp gt

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Case 3: α < 90°