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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