Experiment-3
R-C Half & Full Wave Firing circuit
Aim: To control and test the half wave and full wave rectifier circuit using RC triggering circuit.
Apparatus required:
RC triggering circuit kit, Rheostat (50Ω/25W), C.R.O, Digital multimeter and connecting weirs.
Theory:
Half wave rectifier:
By the RC network, a larger variation in the firing angle can be achieved by changing the phase
and amplitude of the gate current. By varying the value of the resistor, the firing angle can be
controlled from 0 to 180. In the negative half cycle, the capacitor C charges through diode D2 with
lower plate positive to the peak supply voltage. This capacitor votage remains constant until supply
votage attains zero value. Now, as the SCR anode votage passes through zero, and becomes
positive, capacitor C begains to charge through resistor from the intial voltage. When the capacitor
charges to positive voltage equal to gate trigger voltage, SCR is triggered and after this, the
capacitor holds to a small positive votage. During negative half cycle, the diode D1, prevents the
breakdown of the gate to cathode junction.
Full wave rectifier:
Here, the AC line voltage is converted to pulsating DC by the full wave diode bridge. This allows
the SCR to be triggered ON, for both half cycles of the line voltage,which doubles the available
power to the load.
In the circuit, the initial voltage from which the capacitor C charges is almost zero. Capacitor C is
set to this low positive voltage by the clamping action of the SCR gate. When the capacitor
charges to a voltage equal to Vg, SCR triggers and rectified voltage appears across load.
Note: This unit consists of R & C components, thyristor ,Bridge rectifier,and an AC supply of 20
volts required to study the Resistance and Resistance - Capacitance triggering of SCR in Half &
Fullwave rectifier.
RC Half wave Firing Circuit
LOAD
F
U
S
E
Rmin
R
+
BRIGE
RECTIFIER
20V
T
R
D2
~
AC INPUT
D1
C
MAINS
FRONT PANEL DETAILS: 1. Mains
: Mains switch for the circuit with built in indicator.
2. 20Volts
: 20Volts / 0.5A AC supply.
3. Fuse
: 1A glass fuse.
4. Bridge Rectifier
: 6A/200V diode Bridge rectifier for full wave firing.
a) AC input
: Terminals to connect AC input to the bridge rectifier.
b) +
: + ve output point of bridge rectifier.
c) -
: - ve output point of bridge rectifier.
5. T
: SCR-TYN 612 - 12 Amps/600V.
6. D1 & D2
: Diodes-1N 5402
7. R
: Control Potentiometer - 5.0 K ohms
8. R min
: Minimum Resistance for gate current limiting-220 ohms
9. C
: Capacitor 4.7 f / 1000V for R – C triggering.
10. LOAD
: Terminals to connect load.
PROCEDURE for R half wave firing circuit:1) Make the connections as given in the circuit diagram.
2) Connect a Rheostat of 50 ohms between the load points.
3) Vary the control pot and observe the voltage waveforms across load, SCR and at different
points of the circuit.
4) The firing angle can be varied from 00 to 900 only in R triggering.
5) In this triggering the synchronized firing angle can be obtained easily and economically in
the positive half cycle of the supply. But there is a draw back that the firing angle can be
controlled at the most at 900 , because the gate current is in phase with the applied voltage.
A resistor is connected in series with the control pot, so that the gate current does not cross
the maximum possible value - Igmax.
6) Draw the waveforms across load and Thyristor for different firing angle.
VARIOUS WAVEFORMS FOR RC HALF WAVE CONTROL
Vs
π
0
2π
3π
VL
0
α
π
2π
2π + α
3π
0
α
π
2π
2π + α
3π
VT
Tabular column:
4π
Space for rough work:
RC Full wave Firing Circuit
LOAD
F
U
S
E
Rmin
R
+
BRIDGE
RECTIFIER
T
R
~
20V
D2
AC INPUT
D1
_
C
MAINS
Connection Diagram for RC Full wave Firing Circuit
PROCEDURE for R full wave firing circuit:1) Make the connections as given in the circuit diagram.
2) Connect a Rheostat of 100 ohms between the load points.
3) Vary the control pot and observe the voltage waveforms across load, SCR and at
different points of the circuit.
VARIOUS WAVEFORMS FOR RC FULL WAVE CONTROL
Vs
0
π
2π
3π
VL
0
α
π
2π
2π + α
3π
0
α
π
2π
2π + α
3π
VT
Tabular column:
4π
Space for rough work: