Understanding SCR Power Controls and Types of SCR Firing

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Introduction:
Power Control Using SCR
 SCR or Silicon Controlled Rectifier is a 3 pin device, having three basic
terminals- anode, cathode and gate. The gate terminal is the control
terminal for the application of anode-cathode voltage. Typically Silicon
is used because of its low leakage current. The polarity of the voltages
applied to the cathode and anode decides whether the device is in
forward bias or reverse bias and the gate voltage decides the
conduction of the SCR.
 In other words, when forward bias is applied to the SCR, after proper
positive gate voltage is applied, the device starts conducting and is
turned off only when the current through the device is made less than
the holding current. Thus the SCR can be used as a switch.

Block Diagram

SCR Firing
 Triggering or firing is one of the key issues associated with SCRs or
thyristors.
 SCR triggering methods need to be considered when designing a circuit
using one of these components.
 The performance of the circuit and the very way it operates will be
largely dependent upon the SCR triggering or firing method chosen.
 The application of GATE voltage is known as firing.

Types of SCR
firing
 Generally there are two types of firing:
1. Zero Voltage Cross Over firing
2. Phase angle control method

Zero Voltage Cross Over firing:
 Zero-crossing control mode is also called fast cycling, integral cycle, or burst
firing.
 It operates by turning the SCR’s on only when the instantaneous value of
the sinusoidal voltage is zero.
Phase angle control method:
 The phase angle is varied.
 The application of gate pulses is delayed by certain time and the
conduction is controlled.

Firing Circuits
Features of Firing
Circuit:
 Firing circuits should produce trigger pulses for the thyristor at
appropriate instants.
 There needs to be an electrical isolation between firing circuits
and the thyristor.
 It is achieved using a pulse amplifier or an Opto-isolator.

Types of Firing
Circuit:
R-Firing
Circuit:

Types of Firing
Circuit:
RC Firing Circuit:

Types of Firing
Circuit:
UJT Firing Circuit:

Firing Angle:
 The number of degrees from the beginning of the cycle when SCR is
switched on is firing angle.
 Any SCR would start conducting at a particular point on the ac source
voltage.
 The particular point is defined as the firing angle.
 The earlier in the cycle the SCR is gated ON, the greater will be the
voltage applied to the load.

Firing Angle:
SCR controlled Rectifier by Elwood Gillilan

Firing Angle Control
 The firing angle control is achieved by varying the time of application of
Gate pulses to the SCR.
 The voltage to the Gate terminal of the SCR can be applied at a given
time decided by the remote input.
 Controlling the firing angle means to manage the point on the AC signal
waveform when the SCR is going to be triggered.
 The time corresponding to the AC signal waveform when the SCR gate
is going to be given DC supply voltage.
 Normally to trigger a SCR we use optoisolator.

Firing Angle Control
 For simple power application circuit, where no power control is
required.
 Normally zero crossing detectors or optoisolators having zero crossing
detectors can be used.
 The SCR is triggered only at the zero crossing levels of the AC
waveform.
 The gate is triggered using pulses and the firing angle is varied
accordingly to control the switching of the SCR and accordingly the
power of the SCR.

 The variation in firing angle or variation of conduction of SCR
by delaying the application of gate current can be done by two
ways:
1. Phase Shifting Gate control
2. Pulse Triggering

Phase Shifting Gate Control
 It causes 0 to 180⁰ delay of conduction.
 The phase angle of the gate voltage is changed with respect to the
anode cathode voltage.
 The gate voltage is applied out of phase with the anode voltage.
 Usually capacitance or inductances are used for this purpose.
 In the LR combination, current lags the voltage.
 In a RC combination, current leads the voltage.

 The resistor R is varied so as to vary the phase angle by which the gate
voltage delays from the anode voltage.
Digitally Controlled Phase Shift
Different circuits used as a phase shifter

Phase Shift Oscillator

Pulse Triggering
 The gate voltage can also applied by giving pulses to the Gate terminal.
The duty cycle of the pulses can be varied to provide variation in the
conduction.
Pulses can be generated either using UJT or using 555 timers.

Working Example of Firing Angle Control

 The above block diagram represents the system for achieving
power control to induction motor using firing angle control for
back to back SCRs.
 Before going into details about how firing angle control is achieved
in this system.
 Let us have a quick look at back to back connection of SCR.

Firing Angle Control for Back to Back SCRs
 A back to back SCR connection is used to provide AC power to the load
in both half cycles of the AC signal.
 Two optoisolators are connected to each SCR.
 In the first half cycle of the AC signal.
 One of the SCR conducts after getting triggered using an optoisolator
and allows the current to pass through the load.
 In the second half cycle, another SCR connected in reverse direction to
that of the other SCR.

 It gets triggered using another optoisolator and allows current to flow to
the load.
 Thus the load gets AC power in both the half cycles.
 In this system, the SCR is triggered using an optoisolator containing a
combination of a LED and a TRIAC.
 When pulses are applied to the LED.
 It emits light which falls on the TRIAC and it conducts, causing the
output pulses from the optoisolator to the SCR.

 The principle involves controlling the rate of application of pulses by
varying the frequency between adjacent pulses.
 A microcontroller is used to provide pulses to the optoisolator based on the
push button input interfaced to it.
 The number of times the push button is pressed decides the amount of
delay of application of pulses.
 Thus the angle at which the SCR is triggered is controlled accordingly.
 The application of AC power to the load is controlled.

Applications
 Used for AC power control.
 Used for controlling the intensity of a bulb.
 Used for controlling the speed of fan motor.
 Switching, zero-voltage switching.

Conclusion
 SCR firing – ZVS and Phase angle control with 3 firing circuits. Find
about firing angle control and application circuit. Also know about
back to back SCRs to achieve power control.

Understanding SCR Power Controls and Types of SCR Firing

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