Silicon Controlled Rectifier (SCR) | Terms, Working, Thyristors, and Types

Introduction to SCR

SCR is a semiconductor device named as silicon controlled rectifier one of the important members of the thyristor family. SCR is a unidirectional current controlled device. It has three terminal named as the anode(+ve terminal), cathode (-ve terminal), gate(control terminal). Gate terminal controls the flow of current between the anode and cathode.SCR is a special type of rectifier that can withstand at high voltages whereas a normal diode may get destroyed at high voltages.

SCR controls high power and converts high AC currents into DC current. It is used for various applications like rectification, regulation of power and inversion, etc. SCRs are also used in switching circuits, DC motor drivers, AC/DC static switches, etc.

Silicon Controlled Rectifier (SCR) | Terms, Working, Thyristors, and Types
Silicon Controlled Rectifier (SCR) | Terms, Working, Thyristors, and Types

When an ordinary junction and a junction transistor combined in a single unit form a pump device. Three terminals are taken as Anode, cathode, and gate as shown in figure 2.Symbolic representation of SCR is shown in figure 1.

Working of SCR: To explain the working of SCR load is connected in series with the device and anode is always positive with respect to the cathode. Initially, no voltage is applied to the gate, then junction J2 is reverse biased while junction J1 and J3 are forward biased. Here junction J1 and J3 formsanpn transistor with a base open-circuited, hence no current flow and SCR is said to be cut off. When the gate voltage is applied and increased slowly until reverse-biased junction J2 gets breakdown. At this stage, the SCR conducts a large amount of current and called in on state. The amount of applied voltage at which SCR conducts heavily with gate open-circuited is called break overvoltage.

When a small positive voltage is applied at the gate terminal junctionJ3 is Forward bias, junction J2 is reverse bias and the electrons from n-type and holes from p-type starts moving across J3 .electrons from junction J3 get attracted by J2 and gate current start flowing.

Silicon Controlled Rectifier (SCR) | Terms, Working, Thyristors, and Types
Silicon Controlled Rectifier (SCR) | Terms, Working, Thyristors, and Types

As gate current start flowing anode current increases. Increasing anode current makes more electrons available at junction J2. Due to this J2 gets breakdown and SCR starts conducting heavily. This time the gate terminal loses control over the SCR. If the gate voltage is removed a node current does not decrease at all. The only way to stop conduction is to reduce the applied voltage to zero.

Silicon Controlled Rectifier (SCR) | Terms, Working, Thyristors, and Types

There are some important terms related to SCR as described

1) Breakover Voltage

Defined as the minimum forward voltage, the gate being open at which and SCR starts conducting heavily that is turned on

2)Peak reverse voltage

It is defined as the maximum reverse voltage applied to an SCR without conducting in the reverse direction.

3) Holding current

Holding current is defined as the maximum anode current gate being open at which SCR gets turned off from on conditions.

4) Forward current rating

It is the maximum amount of anode current that an SCR is capable of passing without destruction

VI Characteristics of SCR

When the anode is positive with respect to cathode the graph between V and me is called forward characteristics. From the fig.7 region, OBAC shows forward characteristics of SCR with gate current set to zero. When the anode is negative with respect to cathode the graph between and I am known as reverse characteristics. Region OFG shows reverse characteristics whereas the FG portion denoted the avalanche breakdown region.

Silicon Controlled Rectifier (SCR) | Terms, Working, Thyristors, and Types

Different Types of Thyristor

SCR is discussed above.

RCT (Reverse conducting Thyristor) Generally Thyristors allow current in forwarding direction and blocks reverse current. RCT is an SCR integrated with a reverse diode that eliminates unwanted loop inductance and reduces reverse voltage transient. Applications of RCTs are in inverters and DC drivers.RCTs or reverse conducting thyristors applications in frequency inverters and changers, used in the AC controller by using Snubbers circuit.

• Gate turn off thyristors or GTOs

• Emitter turn off thyristors or ET

• Bidirectional Triode Thyristors or TRIACs

• MOS turn off thyristors or MTOs

• Bidirectional phase controlled thyristors or BCTs

• Fast switching thyristors or SCRs

• Light-activated silicon controlled rectifiers or LASCRs

• FET controlled thyristors or FET-CDs

Gate turn off Thyristors (GTOs)

It is one of the special types of high power semiconductor devices is GTO (gate turn-off thyristor). The gate terminal controls the switches to be turned ON and OFF. If +ve pulse applied between the cathode and gate terminals, then the device will be turned ON. Cathode and gate terminals act like a PN junction and there exists a small voltage relatively between the terminals. If –ve triggering is applied between the gate and cathode terminals, then the device will turn OFF

Applications of GTOs

• Variable speed motor drives
• High power inverters and traction
• GTO Application on Variable Speed Driver

Emitter Turn OFF Thyristor

The Emitter turns OFF thyristor will turn ON and turn OFF by using MOSFET. It includes both the advantages of the MOSFET and GTO. It consists of two gates- one gate is used to turn ON and another gate with a series MOSFET is used to turn OFF.


ETO is used for the fault current limiter and solid-state circuit breaker.loss.

Bidirectional Triode Thyristors or TRIACs

TRIAC is a current controlling device. It is a three-terminal semiconductor device. It is derived from the name called Triode for Alternating Current. Thyristors can conduct only in one direction, but TRIAC is able to conduct in both directions.


Used in Domestic light dimmers, small motor controls, electric fan speed controls, controlling of small domestic AC power appliances.

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