Silicon Control Rectifier

• Introduction :                             

   Silicon Control Rectifier is electronic semi-conductor device. Its also known as current controlled device. The name "silicon controlled rectifier" is general electric"s trade name for type of thyristor. The SCR was invented by 

Gordon Hall and Frank W. "Bill" Gutzwiller in 1957. SCR is unidirectional device so that it conduct current only in one direction.

• Construction :

 Silicon Control Rectifier (SCR) is three terminal device(Anode, Cathode, Gate),which consists four layers of semi-conductor, which form NPNP or PNPN structure having  three P-N junction terminal i.e J1, J2, J3.

Anode (A) terminal is connected to p-layer, and Cathode(K) terminal is connected to n-layer  and Gate(G) terminal is connected to p-layer nearest to cathode.

Fig-Symbol Of SCR

Fig-Silicon Control Rectifier

Fig- Actual SCR

• Operation & Working :                

There are three modes of operation for an SCR are follows ;

1. Forward Blocking Mode 
2. Reverse Blocking Mode
3. Forward Conduction Mode

1.Forward Blocking Mode : 

In this mode of operation, here we applied positive voltage to the anode terminal and negative voltage to the cathode, there is no any  supply given to the gate terminal. Due to this junction J1 and J3 are p forward biased and junction J2 are reverse biased. So that there is small leakage current flow through andoe to cathode.

2.Reverse Blocking Mode :

In this mode of operation, applied negative voltage to the anode terminal and positive voltage to the cathode terminal. Due to this junction J2 is forward bias and Junction J1 and J3 are reverse bias. so that reverse leakage current flow through cathode to anode.

3.Forward Conduction Mode :

 SCR operate in conduction mode by two methods :

A)  Increases the positive voltage  through anode to cathode beyond break over voltage.

B) By applying positive pulse at gate terminal.

Once SCR start conducting, no more gate  pulse  is required to keep SCR in ON- state.

For turn OFF SCR reduce the current below the minimum value called holding current.

    V-I  characteristics of SCR :

Dynamic Characteristics :

   Dynamic Characteristics of SCR are follows :

       1.Turn ON characteristics 
       2.Turn OFF Characteristics