SCR (Silicon-Controlled Rectifier)

A Silicon-Controlled Rectifier is a four layer solid state current controlling device. It forms PNPN structure and has three junctions, labeled anode, cathode and gate. Anode terminal is connected to the P-type material, cathode terminal is connected to the N-type material and the gate is connected to the P-type material nearest to the cathode. SCRs are unidirectional devices (i.e. can conduct current only in one direction)

Modes of Operation:

• Forward blocking mode (OFF state)
• Forward conduction mode (ON state)
• Reverse blocking mode (OFF state)

Forward blocking mode

In this mode of operation the anode is given a positive potential while the cathode is given a negative voltage keeping the gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from the anode to the cathode until the applied voltage reaches its breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it starts conducting but below breakover voltage it offers very high resistance to the flow of current and is said to be in off state.

Forward conduction mode

SCR can be brought from blocking mode to conduction mode in two ways - either by increasing the voltage across anode to cathode beyond breakover voltage or by application of positive pulse at gate. Once it starts conducting no more gate voltage is required to maintain it in on state. There is one way to turn it off i.e. Reduce the current flowing through it below a minimum value called holding current.

Figure 1: Silicon-Controlled Rectifier (SCR) Symbol

 

Figure 2: Silicon-Controlled Rectifier (SCR) Schematic Diagram