What is SCR-Silicon Controlled Rectifier ?

What is SCR-Silicon Controlled Rectifier ?

Silicon Controlled Rectifier (SCR)
A silicon controlled rectifier is a semiconductor device that acts as a true electronic controlled switch. It can change AC and at the same instant it can control the quantity of power fed to the load. SCR combines the features of a rectifier and a transistor.
SCR is  a four layers , three terminals (anode - P-layer, cathode - N-layer (opposite end), gate - P-layer near the cathode) & three (J1, J2 & J3) junction device.
This device acts as a switch which is much more cheaper than a relay and is in a position to handle very large amount of power dissipation.

•A gate signal controls switching action of SCR.
Which control from OFF state (high resistance) to  ON state (low resistance).
Working & V-I characteristics of SCR: It is operated in three modes known as a)Forward Blocking(OFF) state b)Forward conduction(ON) state & c)Reverse blocking(OFF) state.

V-I characteristics of SCR means the graph between anode current(Ia) & anode to cathode voltage (VAK) of SCR.
Forward blocking and Conduction state:-
Thyristor (SCR) has three p-n junctions (J1, J2, J3 from the anode).When anode is at a positive potential (VAK) w.r.t cathode with no voltage applied at the gate, junctions J1 & J3 are forward biased, but junction J2 is in reverse biased.
  As J2 is in reverse biased, no conduction takes place, so SCR  is in forward blocking state (OFF state).
  When VAK (forward voltage) is increased , forward leakage current will flow through the device.
  When this forward voltage reaches to breakdown voltage (VBO) of the SCR, forward leakage current will reach saturation and reverse biased junction (J2) will have avalanche breakdown and thyristor starts conducting (ON state), referred to as  forward conducting state .
Reverse blocking state:-
When cathode is made more positive w.r.t anode, Junction J1 & J3 will be reverse biased and junction J2 will becomes forward biased.
  A small reverse leakage current flows, this state is referred as reverse blocking state.

If cathode is made more and more positive, stage is reached when both junctions J1 & J3 will be breakdown, this voltage is referred as reverse breakdown voltage (OFF state), and device is in reverse  blocking state.
SCR Application:
• Gate Turn Off switch (GTO)
• Electronic (pointless) automotive ignition
SCR Specifications:
Forward-breakover voltage (VBO): The voltage at which the SCR enters the forward-conduction region.
Latching Current:-At the breakover voltage the value of anode current is called as latching current(IL).Current starts flow at IL. SCR once turned on, gate potential can be removed and the SCR still conducts called LATCHING of SCR.
Once it  turn on it remains on even after elimination of the gate signal, as long as a minimum current, the holding current (Ih) is maintained in the circuit. To turn off the SCR, anode current(Ia) must be reduced to less than the holding current (Ih).
Holding current(IH): This is the value of anode current below which the SCR switches from the forward-conduction region to the forward-blocking region.
Gate trigger current (IGT) :This amount of gate current necessary to switch the SCR from the forward-blocking state to the forward-conduction state under specified conditions.

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