SILICON CONTROLLED RECTIFIER(SCR)

As the terminology indicates,the SCR is a controlled rectifier constructed of a silicon semiconductor material with a third terminal for control purposes.Silicon was chosen because of its high temperature and power capabilities.The basic operation of the SCR  is different from that of an ordinary two layer semiconductor diode in that a third terminal,called a gate,determines when the rectifier switches from the open-circuit to short-circuit state. It is not enough simply to forward bias the anode-to cathode region of the devices.In the conduction state the dynamic resistance of the SCR is typically 0.01 to 0.1Ω and reverse resistance is typically 100 kΩ or more.It is widely used as a switching device in power control applications.It can control loads y switching on and off up to many thousand times a second.It can switch on for a variable lengths of time duration,thereby delivering desired amount of power to the load.Thus,it possesses the advantage of a rheostat as well as a switch with none of their drawbacks.A schematic diagram and symbolic are shown fig.(a) and (b) respectively.fig(a) SCR is a three terminal four layer semiconductor devices,the layers being alternately of a P-type and N-type.The junctions are marked J1,J2 and J3(junction J1 and J3 operate in forward direction while middle junction j2 operates in the reverse direction) whereas the three terminals are anode (A),cathode (C) and gate (G) which is connected to the inner P-types layer.The function of the gate is to control the firing of SCR.In normal operating conditions,anode is positive with respect to cathode.