SCR in stud mounted package
An SCR is a device with four semiconductor layers (3 PN junctions). A gate control signal switches the SCR from the high resistance OFF state, to the low resistance ON state.
SCR schematic symbol
An SCR has three p-n junctions (J1, J2, J3). When the anode is at a positive potential with no voltage applied at the gate, junctions J1 and J3 are forward biased, but junction J2 is in reverse biased, so the SCR is in the OFF state.
The common method of controlling an SCR is to apply a positive voltage pulse between the gate and the cathode. A small gate current forward biases the J2 PN junction allowing a much greater current to flow through the device.
The SCR stays on even if the gate current is removed (until the anode goes negative in relation to the cathode or power is disconnected).
