The document discusses the symbol, construction, working modes, and applications of SCR (silicon controlled rectifier). It provides details on the following: - SCR has 3 terminals - anode, cathode, and gate. It conducts current in one direction like a diode but requires a gate pulse for triggering. - Internally, it has 4 layers that form 3 PN junctions. It can operate in forward blocking, forward conduction, and reverse blocking modes depending on terminal polarity and gate pulse. - Common applications include motor speed control where SCRs control current to the motor armature. Proper mounting using leads, bolts or studs with insulation is required to dissipate heat generated