Silicon controlled rectifiers (SCRs) are thyristor devices that can control large amounts of power using low control power. SCRs have three terminals - anode, cathode, and gate. Applying a positive pulse to the gate turns on the SCR when the anode is positive with respect to the cathode. SCRs are turned off by reducing the anode current below the holding current. SCRs are commonly used in power electronics and have applications in AC or DC power control systems.