United Airlines Flight Simulator Technician Trade Practice Test

The main idea is that an SCR is a controlled rectifier with a PNPN four-layer structure and three terminals (anode, cathode, gate). It stays off until a gate pulse triggers it; once triggered, it latches on and conducts in the forward direction because the internal junctions provide positive feedback that sustains conduction. It will continue conducting until the current through it falls below a certain holding current, at which point it turns off and can be triggered again with another gate pulse.

This matches a description of a four-layer, three-terminal device that latches on with a gate and turns off when the current drops below the holding value. It’s not just a simple two-terminal diode, nor a transistor switch, and it isn’t bidirectional like a TRIAC, so those other descriptions don’t fit SCR behavior.