Definition
A solid-state semiconductor switching device, similar to a silicon-controlled rectifier (SCR), that is turned on by light striking its sensitive junction rather than by an electrical trigger signal. Once light activates it, it conducts current in one direction and remains conducting until the current through it is interrupted.
Plain English
An electronic switch that closes when light shines on it. Once it switches on, it stays on until the power flowing through it is cut off.
Context Anchor
Seen in aircraft electrical and electronic system discussions, especially in switching, sensing, and optically controlled circuits.
Derivation
Built from three parts: 'photo' (Greek for light), 'thyra' (Greek for door or gate), and the suffix used in 'transistor.' A thyristor is a 'gate' device that controls electrical flow. A photothyristor is a thyristor whose gate is opened by light instead of an electrical signal.
Why Pilots Care
Pilots usually do not operate a photothyristor directly, but understanding the term helps when reading aircraft electrical-system descriptions or maintenance information that explains how a circuit is switched or controlled.
Analogy
It is like a light-sensitive switch: shine light on it and the circuit turns on. Unlike a simple wall switch, it may stay on until the electrical flow through it drops enough.
Intuition Check
Do not read “photo” as meaning it takes pictures. Here, “photo” means light is what triggers the electronic switch.
Example Sentence 1
The technician traced the fault to a failed photothyristor in the optical isolator that links the cockpit control circuit to the high-voltage system.
Example Sentence 2
Replacement of the photothyristor restored proper isolation between the sensor and the cockpit warning panel.