Thyristor Basics Explained

A thyristor is a powerful on-off switch used in both alternating current (AC) and direct current (DC) equipment, primarily within industrial settings. These solid-state devices are made from semiconducting components like transistors and diodes. The name thyristor combines "transistor" and "thyratron," an earlier gas-filled tube with a similar function. Developed in the 1950s, thyristors are also known as silicon-controlled rectifiers (SCR) because they are made from four layers of silicon, a widely used semiconducting material. Thyristors are bistable switches with only two possible states: on or off. They activate when current reaches the control gate and remain on until the current drops to zero or changes direction. Modern thyristors can be deactivated via the control gate, known as gate turn-off (GTO) thyristors. These devices are high gain, meaning the current at the control gate can control a much higher current between the anode and cathode, classifying them as current-operated equipment. Despite their small size, thyristors can control high voltages and currents, making them ideal for high voltage, direct current power transmission lines. They are also used as power switches in factories, vehicle ignition switches, and for controlling the speed of electric motors. Additional applications include liquid level regulators, pressure control systems, and surge protectors. Thyristors are also found in various electrical circuits, such as inverter, oscillator, chopper, power switching, relay replacement, level-detector, logic, phase-control, speed control, and timer circuits. Thyristors typically have three leads: the anode, cathode, and gate. The anode is the positive terminal, the cathode is the negative, and the gate controls the main current. Inside, there are four layers of alternating N-type and P-type semiconductors, creating three junctions. When the gate current is applied, it activates the layers, allowing current to flow from the anode to the cathode. The thyristor remains on until the external current is switched off. The gate current is not needed to maintain the current flow once activated. Transistors are standard electrical components used to switch and amplify electrical signals. They are best suited for low power milliamp currents and do not handle high voltage currents well. Thyristors, on the other hand, can operate with much higher power levels, handling 5-10 amps and hundreds or thousands of volts. Unlike transistors, thyristors can maintain a higher current flow even when the gate current stops, making them ideal for applications like intruder alarms where continuous current is needed after activation. Diodes are simple, inexpensive components with two terminals: an anode and a cathode. They allow electricity to flow in one direction only and are used for switching or converting current. Diodes have two semiconductor layers and a single junction. Thyristors, however, have three terminals and four layers with three junctions, designed for high power applications. While diodes do not require a gate pulse to activate, thyristors do, making them suitable for more complex and high-power tasks. Most thyristors have three leads: an anode, a cathode, and a control gate.