A thyristor (SCR) or triac (bidirectional thyristor) is the core switching component in solid state relays (SSRs). It enables contactless, high-speed switching of AC loads by controlling the gate trigger signal to regulate current flow through the main terminals. The device operates as a latching semiconductor switch: when a small gate current is applied, it turns on and conducts current between its main terminals until the current drops below the holding current. In SSRs, this enables precise control of AC power to loads without mechanical contacts, offering advantages such as fast switching, no arcing, and long operational life. The component is available in various package types (e.g., TO-220, TO-247, ISOTOP) and is characterized by parameters such as rated voltage (600–1600 V), rated current (10–200 A), gate trigger current (5–100 mA), holding current (10–50 mA), on-state voltage (1.2–1.8 V), critical rate of rise of off-state voltage (200–1000 V/µs), critical rate of rise of on-state current (50–200 A/µs), operating temperature range (-40 to 125 °C), isolation voltage (2500–5000 V RMS), and thermal resistance (0.1–1.0 K/W). These parameters are specified under IEC 60747-6 and serve as reference ranges; actual values must be confirmed for the specific model and application. The component is used in industrial equipment, automation, and power control systems where reliable AC switching is required. When selecting a thyristor/triac, engineers must consider load characteristics, cooling requirements, and gate drive compatibility. Verification of model-specific parameters and compliance with applicable standards should be performed with the legal manufacturer or supplier.
The thyristor/triac operates as a latching semiconductor switch. When a small gate current is applied, it turns on and conducts current between its main terminals until the current drops below the holding current. In SSRs, this enables precise control of AC power to loads without mechanical contacts.
