currentlimiting

Current limiting is the practice of constraining electrical current flowing in a circuit to prevent damage, excessive heating, or unwanted tripping of protective devices. It can be achieved with passive components such as fuses, resistors, or thermistors, or with active circuitry that senses current and automatically reduces or interrupts the load current. In power electronics, current limiting is commonly implemented with a sense resistor and a comparator that trips or commands a PWM controller to reduce duty cycle when the sensed current exceeds a predefined limit. Linear regulators embed a current-limit feature that protects the pass transistor during overload. In constant-current sources, the output actively adjusts to hold a set current even as the load changes, up to a maximum voltage compliance.

Common strategies include cycle-by-cycle limit, where the regulator or switcher clamps current on each switching cycle; and foldback or hiccup mode, in which the limiter reduces current to a low level or briefly interrupts, then reattempts after a delay. Inrush current limiters, such as NTC thermistors, limit the surge when power is first applied. Resettable fuses (polyfuses) provide self-resetting protection by increasing resistance under excess current.

Applications span power supplies, battery charging, motor control, and protection of cables and transformers from overheating. Important design considerations include sense accuracy, response time, temperature dependence, and the trade-off between protection level and load continuity. Misplacement of the sense element, parasitic inductance, or slow response can undermine effectiveness. Standards and protection schemes often combine current limiting with overcurrent protection devices to meet safety requirements.

See also: short-circuit protection, overcurrent protection, current limiter, power supply protection.