currentlimiter

A current limiter is a circuit or device designed to restrict the amount of electrical current that can flow through a load in order to protect components, limit inrush, or enforce a maximum operating condition. Current limiters can be implemented as active or passive systems and are common in power supplies, LED drivers, battery chargers, and motor controllers.

Active current limiters use sensing elements, such as a low-value shunt resistor, to monitor load current and a control circuit to enforce the limit. When the sensed current approaches a preset threshold, the circuit reduces the drive to the load, through a linear pass element or a switching regulator, to keep the current at or near the limit. This approach provides a well-defined current limit with adjustable response and is often used to implement constant-current regulation, short-circuit protection, and soft-start behavior to reduce inrush and thermal stress.

Passive current limiters rely on materials whose resistance increases with temperature or current. Positive temperature coefficient (PTC) thermistors and resettable fuses (polyfuses) are common examples; as current rises, their resistance increases, naturally limiting the current. Traditional fuses interrupt current by opening the circuit when fault conditions occur, though they are not resettable.

Applications and considerations: current limiting protects power stages, prevents LED brightness runaway, and safeguards batteries and motors during fault conditions. Design considerations include response time, limit accuracy, temperature effects, power dissipation in the limiter, and interaction with regulation and transients. Properly chosen current limiters balance protection with continued operation under fault conditions.

See also: overcurrent protection, short-circuit protection, soft-start, foldback current limit, polyfuse.