Current limiting

Passive resistive components such as resistors (with power dissipation drawback), or negative temperature coefficient (NTC) thermistors are simple options while the positive one (PTC) is used to limit max current afterward as the circuit has been operating (with cool-down time drawback on both).

Some electronic circuits employ active current limiting since a fuse may not protect solid-state devices.

The schematic represents a simple protection mechanism used in regulated DC supplies and class-AB power amplifiers.

Further, this power dissipation will remain as long as the overload exists, which means that the devices must be capable of withstanding it for a substantial period.

Foldback helps to keep the output transistor within its safe operating area under fault and overload conditions.

It can trigger "lockout" conditions with non-ohmic devices that draw a constant current independent of the supply voltage (such as op-amps).

The safety benefit of reducing the power delivered to a short circuit in the load is proportional to the operating current limit.

Foldback current limiting is most likely to be found in a switch-mode power supply when it is a component in a product that is independently certified to meet regional safety standards.

Active current limiting or short-circuit protection
V-I curves for voltage regulators with different overload handling: foldback , constant current limited , and unlimited .
Power dissipation vs. load resistance for linear voltage regulators with different overload handling. Here V in = 12 V, V OC = 10 V, I max = 1 A, I SC =0.17 A. The maximum dissipation in the foldback design is three times lower than in the constant current limited design.
The inrush current of an incandescent lamp causes a bench power supply to limit its output current with a foldback current limiter.