The 1812L200THPR is a surface-mount resettable PTC (polymeric positive temperature coefficient) fuse from Littelfuse with 2A hold current and 33V maximum voltage in 1812 (4532 metric) package. The device uses a composite of crystalline polymer and conductive carbon particles. At normal temperatures, carbon particles form conductive chains providing low resistance (typically 0.1-0.5 Ohm). When overcurrent occurs, self-heating raises the polymer above its crystalline transition temperature (approximately 125C). The polymer expands, breaking the carbon chains and dramatically increasing resistance (by 3-4 orders of magnitude), which limits current to a low leakage value. When the fault is removed and the device cools, the polymer recrystallizes and carbon chains reform, restoring low-resistance operation. The hold current (2A) is the maximum that does not trip the device; the trip current (approximately 4A at 20C) guarantees tripping.