250 mA hold, 550 mA trip — the PTC resettable fuse for 24 V rails
The Littelfuse 1206L025/24YR is a polymeric PTC (positive temperature coefficient) resettable fuse from the POLY-FUSE 1206L series, designed to protect low-voltage DC power rails up to 24 V. Its 250 mA hold current (Ih) means the circuit operates normally up to that load; above 550 mA trip current (It) the device heats up and latches into a high-resistance state, limiting fault current to protect downstream components. Rated for a maximum fault current of 100 A and a typical trip time of 80 ms at 25 °C, it clears overcurrent events quickly enough to prevent damage to the PCB trace or load before the fault escalates.
Resistance profile and thermal limits
Initial resistance (Ri) is a minimum of 550 mOhms; after tripping, the post-trip resistance (R1) rises to a maximum of 2.3 Ohms. This resistance swing is normal for polymeric PTCs — the device self-heats during a fault and the polymer matrix expands, increasing resistance by roughly 4×. Once power is removed and the device cools, it resets to the low-resistance state automatically. Derate the hold current at elevated ambient temperatures per the POLY-FUSE 1206L series curve — at 85 °C the effective hold current drops to roughly 60% of the 25 °C rating, so a 250 mA hold at 25 °C becomes ~150 mA at the high end of the range.