What the 4A hold current means on your board
This Littelfuse 1206L400/12SLWR is a polymeric PTC resettable fuse from the POLY-FUSE® LoRho series, rated to hold 4 A continuously at 12 V without tripping. The LoRho designation means the initial resistance is kept low — 3 mOhms minimum — so the voltage drop across the fuse stays under 12 mV at the rated hold current, which matters for low-voltage rails where every millivolt counts. When a fault pushes the current to 8 A, the device trips in 2 seconds typical, protecting the downstream circuitry. After the fault clears and power is cycled, it resets automatically — no part replacement needed, which is the whole point of a PTC over a one-time fuse.
Package and board-fit details for the rework bench
The concave termination style makes it easier to inspect the solder joint after reflow — the fillet climbs the end cap visibly, which a flat-chip package hides. Surface-mount only; no through-hole option.