What this 500 mA fast-blow SMT fuse covers
The Littelfuse 0451.500MR is a NANO²® 451 series surface-mount fuse rated 500 mA at 125 V AC and 125 V DC with a fast-blow response. Breaking capacity is 50 A at rated voltage — sufficient for secondary-side protection in power supplies, battery packs, and DC-DC converters where the prospective fault current stays below that threshold. The 2-SMD square end block package measures 6.10 mm x 2.69 mm x 2.69 mm, fitting standard pick-and-place for high-volume assembly.
Lifecycle reality — obsolete, not orphaned
Littelfuse lists the 0451.500MR as obsolete. The NANO² 451 series is a well-known footprint; if a board respin is acceptable, a parametric-equivalent SMT fuse from the same or a competing family may be considered, but no pin-compatible drop-in replacement is documented here.
At 500 mA, this fuse protects low-current branches — sensor supplies, control logic rails, or auxiliary loads — where a hard short pulls well above the 50 A interrupting rating. The fast-blow characteristic means it clears quickly on high overcurrents, limiting let-through energy to the downstream components. Melting I²t is 0.0795 A²s — a key figure for coordinating with downstream semiconductor ratings. Approved by CSA and UL — the part carries the agency marks needed for North American equipment compliance without additional fuse testing.