5 A fast-blow SMD fuse — obsolete, sourced to order
The Littelfuse 0451005.MR is a 5 A fast-blow surface-mount fuse from the NANO²® 451 series, rated for 125 V on both AC and DC circuits. The 50 A breaking capacity at rated voltage means it safely interrupts faults up to that level without rupturing the package — adequate for secondary-side protection in power supplies and control boards where the available fault current stays under 50 A. The 2-SMD square end block package (6.10 mm x 2.69 mm x 2.69 mm) occupies a compact footprint on the PCB, and the surface-mount format suits automated assembly lines.
Lifecycle reality: obsolete — no direct successor on record
Littelfuse has marked the 0451005.MR as obsolete. No official successor order code is recorded in the lifecycle data, so there is no drop-in replacement from the manufacturer. For new designs, a parametric match (5 A fast-blow, 125 V, SMD, 50 A breaking capacity) from Littelfuse or a competitor like Bel Fuse / Eaton Bussmann should be qualified — the 2-SMD square end block footprint narrows the field, but no single pin-compatible alternative is guaranteed without board-level testing.
Melting I²t and inrush handling
The melting I²t of 5.566 A²s tells you how much energy the fuse element absorbs before opening. For a circuit that sees capacitive inrush (e.g., a DC-DC converter input), the inrush energy must stay well below this figure — typically a 20% derating margin — to avoid nuisance blowing on startup. The 0.0125 Ohm DC cold resistance means at 5 A the fuse dissipates about 0.31 W in steady state, which is negligible for most PCB thermal budgets but worth checking in a dense layout.