500 mA fast-blow fuse for secondary-side overcurrent protection
The Littelfuse 0451.500MRL is a 500 mA fast-blow surface-mount fuse from the NANO² 451 series, designed for board-level overcurrent protection where quick clearing on short-circuit is required. The fast-blow response means it opens rapidly under fault conditions, not intended for circuits with high inrush currents that would nuisance-blow a fast-acting element. Rated for 125 VAC and 125 VDC with a breaking capacity of 50 A at rated voltage, this fuse safely interrupts fault currents up to that level without rupturing or sustaining an arc — the limiting factor for secondary-side protection in power supplies, control boards, and instrumentation. The 2-SMD square end block package (6.10 mm × 2.69 mm × 2.69 mm) fits standard pick-and-place assembly and reflow profiles, with the DC cold resistance of 0.42 Ohms contributing minimal voltage drop in the protected circuit.
Melting I²t and thermal coordination
With a melting I²t of 0.0795 A²s, this fuse lets through a defined energy before clearing — critical for coordinating with downstream semiconductor ratings (diodes, MOSFETs, ICs) that have a specific I²t withstand capability. A higher I²t means more energy passes before the fuse opens, so the 0.0795 figure sets the upper bound for the protected device's surge rating.
Active production status and regulatory coverage
No pin-compatible second-source or official successor cross-reference is maintained — the NANO² 451 series is the sole form-factor family for this package and rating.