375 mA fast-blow fuse for secondary-side protection
The 0451.375MRL is a 375 mA fast-blow surface-mount fuse from Littelfuse's NANO² 451 series, rated for 125 V AC and DC with a 50 A interrupting capacity at rated voltage. The 0.0425 melting I²t means it clears quickly on a hard short — the energy let-through stays low enough to protect downstream silicon before the junction overheats.
What the ratings mean for your BOM
Rated 375 mA continuous at 125 V AC or DC — this fuse sits on a secondary-side rail where the steady-state load stays under 300 mA and the fault current can reach 50 A. The fast-blow characteristic means it opens within milliseconds on a dead short, but tolerates brief inrush from a capacitive input filter. The 0.61 Ω cold resistance produces about 86 mW of self-heating at rated current — negligible for the 2-SMD square end block package, but worth factoring into the thermal budget if the fuse is enclosed without airflow.
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm — the same footprint as the rest of the NANO² 451 series, so a single PCB layout serves multiple current ratings across the family. Surface-mount with tape-and-reel or cut-tape packaging — the reel quantity suits automated pick-and-place for production runs; cut tape works for prototype or low-volume builds.
Approvals and compliance
CSA and UL approved — the fuse meets North American safety agency requirements for branch-circuit and supplementary protection in equipment listed to UL 248-14.