4 A fast-blow SMD fuse for secondary-side protection
The Littelfuse 0451004.MRL is a 4 A fast-blow surface-mount fuse from the NANO²® 451 series, rated for 125 V AC and DC circuits. Its 50 A breaking capacity at rated voltage means it can safely interrupt a fault current up to that level without rupturing the package — a key parameter for secondary-side protection in power supplies and DC-DC converters where the available fault current is known. The 2-SMD square end block package (6.10 mm x 2.69 mm x 2.69 mm) fits standard pick-and-place assembly and reflow profiles. The fast-blow characteristic clears overcurrent events quickly, limiting let-through energy to downstream semiconductors — the 3.152 melting I²t figure gives the board designer the data needed to coordinate with downstream traces and components.
Approvals and temperature envelope
The 0451004.MRL carries CSA, PSE, and UL approvals, covering North American and Japanese regulatory acceptance for equipment-level safety certification.