3 A fast-blow SMD fuse for secondary-side protection
The Littelfuse 0451003.MRL is a 3 A fast-blow surface-mount fuse from the NANO²® 451 series, rated for 125 V both AC and DC circuits. Its 50 A breaking capacity at rated voltage means it can safely interrupt a short-circuit fault up to that current without rupturing — critical for clearing a fault on a 125 V rail where the available fault current might hit that level.
What the ratings mean for your board
The 3 A current rating is the continuous current the fuse carries without opening at 25°C ambient. The fast-blow response means it clears quickly on overcurrents — useful for protecting semiconductor circuits that can't tolerate sustained overloads. The 50 A breaking capacity (interrupting rating) at 125 V AC/DC is the maximum fault current the fuse can safely clear; if your power supply's short-circuit output current exceeds 50 A, this fuse may not clear the fault safely, and you'd need a higher-interrupting device. With a melting I²t of 1.65, this fuse tolerates moderate inrush currents without nuisance opening — useful for capacitive input filters or motor start-ups on the secondary side. Surface-mount assembly means it reflows with the board — no hand-soldering required, but also no field-replacement without a soldering iron.
Approvals and compliance
The 0451003.MRL carries CSA, PSE, and UL approvals — accepted in North American, Canadian, and Japanese regulatory jurisdictions.