NANO² 451 — 125 mA fast-blow in a 2-SMD square end block
The 0451.125MRSN is a Littelfuse NANO²® 451 series surface-mount fuse, rated 125 mA at 125 VAC with a fast-blow response time. The 50 A breaking capacity at rated voltage means it safely interrupts a short-circuit fault up to that level without rupturing the package — important for secondary-side protection in a power supply where the upstream transformer can deliver that fault current.
Melting I²t and cold resistance — the coordination numbers
Melting I²t is 0.00286 A²s — the energy required to clear the fuse element. This figure drives coordination with downstream components: a capacitor bank with 0.002 A²s inrush stays below the fuse's melting threshold, so the fuse holds during startup. DC cold resistance is 1.7 Ohms, which at 125 mA produces a voltage drop of about 213 mV — negligible in a 125 V circuit but worth checking if the load is a low-voltage rail.
Temperature range and agency marks
CSA and UR approval marks mean the fuse is accepted for North American safety-agency inspections without additional component-level testing.
Package and lifecycle — active, surface-mount, reel-ready
The 2-SMD square end block measures 6.10 x 2.69 x 2.69 mm, fitting standard pick-and-place nozzles. Available in Tape & Reel for production or Cut Tape for prototypes. Sourced to order against the BOM; availability confirmed at RFQ.