Obsolete — sourcing the 04511.25NR today
The 04511.25NR is marked Obsolete by Littelfuse.
1.25 A fast-blow in a 125 V SMD package
Rated 1.25 A continuous at 125 V AC and 125 V DC. The fast-blow response time means it opens quickly on overcurrent — suited for protecting sensitive semiconductor circuits where a slow-blow would let the fault energy through before clearing. Breaking capacity is 50 A at rated voltage. On a 125 V rail with a prospective short-circuit current up to 50 A, this fuse interrupts the fault without rupturing the package. Above 50 A available fault current, the fuse may not clear safely — check the fault level at the board input.
Melting I²t and cold resistance — the so-what for the BOM
Melting I²t is 0.664. This is the energy needed to melt the fuse element. For a capacitor inrush or a motor start, the I²t of the surge must stay below this value or the fuse opens on turn-on. DC cold resistance is 0.078 Ohms — the voltage drop at 1.25 A is under 100 mV, negligible for most 125 V rails.
Board-mount SMD footprint and approvals
Surface-mount in a 2-SMD, square end block package. Dimensions are 0.240" x 0.106" x 0.106" (6.10 mm x 2.69 mm x 2.69 mm). The NANO² 451 series uses a ceramic body with silver-plated end caps — reflow soldering profile follows standard lead-free profiles. Approved by CSA, cULus, and PSE. cULus listing means it is recognized for use in UL-listed equipment in the US and Canada. PSE covers Japan's electrical appliance and material safety law.