7 A fast-blow SMD fuse for secondary-side protection
The 0451007.MRL is a 7 A fast-blow surface-mount fuse from Littelfuse's NANO² 451 series, rated for 125 VAC and 125 VDC with a 50 A interrupting capacity at rated voltage. The fast-blow response means the element opens quickly on overcurrent — suited for protecting solid-state circuits and PCB traces where high inrush from capacitive loads is not expected. With a DC cold resistance of 0.009 ohms, the voltage drop at rated current is roughly 63 mV, keeping conduction losses low in a 7 A secondary-side rail.
Package, approvals, and operating envelope
Housed in a 2-SMD square end block package measuring 6.10 mm x 2.69 mm x 2.69 mm, the 0451007.MRL mounts on the PCB surface and is available in Tape & Reel (TR suffix) for automated pick-and-place or Cut Tape for prototype quantities. Approved under CSA, PSE, and UL standards — the UL listing certifies the fuse's safety and interrupting performance for North American installations, while PSE covers Japanese market compliance.
Melting I²t and coordination
The melting I²t of 10.32 A²s defines the energy required to open the fuse element — this value is used to coordinate with downstream semiconductor surge ratings (e.g., I²t of a rectifier or MOSFET) so the fuse clears before the semiconductor fails. For a 7 A fast-blow fuse, a 10.32 I²t is typical for this package size; it means the fuse can tolerate brief inrush pulses (e.g., from a bulk capacitor bank) up to roughly 5-6 times rated current for a few milliseconds before opening.