Fuse Resistor
A safety-rated resistor designed to fail safely in an open-circuit, non-flaming manner when over-stressed, combining the in-rush and current-limiting function of a resistor with the fault-isolation function of a fuse.
Definition
A fuse resistor (also called a fusible or flameproof resistor) is the front-line defence in many consumer and industrial power inputs. Under normal operation it behaves as a precision resistor — a few ohms to a megohm — for in-rush limiting, voltage division or current biasing. Under sustained overload or short-circuit (e.g. a downstream rectifier failure), it heats rapidly and the resistive element opens permanently before flames, smoke or molten material can escape, isolating the fault upstream.
Safety standards drive the design. UL 1412 “Fusing Resistors” defines breaking-capacity tests at multiples of rated current; UL94 V-0 sets the flame-retardancy of the coating; IEC 60384 governs the housing and creepage. Typical specs are 0.125 W to 5 W, ±5 % or ±10 %, breaking capacity ≥ 100 A at 250 V, working temperature −55 °C to +155 °C. Construction is usually a wirewound or metal-oxide film element with a flame-retardant epoxy or ceramic coating that contains the failure event.
Fuse resistors are mandatory in switching power supplies between the bridge rectifier and bulk capacitor (replacing both a separate NTC inrush limiter and a fuse), in LED drivers, white goods, audio amplifiers and battery management. They simplify the BOM and reduce PCB space versus a discrete fuse + inrush resistor pair.
Related terms
Surge Withstand
A resistor's ability to absorb high-energy voltage or current surges (lightning, switching transients) without permanent change in value or open-circuit failure, characterised by standard waveforms such as 8/20 μs and 10/350 μs.
Pulse Power
The instantaneous power a resistor can absorb during a short pulse of defined duration and duty cycle, often much greater than its continuous rating because the heat is buffered by the element's thermal mass.
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