Mains Input Resistor, ULW series, for Power Supply
Mains input resistor selection for a small power supply is not easy as conflicting factors are involved. It must survive inrush and transient surges, but also give failsafe fusing in the event of short-circuit failure.
A common choice is a flameproof wirewound in the range 10R to 100R. For certain applications, however, there are further constraints. They needed low impact, silent fusing to avoid consumer concern resulting from visible or audible signs of failure.
In wirewound resistors the ceramic rod at the core acts as a heatsink which delays the fusing, allowing the coating to reach high temperatures. This results in fragmentation and ionisation of the air close to the point of fusing, and can initiate a momentary flashover outside the component body. This releases far more energy than is involved in merely fusing the wire element.
A new coating material was refined to deliver silent fusing performance without adversely affecting safety or cost. It has a flammability rating of UL94-V0 and the ability to absorb the thermal and mechanical stresses of fusing without fragmentation.