UL-recognised mains input resistors

The ULW series wire-wound resistors overcome the problems associated with conventional flameproof resistors caused by the ceramic rod at the resistor's core acting as a heat sink for the wire element. This can delay fusing, resulting in high enough temperatures to fragment the coating and
ionise the air near to where fusing occurs. If ionisation occurs close to the cap edge and at a voltage peak in the mains cycle, it can initiate a momentary flashover outside the component body, releasing far more energy than is required to fuse the wire element. Although the opening of the
circuit is safe for most applications, it's far from silent.

Welwyn's new coating material and process delivers silent fusing performance
without adversely affecting either safety or cost. The new multi-layer
coating has a flammability rating of UL94V-0 and shares the same high
temperature and insulating properties as standard silicone cement. However,
it is more compliant so can absorb thermal and mechanical stresses without
fragmenting. The resistor coating does not burn or emit incandescent
particles under any condition of applied temperature or power overload.

These UL-certified resistors are available in standard values from 10 to
100ohm. Three types are offered: ULW2 is rated 2W and provides a 5s overload
of 10W along with 250V isolation; 3W-rated ULW3 ensures a 15W overload with
350V isolation; and the ULW4 model protects overloads up to 20W with 500V
isolation and a 4W nominal rating.

The devices are designed to withstand inrush and surge conditions. For
example the ULW4-10RJ has an energy limit of 4.3J and a 1.2/50 micro seconds
peak voltage limit of 1.1kV. Fusing performance is characterised by typical
fusing times of 8s at 45W initial power (ULW2) and 10s at 90W initial power
for ULW3 and ULW4 resistors, although rapid fusing in a fraction of a second
is achieved under mains short circuit conditions.

All the ULW series operate in ambient temperatures from -55 to +155°C and
are classified in climatic category 55/200/56 according to IEC 68-1. They
are RoHS-compliant and available in optional surface-mount as well as
standard through-hole leadforms.