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Fusing and Failsafe Wirewound Resistors

Wirewound resistor technology is well known for its robustness with respect to environmental and electrical stress.  The downside to this robustness is that a continuous overload condition can often lead to catastrophic failure.  With continuous line overload, wirewound resistors will overheat and eventually damage the PCB and surrounding components.

This type of failure mode is not acceptable in many applications.  One design option is to decrease the wire diameter of the resistance wire used.  When a continuous line overload occurs, the wirewound resistor will fuse and fail open with low thermal stress to the PCB and surrounding components.

While this is generally a preferred failure mode, the decreased wire diameter reduces the pulse handling of the part.  This pulse handling is often critical in protecting meters, monitoring equipment and motor protection for appliances.

Stackpole has developed proprietary materials that provide reliable and repeatable fusing characteristics without reducing the wire size and thereby maintaining the robust pulse handling.  The SP3A series is an off the shelf product that meets the requirements of UL1412, but Stackpole can also design products to meet specific customer requests in terms of pulse handling and fusing.  The WWF series is a fusible version of the popular WW series axial leaded wirewounds that are designed to pass surges of up to 3KV depending on size and resistance value, but fuse quickly under continuous overload.

 
Applications for fusible wirewound resistors include white goods and appliances, flat panel displays, motor controls, and surge protection devices.