This product highlight will discuss the various adjustable parameters that wirewounds have that give them advantages over film resistors. Frequently, engineers have designs that require a resistor to meet specific characteristics in order to satisfy the unique demands of their applications. Stackpole’s engineering staff has over 30 years experience with tailoring wirewound resistors to fit customer requirements. These requirements range from inductance to power ratings to the usual requirement of minimizing cost. This document will discuss the different attributes that can be adjusted and the effects that these changes may have on the part and its performance. It is critical for our sales force to help customers understand the wide capability range of wirewound technology as well as the unintended effects of making certain changes.
Wirewound resistors are inherently inductive since they are manufactured in the exact same way as wirewound inductors. The amount of inductance can vary widely from size to size and from value to value, but typically ranges from around 10nH to around 10uH. A frequent request for a wirewound resistor is for lower inductance or for it to be non-inductively wound. To achieve a wirewound with lower inductance, we can typically move to a smaller wire size which would have a higher resistivity. This would allow us to reach the resistance value with fewer turns. One side effect of doing this is reduced pulse handling due to lower resistance wire mass.
For non-inductively wound product we do an Ayrton Perry winding. This method uses two elements wound in opposite directions on a single part, with both elements being essentially double the requested resistance value. This helps to cancel the magnetic fields and lowers the inductance to between 0.1nH to 1nH typically. It is important to know that even though this is called a non-inductive part, the part still has some inductance. In addition, since this process requires two precision wind operations, Ayrton Perry winding has a dramatically higher manufacturing cost of around 2X. Non-inductive wirewounds are denoted typically by a preceding N before the series such as WW becoming NWW.
High Power and High Pulse Power Or Energy Withstanding
For higher power or pulse power and energy handling we can increase the wire size which lowers the resistivity of the wire. This can dramatically increase the wire mass available to dissipate the electrical energy on the part. There are two potential side effects from doing this. There will be increased inductance since there will need to be more turns to achieve the resistance value. There may also be some concerns about high voltage handling. High voltage could arc between windings if they are spaced closely together and there is a void present in the coating or molding. Through our extensive experience, Stackpole has been extremely effective in capturing design wins for this type of special part. Pulse handling wirewounds are typically denoted by adding a P after the series such as WW becoming WWP.
High Voltage Withstanding
For high voltage withstanding wirewounds there are two options. The easiest and least expensive option be to would decrease the wire size, raising the resistivity of the wire element. This would allow us to wind fewer turns, creating wider spacing between windings to minimize the risk of internal arcing whether or not there is a void in the coating or molding around the part. This process would also lower the inductance of the part. But this part would have a lower overall pulse power or pulse energy handling capability. These factors may or may not be relevant to a design, but it is important to understand the tradeoffs associated with increasing the voltage handling of a wirewound resistor.
For applications where high voltage handling is required and high pulse energy or power handling is also required, simply decreasing the wire diameter will not work. For these applications is to use element wire that is coated. In this way the part is protected from arcing even if the windings are touching each other. However, coated wire is much more difficult to weld and would have higher manufacturing costs, so the first method is used whenever possible. Higher voltage wirewounds are typically denoted by adding a V after the series such as WW becoming WWV.
Fusing and Intrinsically Safe Failure
One feature of wirewound technology is its robustness. This robustness can cause the part to overheat and potentially catch itself, the PCB, and other components on fire. Stackpole has technological advantages that allow us to build wirewounds with a very reliable and repeatable fusing characteristic. One method to achieve a fusing action is to use coating materials that concentrate the thermal energy and not allow this energy to be dissipated into the ambient air. Then under constant line overload, the part fuses very quickly and typically with very low thermal stress to the PCB and surrounding components. This method does not reduce the part’s ability to withstand short term pulses, which is important to protect metering and monitoring equipment, as well as appliances such as washers, dryers and other white goods. Another method of creating a fusing characteristic is to decrease the wire size. The advantage to this method is that it is still a very straightforward part to manufacture and is therefore less expensive than the first option. One limitation to this method is that the component will have lower short term pulse handling capability. Fusible versions of various series can be denoted by adding an F after the series such as WW becoming WWF.
For more information about Stackpole products, contact Stackpole Electronics, Inc. at
Stackpole Electronics, Inc.
2700 Wycliff Road Suite 410,
Raleigh NC 27607
Stackpole Electronics Inc. is a leading global manufacturer of resistors supplying to the worlds largest OEMs, contract manufacturers and distributors. Headquartered in Raleigh, N.C., the privately held company began manufacturing in 1928 as part of Stackpole Carbon Company in St. Mary's, Pennsylvania. Now part of the Akahane Stackpole Manufacturing Group (ASMG), Stackpole has manufacturing facilities in Japan, Taiwan, China and Mexico; warehousing facilities in El Paso, Shenzhen and Japan; and international sales offices in Tokyo, Taipei, London, Hong Kong and Shenzhen.