Wirewound Applications And Customizable Wirewounds
This product highlight will discuss the various adjustable parameters of wirewound resistors 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 has the ability to tailor wirewound resistors to fit requirements such as inductance, high voltage, pulse handling, and 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 important for our customers to understand our wide range of capabilities using wirewound technology as well as the unintended effects of making certain changes.
Wirewound resistors are inherently inductive. In fact, 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 lower inductance with a wirewound, we can typically move to a smaller wire size which will have a higher resistivity. This thinner wire allows us to reach the desired resistance value with fewer turns. However, one side effect of doing this is reduced pulse handling due to lower resistance wire mass.
Non-inductively wound resistors require an Ayrton Perry winding. This method uses two elements wound in opposite directions on a single part, with the outside element 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 significantly higher manufacturing cost. 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 (left figure above). 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 is to decrease the wire size, raising the resistivity of the wire element (right figure above). This results in winding fewer turns, creating wider spacing between windings to minimize the risk of internal arcing. This process would also lower the inductance of the part. However it should be noted that 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 making the change to higher resistivity and smaller diameter wire.
For applications where high voltage handling is required and high pulse energy or power handling is also desired, simply decreasing the wire diameter will not work. For these applications typically a coated element wire is used. In this way the part is protected from arcing even if the windings are touching each other. However, coated wire is more expensive and is more difficult to weld, resulting in higher manufacturing costs. Higher voltage wirewounds are typically denoted by adding a V after the series such as WW becoming WWV.
Fusing and Intrinsically Safe Failure
One benefit of wirewound technology is its robustness. A downside to this robustness is that it can 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. When a part with this type of coating is subjected to 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 therefore less expensive than the first option. One limitation of this method however is that the components built this way 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.