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issue: June 2005 APPLIANCE Magazine

Technology Report
Non-Inductive Resistors


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IRC Inc. of Boone, NC, U.S. has developed the NT Series of wirewound resistors to provide design engineers with a non-inductive power resistor capable of withstanding repetitive high energy pulses.

Designed for high-surge applications such as defibrillators, the NT series resistors employ a heavy-gauge resistance wire that is wound in two layers. The second layer is wound in the opposite direction from the first, reducing the inductance. The additional wire mass keeps the resistance wire peak temperature lower during the surge, giving the resistor increased surge capabilities.

The devices are designed for high-energy circuits in defibrillators and other medical electronics applications, and for inrush limiters in power supplies and motor controllers.

According to Keith Chipman, product manager at IRC’s Wire and Film Division, the NT series is particularly good at withstanding pulse and inrush currents associated with capacitor discharge applications and power supply inrush currents. “The NT is able to withstand high levels of pulse energy and survive short time overloads of several times its nominal power rating,” he tells APPLIANCE. “These wirewound resistors are engineered with larger diameter resistance wire than would normally be used. The additional wire mass will absorb more energy before reaching its temperature limit. A second layer is wound over the first, further increasing the wire mass and creating a non-inductive part.”

Most notably, the NT series resistors can be used in the demanding environment of automatic external defibrillators (AEDs), which require a reliable resistor capable of handling repeated energy surges. Depending on the application, Mr. Chipman says the NT Series resistors can be used to divide the voltage or limit the amount of energy delivered to a patient. In this type of circuit, the resistor is an essential part of the energy delivery system to the patient.

The resistors are also said to offer excellent thermal conductivity for even distribution of heat within the part. According to Mr. Chipman, this is possible because the core on which the resistance wire is wound is a high-quality ceramic. “This material does an excellent job of absorbing heat from the resistance wire and spreading it across the body of the resistor,” he explains. “These properties contribute to the resistor’s ability to withstand overloads.”

All resistance wire connections to the stainless-steel end caps and the axial leads are welded for reliability and stability. The resistor body also features a high-temperature silicone coating for further protection.

The NT Series resistors are available in power ratings up to 10 W, with resistance values from 0.1 to 125K Ω. Tolerances down to ±0.1 percent and standard TCRs down to ±20 ppm/°C are available. Operating temperature range is from -55°C to +275°C.

 

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