NEPP - Magnetics

For many power supply and RF applications in satellites, it is not possible to find a military specification coil, inductor, or transformer that meets the need of the application. In the mid 1980's NASA, through the NASA Standard Parts Program, developed MIL-STD-981. The purpose was to give design engineers guidelines for custom inductors and transformers for space flight use. It defines construction techniques, such as minimum wire sizes, and screening and qualification procedures. MIL-STD-981 assumes that the design engineer had knowledge of the other aspects of magnetic design, such as choosing the appropriate core material and an insulation system that meets the temperature requirements of the application.

Technical Information

In the world of commercial electrical and electronic products, Underwriters Laboratories (UL) sets technical standards to guarantee a minimum standard of safety for product performance in the United States. In the area of magnetic devices (coils, inductors, transformers), they have established a series of temperature-based classes for these parts (UL1446, UL Standard for Safety for Systems of Insulating Materials-General). In the reference section later on this page, you will find a reference to a Dupont web page that explains how systems of wire, bobbins, etc. can be chosen to meet a specific temperature rating.

The class designations used in MIL-STD-981 and other specifications are not the same as those used by UL. If you are designing a custom magnetic device, the following information will help pick appropriate parts from commercial catalogs. Please remember that these commercial parts may have to be tested to guarantee compliance with out-gassing and radiation requirements. In the case of radiation exposure, the materials may become brittle.

Class Designations for Insulation Systems

MIL-PRF-27	MIL-PRF-15305 & MIL-T-55361	UL1446	Maximum Operating Temperature, °C
Q	O	–	85
R	A	A	105
–	–	E	120
–	B	–	125
–	C	–	>125
S	–	B	130
V	–	F	155
T	–	–	170
U	–	–	170, as specified
–	–	H	180
–	–	N	200
–	–	R	220
–	–	S	240

The Federal Specification for Magnet Wire, J-W-1177, was cancelled in 1996 and replaced by the National Electrical Manufacturers Association (NEMA) Standard MW-1000 for Magnet Wire. MW-1000 offers numerous wire types that comply with the various UL thermal classes.

EEE-INST-002 has been issued. It can be downloaded from the NEPP web site. This document replaces 311-INST-001