MIL-PRF-27G
6.10.2.2 Temperature rise. Temperature rise is normally measured and rated at sea level. At higher altitudes, the
temperature rise will increase and should be compensated for in the equipment design.
6.10.2.3 Ambient temperature. It is not recommended that a higher operating ambient temperature be specified
than that to which the transformer or inductor will actually be exposed. To do so may result in a larger and heavier unit
than is needed. In the absence of a specified ambient temperature in the individual document, the following ambient
temperatures may be used for the temperature rise test; class Q (65°C), class R (65°C), and class S (85°C).
6.11 Overload. Transformers and inductors designed in accordance with this specification are capable of
withstanding an overload of 12 percent for 48 hours.
6.12 Environmental characteristics. Care should be exercised in specifying environmental test requirements to
insure that the unit should be tested in a manner compatible with the environment actually present. Thus, for example,
if the end equipment is to be shock mounted, sealed, or will include cooling facilities, the transformer or inductor may
encounter a less stringent environment. However, it should be noted that the effect on a transformer or inductor of the
conditions to which an equipment is subjected, because of position or method of mounting, may be entirely different
from the effect on the end equipment as a whole.
6.13 Dielectric withstanding voltage (at atmospheric pressure) (points of application of test voltage, winding to case
or core). Grade 5 units (when applicable) must be wrapped lightly with metal foil on as many surfaces as practicable in
lieu of a metal case. For some SMD units, points of application are between the terminals and the surface opposite the
terminals.
6.14 Laminated phenolic materials (see 3.3.3.1). It is recommended that MIL-I-24768/17, L-P-513,
MIL-I-24768/1, or MIL-I-24768/9 be considered for laminated phenolic materials.
6.15 Molded phenolic or melamine materials (see 3.3.3.2). It is recommended that ASTM D 5948-96 to be
considered for molded phenolic or melamine materials.
6.16 Ceramic (see 3.3.3.3). It is recommended that MIL-I-10 to be considered for ceramic materials.
6.17 Magnet wire (see 3.3.4.1). It is recommended that MW-1000 to be considered for magnet wire.
6.18 Insulated wire (see 3.3.4.2). It is recommended that MIL-W-76 or MIL-W-16878 to be considered for use
when the insulated wire is being used as wire terminals.
6.19 Solder and soldering flux (see 3.3.5). It is recommended that solder and soldering flux to be in accordance
with J-STD-004 and J-STD-006.
6.20 International standardization agreement. Certain provisions of this specification are the subject of
international standardization agreement (NEPR No. 20). When amendment, revision, or cancellation of this
specification is proposed which will affect or violate the international agreement concerned, the preparing activity will
take appropriate reconciliation action through international standardization channels including departmental
standardization offices, if required.
6.21 Tin whisker growth. The use of alloys with tin content greater than 97 percent, by mass, may exhibit tin
whisker growth problems after manufacture. Tin whiskers may occur anytime from a day to years after manufacture
and can develop under typical operating conditions, on products that use such materials. Conformal coatings applied
over top of a whisker-prone surface will not prevent the formation of tin whiskers. Alloys of 3 percent lead, by mass,
have shown to inhibit the growth of tin whiskers. For additional information on this matter, refer to ASTM-B-545
(Standard Specification for Electrodeposited Coatings of Tin).
6.22 Environmentally preferable material. Environmentally preferable materials should be used to the maximum
extent possible to meet the requirements of this specification. As of the dating of this document, the U.S.
Environmentally Protection Agency (EPA) is focusing efforts on reducing 31 priority chemicals. The list of chemicals
is available on their website at https://epa.gov/epaoswer/hazwaste/minimize/chemlist.htm. Further information is
available at the following EPA site: https://epa.gov/epaoswer/hazwaste/minimize/. Included in the EPA list of 31
priority chemicals are cadmium, lead, and mercury. Use of the materials on the list should be minimized or eliminated
unless needed to meet the requirements specified herein (see Section 3).
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