MIL-PRF-39009 Resistors, Fixed, Wire-Wound \(Power Type, Chassis Mounted\), Nonestablished Reliability, Established Reliability, General SpecificationThis specification covers the general requirements for nonestablished reliability \(non-ER\), and established reliability \(ER\), power type, wire wound, fixed resistors which utilize the principal of heat dissipation through a metal mounting surface. These resistors have initial resistance tolerances of ??1 percent. These resistors are suitable for continuous full load operation at an ambient temperature of +25 degrees C, and when derated up to +250 degrees C \(see figure 1\). These resistors should not be used in circuits where their ac performance is of critical importance in the operation of such circuits. However, provisions have been made in particular styles to minimize inductances \(see 3.1\). These resistors will have life failure rate levels \(FRL\) ranging from 1.0 percent to 0.001 percent per 1,000 hours \(see 1.2.1.4\). These FRL are established at a 60 percent confidence on the basis of the life tests. The FRL, identified by the appropriate symbol, is referred to operation at full rated wattage at +25 degrees C, with a permissible change in resistance of ??2 percent as the criteria for failure.

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MIL-PRF-39009E

3.4 Material. The material shall be as specified herein. However, when a definite material is not

specified, a material shall be used which will enable the resistors to meet the performance requirements of

this specification. Acceptance or approval of any constituent material shall not be construed as a

guarantee of the acceptance of the finished product.

3.5 Interface and physical dimension. Resistors shall meet the interface and physical dimensions

specified (see 3.1). Resistors shall be wound with a resistance wire (round or flat wound ribbon), except

as required for noninductive performance of applicable styles (see 3.1). When applicable, in order to

minimize inductance, resistors shall be wound by one of the following methods:

a. Ayrton-Perry.

b. Bifilar.

c. Or an equivalent method.

3.5.1 Protective coating or enclosure. The resistance element and caps shall be protected by a coating

or an enclosure which shall completely cover the exterior of the resistance element and caps. The

enclosure shall be free from holes, fissures, chips, and other faults. Small cracks and molding flaws which

do not adversely affect the unit's ability to meet all environmental requirements of this specification will not

be considered a cause for rejection. The exposed end of the enclosure shall prevent the entrapment of

moisture.

3.5.2 Housing. The housing shall protect against corrosion. All fasteners shall be plated. Unplated

copper bearing metals shall not be used in contact with aluminum.

3.5.3 Terminals. Terminal shall be treated to facilitate soldering. When a coating tin is used, the tin

content shall range between 40 percent and 70 percent. The terminals shall be firmly secured and shall

not be solely dependent on the protective coating or enclosure for mechanical anchorage.

3.5.3.1 Tin plated finishes. Uses of tin plating are prohibited as a final finish, and as an undercoat (see

6.7). Use of tin-lead (Sn-Pb) finishes is acceptable provided that the minimum lead content is 3 percent.

3.5.4 Solder dip (retinning) of leads. The manufacturer (or their authorized category B or category C

distributor) may solder dip/retin the leads of product supplied to this specification provided the solder dip

process (see A.5 of the appendix) has been approved by the qualifying activity.

3.5.5 Weight. Resistors shall not exceed the maximum weight specified (see 3.1).

3.6 Voltage rating. Resistors shall have a rated dc continuous working voltage or an approximate

sine-wave root-mean-square (rms) continuous working voltage at commercial line frequency and

waveform corresponding to the power rating, as determined from the following formula:

E = PR

Where:

E = Continuous rated dc or rms working voltage in volts.

P = Rated wattage in watts (see 3.1).

R = Nominal resistance in ohms.