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

4.8 Methods of inspections.

4.8.1 Visual and mechanical inspection. Resistors shall be inspected to verify that the materials,

design, construction, physical dimensions, and workmanship are in accordance with the acceptable

requirements (see 3.1, 3.4 to 3.5.5 inclusive, and 3.24 to 3.27 inclusive).

4.8.2 Conditioning (see 3.8). Resistors shall be conditioned in accordance with method 108 of MIL-

STD-202. The following details and exceptions shall apply:

a. Method of mounting: Supported by their terminal leads (resistor not mounted on life test chassis).

Resistors shall be so arranged that the temperature of any one resistor shall not appreciably

influence the temperature of any other resistor. There shall be no undue draft on the resistors.

b. Test temperature and tolerance: +25�C, +15�C, -0�C.

Initial measurements: Initial resistance shall be measured as specified in 4.8.3 at +25�C +15�C,

-0�C. This initial measurement shall be used as the reference for all subsequent measurements.

d. Operating conditions: Rated dc continuous working voltage or rated working voltage from an

alternating current (ac) power supply at commercial line frequency and waveform, shall be applied

intermittently, 1 hour 30 minutes on and 30 minutes hour off for 96 hours +48 hours, -0 hours.

Each resistor shall dissipate a wattage equal of the power rating (free air) of the resistor. (See 3.1)

e. Measurement after condition: Resistance shall be measured at the end of 96 hours +48 hours, -0

hours as specified in 4.8.3 after load has been removed and the resistors stabilized.

Examination after conditioning: Resistors shall be examined for evidence of mechanical damage.

4.8.3 DC resistance (see 3.9). Resistors shall be tested in accordance with method 303 of

MIL-STD-202. The following details and exceptions shall apply:

a. Measuring apparatus: Different types of measuring test equipment (multimeters, bridges, or

equivalent) are permitted to be used on the initial and final readings of this test, provided the

equipment is the same style, model, or if it can be shown that the performance of the equipment is

equivalent or better.

b. The same measuring apparatus shall be used for any one test, but not necessarily for all tests.

c. Test voltage: Measurements of resistance shall be made using a dc potential resulting in not more

than 1 percent of rated wattage. This same voltage shall be used whenever a subsequent

resistance measurement is made.

d. Measurement energy for electronic test equipment: The measurement energy applied to the unit

under test shall not exceed 10 percent of the 25�C rated wattage times 1 second.