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

4.8.13 Life (see 3.19). Resistors shall be tested in accordance with method 108 of MIL-STD-202. The

following details and exceptions shall apply:

a. Method of mounting: Resistors shall be mounted on lightweight terminals, on an aluminum

chassis of the dimensions specified (see 3.1), with the longitudinal axis of the chassis. The

chassis shall be horizontally supported by a material having low thermal conductivity. The method

of attaching the resistor leads to the terminals shall be at the option of the manufacturer. The

integrity of the terminations shall be determined at each measurement interval. The voltage

applied to any resistor shall not be less than 95 percent of the dc, ac line or true rms rate

continuous working voltage. Resistors shall be arranged so that the temperature of any one

resistor shall not appreciably influence the temperature of any other resistor. There shall be no

circulation of air over the resistors other than that caused by the heat of the resistors.

b. Test temperature: +25�C �5�C.

Initial measurements: Initial resistance shall be measured after mounting at +25�C �5�C. This

initial measurement shall be used as the reference temperature for all measurements.

d. Operating conditions: Resistors shall be operated at full rated wattage (chassis mounted) by

applying dc continuous working voltage, or ac rated continuous working voltage, from an ac

supply at commercial line frequency and waveform, intermittently, 1 hour and 30 minutes "on" and

30 minutes "off" for the applicable number of hours. "On time" shall be 75 percent of the total

elapsed time. The actual test time shall be recorded.

e. Test condition: 2,000 hours for qualification inspection with all samples continued on test to

10,000 hours. Ten thousand hours for FR level determination.

Measurements during test:

(1)

Qualification inspection: Resistance (see 4.8.3) shall be measured at the end of the 30

minutes "off" periods after 250 hours +72 hours, -24 hours, 500 hours +72 hours, -24 hours,

1,000 hours +72 hours, -24 hours, and 2,000 hours +96 hours, -24 hours have elapsed.

Units continued on test shall be measured at intervals above 2,000 hours +96 hours, -24

hours in accordance with 4.8.13f(2).

(2)

Extended life testing: Resistance (see 4.8.3) shall be measured at the end of the 30

minutes "off" periods after 250 hours +72 hours, -24 hours, 500 hours +72 hours, -24 hours,

1,000 hours +72 hours, -24 hours, 2,000 hours +96 hours, -24 hours and every 2,000 hours

+96 hours, -24 hours thereafter, until the required extended life period (10,000 hours +120

hours, -0 hours) has elapsed. Measurements shall be made as near as possible to the

specified time but may be adjusted so that measurements need not be made during other

than normal working days.