MIL-PRF-39015 Resistors, Variable, Wirewound, \(Lead Screw Actuated\), Nonestablished Reliability, And Established Reliability, General Specification ForThis specification covers the general requirements for nonestablished reliability \(non-ER\) and established reliability \(ER\), lead screw actuated; wirewound variable resistors with a contact bearing uniformly over the surface of the entire resistive element, wound linearly, when positioned by a multiturn lead screw actuator. These resistors are capable of full load operation \(when the maximum resistance is engaged\), at a maximum temperature of 85OC and are capable for continuous operation, when properly derated, at maximum temperature of 150OC \(see 3.6\), with a resistance tolerance of ??5 percent. The ER resistors have life failure rate \(FR\) levels ranging from 1.0 percent to 0.001 percent per 1,000 hours \(see 1.2.1.4\). These FR levels are established at a 60 percent confidence level on the basis of life tests.

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MIL-PRF-39015E

d. Operating conditions: Rated dc or ac continuous working voltage (RCWV) at commercial line

frequency and waveform shall be applied intermittently to the wiper and active terminal of the

resistor such that 95 percent �2 percent of the rated power (see 3.1) is dissipated by the resistor.

The voltage shall be applied to the resistor, one and one-half hours "on" and one-half hours "off"

for time duration specified in 4.8.25f at the test temperature. Adequate precaution shall be taken

to monitor the constant voltage on the resistor.

e. Test condition: 2,000 hours for qualification with all samples continued to 10,000 hours.

f. Measurements during test:

(1) Qualification inspection: Resistance (see 4.8.4), shall be measured at the end of the 0.5 hour

"off" periods after 168 hours +72 hours, -24 hours; 504 hours +72 hours, -24 hours; 1,008

hours +72 hours, -24 hours; and 2,016 hours +96 hours, -24 hours have elapsed. Units

continued on test shall be measured at intervals above 2,000 hours in accordance with

(2) Extended life testing: Resistance (see 4.8.4), shall be measured at the end of the 0.5 hour

"off" periods after 168 hours +72 hours, -24 hours; 504 hours +72 hours, -24 hours; 1,008

hours +72 hours, -24 hours; 2,016 hours +96 hours, -24 hours, and every 2,000 hours

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

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.

(3) Measurements outside of chamber: When measurements are made outside the chamber,

resistors shall be outside of the chamber for a minimum of 45 minutes and stabilized at room

temperature before measurement.

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

4.8.26 Resistance to solvent (see 3.32). Resistors shall be tested in accordance with method 215 of

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

a. The marked portion of the resistor body shall be brushed.

b. The number of sample units shall be as specified in table V and table IX, as applicable.

Resistors shall be examined for mechanical damage and legibility of markings.

4.8.27 Fungus (see 3.33). Resistors shall be tested in accordance with method 508 of MIL-STD-810.

Resistors shall be examined for evidence of fungus.