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

3.25 Low temperature operation. When resistors are tested as specified 4.8.19, they shall meet the

following requirements:

Change shall not exceed 0.5 percent plus the specified maximum

Setting stability:

resolution (see 3.1).

Total resistance:

Change shall not exceed �1 percent for resistance values of 100 ohms

or greater, and �(1.0 percent +0.05 ohm) for values below 100 ohms.

Visual examination:

There shall be no evidence of mechanical damage.

3.26 Low temperature storage (for qualification only). When resistors are tested as specified in 4.8.20,

the change in total resistance shall not exceed �1 percent for resistance values of 100 ohms or greater,

and �(1.0 percent +0.05 ohm) for values below 100 ohms. There shall be no evidence of mechanical

damage.

3.27 High temperature exposure. When resistors are tested as specified 4.8.21, they shall meet the

following requirements:

Change shall not exceed 0.5 percent plus the specified maximum

Setting stability:

resolution (see 3.1).

Total resistance:

Change shall not exceed �1 percent for resistance values of 100 ohms

or greater, and �(1.0 percent +0.05 ohm) for values below 100 ohms.

Dielectric withstanding voltage

As specified in 3.14.

(atmospheric pressure):

Insulation resistance:

Shall not be less than 1,000 megohms.

Visual examination:

There shall be no evidence of mechanical damage.

3.28 Integrity of shaft. When resistors are tested as specified 4.8.22, the shaft shall remain in one piece.

3.29 Rotational life. When resistors are tested as specified in 4.8.23, the change in total resistance

shall not exceed �2 percent, and there shall be no evidence of mechanical damage. The failure of one

pair of units being tested shall not count as a failure for the companion unit.

3.30 Terminal strength. When resistors are tested as specified in 4.8.24, there shall be no evidence of

mechanical damage and resistors shall be electrically continuous.

3.31 Life.

3.31.1 Qualification. When resistors are tested as specified in 4.8.25, there shall be no evidence of

mechanical damage. The change in resistance between the initial measurement and any of the

succeeding measurements up to and including 2,000 hours shall not exceed �2 percent plus specified

maximum resolution (see 3.1).

3.31.2 FR level determination. When resistors are tested as specified in 4.8.25, there shall be no

evidence of mechanical damage to the resistance element or enclosure. The change in resistance

between the initial measurement and any of the succeeding measurements shall not exceed �3 percent

plus the specified resolution. This single failure criterion shall be applicable to all measurements during

the life test for purposes of determining FR level qualification and is applicable as a parallel requirement

with 3.31.1 to the measurements made during the life test specified for the qualification test.