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

3.5.3.1 Terminal identification and circuit diagram.

3.5.3.1.1 Terminal identification.

3.5.3.1.1.1 Terminals P, W, X, and Y type resistors. For terminals P, W, X, and Y type resistors,

identification shall be by one of two methods: The numerical designating terminal identification may be

marked adjacent to the terminals, or the circuit diagram (see 3.5.3.1.2) may be used, provided that such

information clearly indicates the applicable terminals.

3.5.3.1.1.2 Terminal "L" type resistors. For terminal type L resistors, the insulation of the flexible leads

shall be color-coded as shown on figure 1.

3.5.3.1.2 Circuit diagram. The circuit diagram shall be marked on any surface of the resistor in a

legible manner, as shown on figure 1.

3.5.3.1.3 Legibility. Marking shall remain legible after all tests.

FIGURE 1. Circuit diagram.

3.5.4 Securing of screw-thread assemblies. Screw-thread assemblies shall not loosen as a result of

the tests specified herein.

3.5.5 Lead screw actuator. The lead screw actuator shall be of corrosion resistant material with the

head insulated from all electrical parts of the resistor.

3.5.6 Contact-arm assembly. Contact pressure on the resistance element shall be maintained by

positive pressure and shall permit smooth electrical and mechanical control of the resistor over the entire

range. The moving contact shall have continuous electrical contact with its terminal throughout the entire

mechanical travel and shall be insulated from the lead screw actuator head and case.

3.5.7 Clutches. Resistors shall contain clutches which permit the contact arm to idle at either end of

the resistance element without electrical or mechanical malfunction (see 3.16.2).

3.5.8 Solder dip (retinning) leads. The manufacturer (or his 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 appendix A) has been approved by the qualifying activity.

3.5.9 Pure tin. The use of pure tin, as an underplate or final finish is prohibited both internally and

externally. Tin content of resistor components and solder shall not exceed 97 percent, by mass. Tin

shall be alloyed with a minimum of 3 percent lead, by mass (see 6.13).