MIL-PRF-94 Resistors, Variable, Composition General Specification ForThis specification covers the variable resistors having a composition resistance element shaped in an arc, and a contact bearing uniformly thereon, so that a change in resistance is produced between the terminal of the contact and the terminal of either end of the resistance element when the operating shaft is turned. These resistors are capable of full load operation \(where maximum resistance is engaged\) at a maximum ambient temperature of 70 degrees C and are capable for continuous operation when properly derated, at a maximum temperature of 120 degrees C \(see 6.9\).

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MIL-PRF-94G

FIGURE 4. Counterclockwise taper.

3.4.3 Contact arm assembly. Contact pressure on the resistance element shall be maintained uniformly by

positive pressure and shall be such as to permit smooth electrical and mechanical control of the resistor over the

entire range of electrical rotation (see figure 2). The rotating contact shall have continuous electrical contact with its

terminal throughout the entire electrical rotation and shall be insulated from the rotating operating shaft and the

resistor housing. The entire contact arm assembly shall be sufficiently sturdy to preclude the necessity of adjustment

during the life of the resistor.

3.4.3.1 Stops. Stops employed to limit the mechanical rotation of the contact arm assembly may be part of, but

not complete any electrical circuit.

3.4.4 Terminals. Resistors and attached switches shall be supplied with external terminal lugs or terminal loops of

such size and styles as to permit accommodating and soldering of three 0.032 inch (0.81 mm) diameter (AWG size

22, stranded) wires for resistors with 0.250 inch (6.35 mm) diameter operating shaft, and three 0.025 inch (0.64 mm)

diameter (AWG 22, solid) wires for resistors with 0.125 inch (3.18 mm) diameter operating shafts. Terminals shall be

treated to facilitate soldering.

3.4.4.1 Solder dip (retinning) lead. The manufacturer may solder dip/retin the leads of the product supplied to this

specification provided the solder dip process has been approved by the qualifying activity. See appendix A,

3.4.4.2 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 6.4 and appendix A).

3.4.5 Solder. Solder for electrical connections shall be in accordance with J-STD-006.

3.4.6 Mounting. Resistors shall be mounted by the single hole method with previsions for the nonturn device as

specified (see 3.1).

3.4.6.1 Hardware. Each resistor shall be furnished with a corrosion resistant, internal tooth lock washer, a

hexagonal mounting nut and a lock washer (if applicable), as specified (see 3.1). Hardware shall not be assembled

on the resistor unless otherwise specified (see 6.2d). For direct Government orders, hardware shall be assembled on

the resistor.