MIL-PRF-27208 Resistor, Variable, Wirewound, Nonprecision General Specification ForThis specification covers the general requirements for multiturn, lead-screw actuated and single turn wirewound, variable resistors with a contact bearing uniformly over the entire surface of the entire resistive element, wound linearly, when positioned by an actuator. These resistors are capable of full load operation \(where maximum resistance is engaged\) at maximum ambient temperature of 85 Degrees C, and are capable for continuous operation when properly derated, at a maximum temperature of 150 Degrees C \(see figure 2\). These resistors have a resistance tolerance of ┬▒5 percent \(see 3.1\).

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MIL-PRF-27208F

3.4.3 Terminals. Terminals shall be as specified in table III. All terminals shall be fastened securely. Terminals

shall be coated to meet the solderability requirements of 3.26 (see 6.11).

3.4.3.1 Terminal identification and circuit diagram.

3.4.3.1.1 Terminal identification.

3.4.3.1.1.1 Terminals P, W, X and Y. For terminals P, W, X, and Y type resistors, identification shall be by one of

two methods. The numerals designating terminal identification may be marked adjacent to the terminals or the circuit

diagram (see 3.4.3.1.2) may be used, provided that such identification clearly indicates the applicable terminals.

3.4.3.1.1.2 Terminal L. For terminal L type resistors, the insulation of the flexible lead shall be color coded as

FIGURE 1. Circuit diagram

3.4.3.1.2 Circuit diagram. The circuit diagram shall be marked on any surface of the resistor in a legible manner,

as shown on figure 1.

3.4.3.1.3 Legibility. Marking shall remain legible after all tests.

3.4.3.2 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 Securing of screw thread assemblies. Screw thread assemblies shall not loosen as a result of the tests

specified herein.

3.4.5 Operating shaft. The operating shafts shall be of corrosion resistant material with the head insulated from all

electrical parts of the resistor. The operating shaft head shall be slotted as specified (see 3.1).

3.4.6 Contact arm assembly. Uniform 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 operating shaft head and case.

3.4.7 Mechanical limits.

3.4.7.1 Clutches (applicable only to multiturn lead screw actuated units, unless otherwise specified) (see 3.1).

Clutches shall be furnished to permit the contact arm to idle at either end of the resistance element without electrical

or mechanical malfunctions (see 3.13.2).

3.4.7.2 Stops (applicable to single turn units, unless otherwise specified) (see 3.1). Stops employed to limit the

mechanical rotation of the assembly are permitted to be part of, but shall not complete an electrical circuit (see