MIL-PRF-39002 Resistors, Variable, Wirewound, Semi-Precision, General Specification ForThis specification covers the general requirements for semi-precision, wirewound, variable, resistors having a resistance element of wire, wound on an arc shaped insulated form, so that a contact bears uniformly on the resistance element, when adjusted by a contact shaft. The electrical output \(in terms of percent of applied voltage\) is linear with respect to the angular position of the contact arm. These resistors are capable of full-load operation \(when the maximum resistance is engaged\) at a maximum ambient temperature of 85 Degrees C, and are suitable for continuous operation, when properly derated, at a maximum temperature of 135 Degrees C. These resistors have a tolerance of ??5 percent.

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MIL-PRF-39002C

4.6.20.2 Procedure. Total resistance and setting stability shall be measured as specified in 4.6.2.1 and 4.6.11.1,

respectively. The resistors shall be placed in a chamber at room temperature. The temperature shall be gradually

decreased to -55 C +0 C, -5 C within a period of not less than one and one-half hours. For conformance inspection

only, and at the option of the supplier, the resistors may be placed in the chamber when the chamber is already at the

extreme low temperature. After one hour of stabilization at this temperature, setting stability shall be measured as

specified in 4.6.11.1. Full rated continuous working voltage (see 3.1 and 3.6) shall be applied for 45 minutes. The

resistors may be loaded individually or in parallel. Fifteen minutes +5 minutes, -0 minute after the removal of voltage,

setting stability and torque shall be measured as specified in 4.6.11.1 and 4.6.10, respectively. The temperature in

the chamber shall be gradually increased to room temperature within a period of not more than eight hours. The

resistors shall be removed from the chamber, and maintained at a temperature of 25 C �5 C for a period of

approximately 24 hours. Total resistance shall again be measured as specified in 4.6.2.1. Resistors shall be

examined for evidence of mechanical damage.

4.6.21 High temperature exposure (see 3.26).

4.6.21.1 Mounting. Resistors shall be mounted in such a manner as to allow electrical connections to be made to

the terminals.

4.6.21.2 Procedure. Total resistance and setting stability shall be measured as specified in 4.6.2.1 and 4.6.11.1,

respectively. The resistors shall then be exposed to an ambient temperature of 135 C +5 C, -0 C for a period of

250 hours �8 hours. Not less than two hours after the end of the exposure period, setting stability and total

resistance shall be measured as specified in 4.6.11.1 and 4.6.2.1, respectively; torque shall be measured as

specified in 4.6.10, except that it shall be determined during the movement of the contact arm from the position for

setting stability to the position for total resistance; dielectric withstanding voltage (at atmospheric pressure),

independent linearity tolerance and insulation resistance shall be measured as specified in 4.6.7.1, 4.6.8, and 4.6.9,

respectively,. Resistors shall then be examined for evidence of mechanical damage.

4.6.22 Rotational life (see 3.27).

4.6.22.1 Mounting. Resistors shall be mounted as specified in 4.6.19a. Care should be taken to insure that side

thrust is minimized by the proper alignment of the resistor shafts with the drive shafts. The resistors shall be ganged

in pairs and each pair shall be connected in series as shown on figure 4, so that a nominally constant current flows

through the resistor, irrespective of the contact arm position during the cycling of the shafts. The shafts shall be so

connected mechanically that they will turn simultaneously in the same direction. When performing this test on

locking-bushing types, the locking nut shall be removed.

FIGURE 4. Rotational life test circuit.