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

4.6.18 Resistance to soldering heat (see 3.23). Resistors shall be tested in accordance with method 210 of

MIL-STD-202. The following details shall apply:

Measurement before test: DC resistance shall be measured as specified in 4.6.2.

Test condition letter: A.

Depth of the immersion in the molten solder: To a point within .125 inch to .188 inch

(3.18 mm to 4.78 mm) from the resistor body.

Measurement after test: Twenty-four hours �4 hours after completion of the test, the dc resistance shall be

measured as specified in 4.6.2. Resistors shall be examined for evidence of mechanical damage.

4.6.19 Life (see 3.24). Resistors shall be tested in accordance with method 108 of MIL-STD-202. The following

details and exceptions shall apply:

Method of mounting: Resistors with a diameter of 2 inches (50.8 mm) or less shall be mounted on a 4-inch

(101.6 mm) square, .050 inch (1.27 mm) thick, steel panel in still air. Still air is surrounding air with no

circulation other than that created by the heat of the resistor being operated. Resistors with a diameter

greater than 2 inches (50.8 mm) shall be mounted on a 6-inch (152.4 mm) square, .050 inch (1.27 mm)

thick, steel panel in a similar manner, no shielding shall be located closer than 12 inches (304.8 mm) from

the panel.

Test temperature and tolerance: 85 C �5 C.

Initial measurements: Total resistance and setting stability shall be measured as specified in 4.6.2.1 and

4.6.11.1, respectively. After the resistors have been stabilized at 85 C �5 C for at least 8 hours, the

resistance between the end terminals, with the contact arm in position for setting stability, shall be

measured.

Operating conditions: Rated dc continuous working voltage (see 3.1) shall be applied intermittently to the

end terminals of the resistors, one and one-half hours on and one-half hour off, for a total of 1,000 hours, at

the test temperature. Each resistor shall dissipate rated wattage, but shall not exceed maximum voltage.

Adequate precaution shall be taken to maintain constant voltage on the resistor.

Test condition letter: D.

Measurements during test: While the resistors are still in the oven, resistance shall be measured between

the end terminals, at the end of the one-half hour off periods, after 50 hours �8 hours; 96 hours +48 hours,

-0 hours; 500 hours +48 hours, -0 hours; 750 hours +48 hours, -0 hours; and 1,000 hours +48 hours,

-0 hours have elapsed and compared to the similar reading taken in 4.6.19c.

Measurements after test: After the resistors have been removed from the oven and returned to room

temperature, setting stability, total resistance, dielectric withstanding voltage (at atmospheric pressure),

independent linearity tolerance, and torque shall be measured as specified in 4.6.11.1, 4.6.2.1, 4.6.7.1,

4.6.8, and 4.6.10, respectively.

Examination after test: Resistors shall be examined for evidence of mechanical damage.

4.6.20 Low temperature operation (see 3.25).

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

the terminals.