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

4.6.8 Independent linearity tolerance (see 3.13). Independent linearity measurements shall be of the continuous

output type or performed at intervals of not more than 10 percent of the effective electrical travel. The resistor shall

be tested without load. Measurements shall be performed by using a dc potential of low enough value to insure that

its application for the time necessary to obtain readings does not appreciably affect the measured values. The

method employed for the measurement of linearity shall be such that the combined inherent errors of the measuring

instruments shall not exceed 0.10 percent.

4.6.9 Insulation resistance (see 3.14). Resistors shall be tested in accordance with method 302 of MIL-STD-202.

The following details shall apply:

Test condition letter: A.

Special preparation: As specified in 4.6.7.1a.

Points of measurement: As specified in 4.6.7.1d.

4.6.10 Torque (see 3.15).

4.6.10.1 Starting. The maximum torque required to move the contact arm on the resistance element shall be

determined at approximately 10 percent, 50 percent, and 90 percent of actual effective-electrical travel by the torque-

wrench method or by any other method satisfactory to the Government.

4.6.10.2 Stop. Resistors shall be mounted on a metal panel by their normal mounting means. The total

mechanical travel shall be measured. The contact arm shall then be rotated to each extreme limit of mechanical

travel, and the 2 pound-inches static torque shall be applied through the operating shaft to the stops. The total

mechanical travel shall again be measured, and the contact arm and stop shall be examined for damage.

4.6.10.3 Locking. Resistors shall be mounted on a metal panel by their normal mounting means and the locking

device tightened with a torque of 8 pound-inches. With the locking device tightened, the operating shaft shall

withstand a torque of 20 ounce-inches. Movement of the operating shaft shall be checked visually and resistors shall

be examined for evidence of damage.

4.6.11 Thermal shock (see 3.16). Resistors shall be tested in accordance with method 107 of MIL-STD-202. The

following details and exceptions shall apply:

Test condition letter: B.

Measurements before cycling: Total resistance and setting stability shall be measured as specified in

Measurements after cycling: Setting stability, independent linearity tolerance and total resistance shall be

measured as specified in 4.6.11.1, 4.6.8, and 4.6.2.1, respectively. Continuity of the contact arm shall be

verified by connecting a vacuum-tube voltmeter of other suitable indicating device, between the contact-arm

terminal and the counterclockwise end terminal. The applied voltage shall be in accordance with table VI.

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