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.

Pages 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12 - 13

MIL-PRF-39002C

4.6.2.1 Total resistance (see 3.7.1). Total resistance shall be measured as specified in 4.6.2, between the

resistance-element end terminals (terminal 1 and terminal 3 on figure 1) with the contact arm positioned against a

stop. The positioning of the contact arm and terminal shall be the same for all subsequent measurements of the total

resistance on the same specimen.

4.6.2.2 Absolute minimum resistance (not applicable to style RK09) (see 3.7.2). The contact arm shall be

positioned near one end of the resistance element, so that a minimum value of resistance shall be measured as

specified in 4.6.2 between the contact arm and the respective end terminal. The same procedure shall be followed

for the other end of the resistance element. Rated current through the resistance element shall not be exceeded

during this measurement.

4.6.2.3 End resistance (see 3.7.3). The contact arm shall be positioned at the extreme counter-clockwise limit of

mechanical travel, and the resistance shall be measured as specified in 4.6.2 between the contact arm and the

respective end terminal. The contact arm shall then be positioned at the extreme clockwise limit of mechanical travel,

and the resistance shall be measured as specified in 4.6.2 between the contact arm and the respective end terminal.

During this test, precaution shall be taken to insure that rated current of the resistance element is not exceeded.

Clockwise and counterclockwise signify the direction of rotation of the operating shaft when the resistor is viewed

from the shaft end.

4.6.3 Continuity (see 3.8). The operating shaft shall be rotated at a uniform rate such that the wiper traverses the

effective electrical travel in both directions within 1 minute � .25 minute. During rotation, a suitable electrical device

shall be connected between the wiper and either end terminal and monitored for uni-directional change in voltage or

resistance. Precaution shall be exercised to prevent excessive current flow in the resistor during the test.

4.6.4 Mechanical travel (see 3.9). Resistors shall be placed in a suitable angle indicating device and the operating

shaft shall be rotated from one stop to the other and held against the stops with the torque of approximately

1 pound-inch. The mechanical travel shall be determined from the number of degrees traversed between stops.

4.6.5 Actual effective-electrical travel (see 3.10). The actual effective-electrical travel shall be measured by

placing the resistor in a suitable device and circuit which will indicate both angular position of the contact arm and

voltage output. The actual effective-electrical travel will be the degree of rotation of the operating shaft in which a

change in contact-arm position gives a measurable change in voltage output.

4.6.6 Peak noise (see 3.11). Peak noise resistance shall be measured with the measuring circuit shown on

figure 3, or its equivalent. During this test, the contact arm shall be rotated in both directions for a total of ten cycles,

through 90 percent of the actual effective-electrical travel, at the rate of 1 cycle for 5 seconds, minimum, to 2 minutes,

maximum. The equivalent resistance shall be measured during the tenth cycle and calculated using the following

formula:

Epn

ohms

Noise =

0.001

Where: Epn = the peak-noise signal voltage presented on the oscilloscope screen.