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

4.7.5 Peak noise (see 3.10). Peak noise resistance shall be measured with the measuring circuit shown in

figure 3, or it's equivalent. The operating shaft shall be rotated in both directions through 90 percent of the actual

effective travel for a maximum of 6 cycles. If the product passes on any one of the first three cycles, the product is

acceptable. Only the last three cycles shall be counted in determining whether or not a noise is observed at least

twice in the same location. Acceptance may be determined based on one cycle minimum where compliance to the

specification is demonstrated. The rate of rotation of the operating shaft shall be such that the wiper completes 1

cycle in 5 seconds, minimum, to 2 minutes, maximum. The equivalent resistance shall be calculated using the

following formula:

Epn

ohms

Noise =

0.001

where:

Epn = The peak noise signal voltage presented

on the oscilloscope screen.

NOTES:

1. RX is the test specimen.

2. Oscilloscope bandwidth:

DC to 50 kHz.

3. Minimum input impedance:

1.0 megohm at 400 hertz.

FIGURE 3. Peak noise circuit.

4.7.6 Dielectric withstanding voltage (see 3.11).

4.7.6.1 Atmospheric pressure. Resistors shall be tested in accordance with method 301 of MIL-STD-202. The

following details and exceptions shall apply:

a. Special preparation: Resistors shall be clamped, or otherwise mounted on metal plates of sufficient size to

extend beyond the resistor extremities, and in such a manner that measurements can be made between

terminals tied together and any other external parts.

b. Magnitude of test voltage: 900 volts root mean square (rms).

c. Nature of potential: From alternating current (ac) supply at commercial line frequency and waveform.

d. Points of application of test voltage: Between the resistor terminals tied together and all external metal

portions of the resistors and metal mounting plate.

e. Examination after test: During the tests, the leakage current shall be monitored and the resistors examined

for evidence of arcing and breakdown. At the conclusion of the test, resistors shall be examined for evidence

of damage.