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

d. Polarization and loading voltage: The resistors shall be divided in two equal groups; one group shall be

subjected to polarization and the other group to load.

(1) Polarization: During steps 1 to 6 inclusive, a 100 volt dc potential shall be applied with the positive lead

connected to the resistor terminals tied together, and the negative lead connected to the mounting plate.

(2) Loading voltage: During the first two hours of steps 1 and 4, a dc potential equivalent to 100 percent

rated wattage, but not exceeding the maximum rated voltage (see 3.1), shall be applied to the resistors.

e. Final measurements: Upon completion of steps 6 of the final cycle, the resistors shall be removed from the

chamber and air dried for one half hour at room ambient conditions. Samples shall not be subjected to forced

air drying. The total resistance and insulation resistance shall then be measured 30 to 60 minutes after

removal from the humidity chambers, as specified in 4.7.2.1 and 4.7.7, respectively. The subsequent 24-hour

conditioning period and measurements do not apply.

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

4.7.12 Shock (specified pulse) (see 3.17). Resistors shall be tested in accordance with method 213 of

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

a. Special mounting means: Resistors shall be mounted by their normal mounting means, with their bodies

restrained from movement on an appropriate mounting fixture. The mounting fixture shall be constructed in

such a manner as to insure that the mounting supports remain in a static condition with reference to the

shock test table. Resistors shall be mounted in relation to the test equipment in such a manner that the

stress applied is in the direction which would be considered most detrimental.

b. Test leads: Test leads used during this test shall be no longer than average wire gauge (AWG) size 22

stranded wire, so that the influence of the test lead on the resistor shall be held to a minimum. The test lead

length shall be no longer than necessary.

c. Measurements before shock: Total resistance and setting stability shall be measured as specified in 4.7.2.1

and 4.7.9.1, respectively.

d. Test condition: I.

e. Measurement during shock: Each resistor shall be monitored to determine electrical discontinuity of the

resistance element, and between the contact arm and element, by a method that shall at least be sensitive

enough to monitor or register, automatically, any electrical discontinuity of 0.1 millisecond or greater duration.

f. Measurement after shock: Setting stability and total resistance shall be measured as specified in 4.7.9.1 and

4.7.2.1, respectively.

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