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

4.7.9.1 Setting stability. The contact arm shall be set at approximately 40 percent of the actual effective electrical

travel. An adequate dc test potential shall be applied between end terminals. The voltage between the end

terminals, and the voltage between one end terminal and the contact arm, shall be measured and applied to the

following formula:

Setting stability = E1 x 100

where:

E1 = Voltage across one end terminal and the contact arm terminal.

E2 = Voltage across the end terminals.

The difference between the initial measurement made before the environmental test and the measurement made

after the test indicates the setting stability in percent.

4.7.10 Resistance temperature characteristic (see 3.15). Resistors shall be tested in accordance with

method 304 of MIL-STD-202. The following details shall apply:

a. Test temperatures: As specified in table XI.

b. Measurements at the end of each period: Total resistance shall be measured as specified in 4.7.2.1, at the

temperature maintained during the period.

TABLE XI. Resistance temperature characteristic test temperature.

Temperature �3oC

Sequence

25 2/

2 1/

-15

-55

25 2/

5 1/

150

1/ Not applicable to conformance inspection.

2/ This temperature shall be considered the reference temperature

for each of the succeeding temperatures.

NOTE: At the option of the manufacturer the reverse sequence may be as follows:

Room temperature

+150 C �3 C

Room temperature

-55oC �3oC

4.7.11 Moisture resistance (see 3.16). Resistors shall be tested in accordance with method 106 of MIL-STD-202.

The following details and exceptions shall apply:

a. Mounting: Clamped, or otherwise mounted with the terminals exposed on a stainless steel panel of sufficient

size to extend beyond the resistor extremities, and in such a manner as to allow electrical connections to be

made to the terminals.

b. Initial measurement: Immediately following the initial drying period, total resistance shall be measured as

specified in 4.7.2.1.

c. Steps 7a and 7b are not applicable.