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

4.7.6.2 Barometric pressure. Resistors shall be tested in accordance with method 105 of MIL-STD-202. The

following details and exceptions shall apply:

a. Method of mounting: As specified in 4.7.6.1a.

b. Test condition: C.

c. Period of time at reduced pressure prior to application of potential: One minute.

d. Tests during subjection to reduced pressure: A potential of 350 volts from an ac supply at commercial line

frequency and waveform shall be applied for one minute.

e. Points of application: As specified in 4.7.6.1d.

f. Examination and measurement: As specified in 4.7.6.1e.

4.7.7 Insulation resistance (see 3.12). Resistors shall be tested in accordance with method 302 of MIL-STD-202.

The following detail shall apply:

a. Test condition: C.

b. Special preparation: As specified in 4.7.6.1a.

c. Point of application: As specified in 4.7.6.1d.

4.7.8 Torque.

4.7.8.1 Operating (see 3.13.1). The maximum operating torque required to move the contact arm on the resistive

element shall be determined at approximately 10, 50, and 90 percent of the actual effective electrical travel by the

torque wrench or by any other method acceptable to the government.

4.7.8.2 Clutch (see 3.13.2). The contact arm shall be adjusted to each extreme limit of mechanical travel, and

sufficient torque shall be applied to the operating shaft to permit the contact arm to idle for 25 complete mechanical

turns of the lead screw actuator. During idle, a electrical indicating device connected between the contact arm

terminal and the adjacent end terminal shall be observed for electrical continuity. After idle, the operating shaft shall

be rotated in the opposite direction and the indicating device observed to determine if the contact arm reversed

direction.

4.7.8.3 Stop (see 3.13.3). Resistors shall be mounted by their normal mounting means. The contact arm shall

then be rotated to each extreme limit of mechanical rotation, and the specified torque (see 3.1), applied through the

operating shaft to the stop.

4.7.9 Thermal shock (see 3.14). Resistors shall be tested in accordance with method 107 of MIL-STD-202. The

following details shall apply:

a. Test condition: B.

b. Measurement before cycling: Setting stability and total resistance shall be measured as specified in 4.7.9.1

and 4.7.2.1, respectively. Continuity of the contact arm shall be verified by connecting a vacuum tube

voltmeter or another indicating device, between the contact arm terminal and the counterclockwise end

terminal. The applied voltage shall be in accordance with table X.

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