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

FIGURE 4. Rotational life test circuit.

4.7.19.2 Procedure. Total resistance shall be measured as specified in 4.7.2.1. A dc potential, equivalent to that

required to dissipate rated wattage across the entire resistive element of resistors having the same total resistance as

those under test, but not exceeding the maximum rated voltage, shall then be applied as shown in figure 4. The

operating shaft shall be continuously cycled through not less than 90 percent of the actual effective electrical travel, at

the rated 1 cycle for 2 minutes �0.5 minutes, for a total of 200 cycles. A cycle shall consist of travel through 90

percent of actual effective electrical travel and return to the starting point. After rotation, total resistance shall be

measured as specified in 4.7.2.1. Resistors shall then be examined for evidence of mechanical damage.

4.7.20 Terminal strength (see 3.25).

4.7.20.1 Pull (applicable to all terminal types). Resistors shall be tested in accordance with method 211 of

MIL-STD-202. The following detail and exception shall apply:

a. Test condition A: Applied force 2 pounds. Resistors clamped by the resistor body, force applied to each

lead individually.

b. Measurement after test: Resistors shall be examined for evidence of mechanical damage, and tested for

electrical continuity.

4.7.20.2 Push (applicable to all terminals except type L). Resistors shall be tested in accordance with method 211

of MIL-STD-202. The following detail and exception shall apply:

a. Test condition: A, except force shall be applied in the direction towards the resistor body. Applied force 2

pounds. Resistors clamped by the resistor body, force applied to each lead individually.

b. Measurement after test: Resistors shall be examined for evidence of mechanical damage, and tested for

electrical continuity.

4.7.20.3 Bend (applicable to terminal types P, W, X, and Y). Resistors shall be firmly clamped and each terminal

shall be bent through 90 degrees at a point 0.125 inch from the body of the resistor, with the radius of the curvature at

the bend approximately 0.03125 inch. The pin shall be returned to the original position, bent 90 degrees in the

opposite direction, and again returned to the original position. At the conclusion of the test, the resistors shall be

examined for evidence of mechanical damage and tested for electrical continuity.

4.7.21 Solderability (applicable to terminal types P, W, X, and Y) (see 3.26). Resistors shall be tested in

accordance with method 208 of MIL-STD-202. The three pin terminal of each resistor shall be tested.