MIL-PRF-22 Resistors, Variable, Wirewound, Power Type, General Specification ForThis specification covers the general requirements for power type variable resistors having a resistance element of wire, wound linear on an insulating strip shaped in an arc, such that a contact bears uniformly on the resistance element when adjusted by a control shaft. The power ratings \(see 3.1\) cover a range from 6.25 watts to 1,000 watts, inclusive \(see 6.2\).

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MIL-PRF-22E

4.7.7 Load life. Load life shall be conducted at any ambient temperature from 20 C to 40 C. The total resistance

shall be measured as specified in 4.7.3 to 4.7.3.1.2, as applicable. With the resistors mounted as specified in 4.7.6.1

and with the movable contact adjusted for total resistance, the applicable rated wattage, in accordance with figure 1,

shall be applied intermittently, one and one-half hours "on" and one-half hour "off", for a total of 1,000 hours. Circuit

connections shall be such that current flows through the movable contact. Total resistance measurements (see 4.7.3

to 4.7.3.1.2, as applicable) shall be made at the end of the one-half hour "off" period after 50 hours 8 hours, 100

hours 8 hours, 250 hours 8 hours, 500 hours 12 hours, 750 hours 12 hours, and 1,000 hours 12 hours have

elapsed. Before the final resistance measurement is made, the contact arm shall be rotated 25 cycles. Resistors

shall be examined for evidence of mechanical damage (see 3.10).

NOTE: Operation of these resistors at ambient temperatures greater than 125 C can damage the metal plating, shaft

lubricant, etc. of the resistor.

FIGURE 1. Power rating curves for continuous duty.

4.7.8 Vibration, low frequency (when applicable). Resistors shall be tested in accordance with method 201

of MIL-STD-202 (see 3.11). The following details shall apply:

a. Method of mounting: With the contact arm set at approximately 50 percent of the effective rotation. Locking

bushing type resistors shall have the locking nut tightened with a torque not greater than 8 pound-inches.

b. Measurement after vibration: Total resistance shall be measured as specified in 4.7.3 to 4.7.3.1.2, as

applicable. The position of the contact arm shall be measured in degrees and the resistance between the

contact arm terminal and either terminal.

c. Measurement during vibration: Each resistor shall be monitored to determine electrical discontinuity or

movement of contact area.

d. Measurement after vibration: As specified in 4.7.8b. Resistors shall be examined for mechanical damage.