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

4.7.13 Vibration, high frequency (see 3.18). Resistors shall be tested in accordance with method 204 of

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

a. Mounting: As specified in 4.7.12a.

b. Test leads: As specified in 4.7.12b.

c. Measurement before vibration: As specified in 4.7.12c.

d. Test condition: D.

e. Measurement during vibration: As specified in 4.7.12e.

f. Measurement after vibration: As specified in 4.7.12f, and torque as specified in 4.7.8.1.

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

4.7.14 Salt spray (corrosion) (see 3.19). Resistors shall be tested in accordance with method 101 of

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

a. Special mounting: As specified in 4.7.11a.

b. Test condition: A.

c. Examination after exposure: Resistors shall be examined for corrosion.

4.7.15 Resistance to soldering heat (applicable to terminal types P, W, X, and Y) (see 3.20). Total resistance

shall be measured as specified in 4.7.2.1. All resistors terminals shall be immersed in a pot of molten solder at a

temperature of 350oC �10oC, for a period of 3 seconds �0.5 seconds, to a point 0.125 inch from the entry of the

terminal into the resistor body. After a minimum of 3 hours have elapsed, total resistance shall be measured as

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

4.7.16 Life (see 3.21). Resistors shall be tested in accordance with method 108 of MIL-STD-202. The following

details and exceptions shall apply:

a. Method of mounting: Resistors shall be mounted by their normal mounting means, on a 0.0625 inch thick,

glass base, epoxy laminate. The resistors shall be so arranged that the temperature of any one resistor shall

not appreciably influence the temperature of any other resistor. There shall be no circulation of air over the

resistors other than that caused by heat of the resistors.

b. Test temperature and tolerance: 85 C �5 C.

c. Initial measurements: Measurements may be made inside or outside the chamber. Total resistance and

setting stability shall be measured as specified in 4.7.2.1 and 4.7.9.1, respectively.

d. After resistors have been stabilized at their respective temperatures for at least 8 hours, the resistance

between the end terminals, with the contact arm in the position for setting stability, shall be measured.

e. Operating conditions: Rated dc or ac continuous working voltage at commercial line frequency and

waveform, shall be applied intermittently to the end terminals of the resistors, 1.5 hours "on" and 0.5 hour

"off", for a total of 1,000 hours, at the test temperature. Each resistor shall dissipate rated wattage, and shall

not exceed maximum voltage. Adequate precautions shall be taken to maintain constant voltage on the

resistor.

f. Test condition: D.