MIL-R-19365 Resistor, Adjustable, Wirewound, PowerThis specification covers power, adjustable, wirewound, resistors for use in electrical, electronic, communication, and associated equipment. These resistors have a resistance range of 1 ohm to 15,000 ohms, inclusive, and a resistance tolerance of ┬▒5 percent. The power ratings cover a range of 11 watts to 210 watts, inclusive. These power ratings are applicable only when the maximum resistance is engaged in the circuit. When only a portion of the resistance element is engaged, the power rating is reduced in approximately the same proportion as the resistance.

Pages 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12 - 13 - 14 - 15 - 16 - 17 - 18 - 19 - 20 - 21 - 22 - 23 - 24 - 25 - 26

MIL-R-19365E

4.7.10 Moisture resistance (see 3.15). Resistors shall be tested in accordance with method 106 of MIL-STD-202 . The following details and exceptions shall apply:

a. Mounting: All resistors shall have their adjustable terminal positioned within ±5 percent of electrical center.

Resistors under load shall be mounted horizontally not less than 2.5 inches apart and with their terminals pointing upward, as follows:

(1) One half of the resistors shall be mounted by means of their associated hardware (see 3.1), with supplementary insulation.

(2) The remaining half of the resistors shall be mounted, without supplementary insulation, between two metal plates normal to the longitudinal axis of the resistor (one plate at each end) held firmly against the ends of the resistor by through bolts. These plates shall be of sufficient size to extend beyond the extremities of the resistor terminals.

b Initial measurements: Resistance between adjustable terminal and fixed end terminals, total resistance, and insulation resistance shall be measured as specified in 4.7.1, 4.7.1.1, and 4.7.6, respectively.

c. Load and polarization: One half of the resistors shall be subjected to load (see 4.7.10a.1) and the remaining half to polarization (see 4.7.10a.2).

(1) Load: During the first 2 hours of each of steps 1 and 4, one half the rated wattage from a dc supply, derated in accordance with the derating curve shown on figure 1 to the temperature attained at the end of the 2 hour period, shall be applied to each half of the resistor with the positive lead connected to the adjustable terminal, and a negative lead to each of the fixed end terminals. The negative terminals shall be electrically grounded to the mounting surface.

(2) Polarization: During steps 2 and 3, and 5 and 6 a potential of 100 volts dc shall be applied with the positive lead connected to the terminals tied together, and the negative lead connected to the mounting hardware.

d. Final measurements: At the end of the final cycle and while the resistors are still in the humidity chamber at the high humidity condition, the resistance between the adjustable terminal and fixed end terminals, total resistance, and insulation resistance shall be measured as specified in 4.7.1, 4.7.1.1, and 4.7.6, respectively. Resistors shall then be examined for evidence of mechanical damage, breaking, cracking, spalling, loosening of terminals or mounting hardware. (The subsequent 4 hours to 24 hours conditioning period and measurements do not apply).

4.7.11 Mechanical strength (see 3.16). Resistors shall be supported .125 inch (3.18 mm) from each end and subjected to a transverse load of 50 pounds applied at the center of the resistor through a fulcrum having a radius of not less than .250 inch (6.35 mm) nor more than .375 inch (9.53 mm). The load shall not be applied on the exposed portion of the resistor. Resistors shall then be examined for evidence of mechanical damage.

4.7.12 Terminal strength (see 3.17). Resistors shall be firmly clamped and a direct pull of 10 pounds shall be applied to the hole of each fixed end terminal for at least 30 seconds, one terminal at a time, in the direction away from the resistor and parallel to the longitudinal axis. Resistors shall then be examined for evidence of mechanical damage, and breaking or loosening of terminals.

4.7.13 Life (see 3.18).

4.7.13.1 Test conditions. This test shall be conducted at an ambient temperature of 25OC ±2OC. Resistors shall be mounted by means of their associated hardware, and wire leads shall be soldered to their terminals to make connection for arranged that the temperature of any one resistor will not appreciably influence the temperature of any other resistor. There shall be no circulation of air over the resistors other than that caused by the heat of the resistors. The voltage used shall be the rated continuous working voltage from an ac waveform (see 3.6), and adequate precaution shall be taken to maintain constant voltage on resistors under test.