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

3.4.8 Pure tin. The use of pure tin, as an underplate or final finish is prohibited both internally and externally. Tin

content of resistor components and solder shall not exceed 97 percent, by mass. Tin shall be alloyed with a

minimum of 3 percent lead, by mass (see 6.11.1).

3.5 Power rating. The resistors shall have a power rating based on continuous full load operation at the ambient

temperature specified for the applicable characteristic (see 3.1 and table II). The power rating is dependent on the

ability of the resistors to meet the life requirements specified in 3.21. For temperatures in excess of those specified,

the load shall be derated as shown in figure 2. Power ratings as specified (see 3.1) are applicable only when the

maximum resistance is engaged in the circuit. When only a portion of the resistance element is engaged in the

circuit, the power rating is reduced in approximately the same proportion as the resistance.

FIGURE 2. Derating curve for high ambient temperatures.

3.6 Voltage rating. The theoretical calculated rated continuous working voltage or the voltage equivalent to power

rating (rated wattage) shall be determined from the following formula:

E = PR

Where:

E = Rated dc or root mean square (rms) continuous working voltage.

P = Power rating.

R = Nominal total resistance.

The rated voltage shall not exceed the maximum continuous working voltage (see 3.1).

3.7 DC resistance.

3.7.1 Total resistance. When measured as specified in 4.7.2.1, the total dc resistance shall not deviate from the

specified nominal resistance (see 3.1) by more that �5 percent.

3.7.2 Minimum resistance. When measured as specified in 4.7.2.2, the absolute minimum resistance shall not

exceed 0.25 percent of the nominal resistance value or 1 ohm, whichever is greater.