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

3.7.3 End resistance. When measured as specified in 4.7.2.3, the end resistance for all characteristics shall not

exceed 2 percent, of the nominal total resistant value, or 1 ohm, whichever is greater.

3.7.4 Resistance value deviations. All maximum deviations as specified in this section are to be considered

absolute limits with the exception of the contact resistance adjustments.

3.8 Continuity. When resistors are tested as specified in 4.7.3, the change in voltage or resistance shall be

smooth and unidirectional throughout the effective travel.

3.9 Actual effective electrical travel. When resistors are tested as specified in 4.7.4, the number of turns or

degrees of the operating shaft necessary for the contact arm to traverse the resistance element shall be as specified

3.10 Peak noise. When resistors are tested as specified in 4.7.5, the peak noise resistance shall not exceed 500

ohms, with the following exceptions.

In group I of table IV:

Shall not exceed 100 ohms.

In group III of table IV:

Shall not exceed 200 ohms.

Shall not exceed 100 ohms.

3.11 Dielectric withstanding voltage. When resistors are tested as specified in 4.7.6, there shall be no evidence of

damage, arcing, or breakdown. The leakage current shall not exceed 1 milliampere.

3.12 Insulation resistance. When resistors are tested as specified in 4.7.7, the insulation resistance shall not be

less than 1,000 megohms.

3.13 Torque.

3.13.1 Operating. When resistors are tested as specified in 4.7.8.1, the torque required to move the contact arm

shall be as specified (see 3.1).

3.13.2 Clutch (applicable only to multiturn, lead screw actuated units, unless otherwise specified (see 3.1)). When

resistors are tested as specified in 4.7.8.2, the contact arm shall idle against the stop without electrical discontinuity

or evidence of mechanical damage. The travel of the contact arm shall be capable of reversing direction.

3.13.3 Stops (applicable only to single turn units, unless otherwise specified). When resistors are tested as

specified in 4.7.8.3, they shall withstand the stop torque specified (see 3.1), with out damage to the contact arm or

stop.

3.14 Thermal shock. When resistors are tested as specified in 4.7.9, the change in total resistance shall not

exceed �(1 percent + 0.05 ohms). The change in setting stability shall not exceed 1 percent plus the specified

maximum resolution (see 3.1) and there shall be no electrical discontinuity or evidence of mechanical damage.

3.15 Resistance temperature characteristic. When resistors are tested as specified in 4.7.10, the resistance

temperature characteristic, at each of the test temperatures specified in 4.7.10a, referred to an ambient temperature

of 25 C, shall not exceed �0.005 percent per C.