MIL-PRF-39002 Resistors, Variable, Wirewound, Semi-Precision, General Specification ForThis specification covers the general requirements for semi-precision, wirewound, variable, resistors having a resistance element of wire, wound on an arc shaped insulated form, so that a contact bears uniformly on the resistance element, when adjusted by a contact shaft. The electrical output \(in terms of percent of applied voltage\) is linear with respect to the angular position of the contact arm. These resistors are capable of full-load operation \(when the maximum resistance is engaged\) at a maximum ambient temperature of 85 Degrees C, and are suitable for continuous operation, when properly derated, at a maximum temperature of 135 Degrees C. These resistors have a tolerance of ??5 percent.

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MIL-PRF-39002C

3.4.3.2 Contact-arm assembly. Uniform contact pressure on the resistance element shall be maintained by

positive pressure and shall permit smooth electrical and mechanical control of the resistor over the entire range. The

rotating contact shall have continuous electrical contact with its terminal throughout the entire mechanical travel and

shall be electrically insulated from the operating shaft, bushing, and resistor housing.

3.4.4 Stops. A stop shall be employed to limit the mechanical rotation of the operating shaft. The contact arm

shall not engage the stop.

3.4.5 Terminals. Terminals shall be a solder lug. Connection of terminals to the resistance element shall be

mechanically strong. All terminals shall be fastened securely. Pressure-type connections between end terminals and

the resistance element shall not be used. Terminals shall be protected by a corrosion-resistant metallic coating. All

terminals shall be suitably treated to facilitate soldering.

3.4.5.1 Terminal designation. Resistor terminations shall be color coded or numbered (see 3.1) and the circuit

diagram shall be marked as applicable, on the resistor in a legible manner, as shown on figure 1. Clockwise signifies

the direction of rotation of the operating shaft when the resistor is viewed from the shaft end.

FIGURE 1. Circuit diagram.

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

temperature of 85 C. The power rating is dependent on the ability of the resistors to meet the life requirements

specified in 3.24. For temperatures in excess of that specified, the load shall be derated as shown on figure 2.

Power rating as specified (see 3.1) is 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.

135

100

-65 -50

-25

100

125

150

170

Ambient Temperature ( C)

FIGURE 2. Derating curves for high-ambient temperature.