scope:

This specification covers the general requirements for variable resistors having a resistance element of wire, wound on an insulating strip shaped in an arc, so that a contact bears uniformly on the resistance element when adjusted by a control shaft. These resistors are capable of full-load operation at an ambient temperature of 40°C and are suitable for continuous operation when properly derated at any ambient temperature up to 105°C.

The type designation shall be in the following form, and as specified (see 3.1 and 6.1 ):

The style is identified by the two-letter symbol "RA" followed by a two-digit number; the letters identify low-operating-temperature, wire-wound, variable resistors and the number identifies the size and power rating.

The type of bushing is identified by a single letter in accordance with table I. TABLE I. Type of bushing.

Symbol Bushing ----------------------------------- N - - - Standard L - - - Locking S - - - Shaft and panel seal (standard) T - - - Shaft and panel seal (locking)

The type of an attached switch or absence of a switch is identified by a single letter in accordance with table II. TABLE II. Switch.

Symbol Switch -------------------------------------------- A - - - - No switch B^1/--- Single-pole, single-throw switch which is actuated to the on position at the start of clockwise rota- tion ^1/For replacement purposes only, not for new design.

The operating-shaft styles and lengths are identified by a two letter symbol; the first letter indicates operating-shaft style in accordance with table III, and the second letter indicates operating-shaft length as specified (see 3.1). TABLE III. Style of operating shaft.

Symbol Shaft ------------------------- S - - - - - Slotted

The nominal total resistance value expressed in ohms is identified by a three-digit number; the first two digits represent significant figures and the last digit specifies the number of zeros to follow. When fractional values of an ohm are required, the letter "R" shall be substituted for one of the significant digits to represent the decimal point, and the succeeding digits of the group shall represent significant figures; e.g., 3R0 signifies 3.0 ohms.

The resistance characteristic is identified by a single letter in accordance with table IV. TABLE IV. Resistance characteristic.

Symbol Resistance taper Resistance tolerance (see figures 2 and 3) -------------------------------------------------------------- Percent (±) A - - A 10 C - - C 10 E - - F 10

The type designation RA20NASK502A signifies:

RA20 - A 2-watt, low-operating temperature, wire-wound, variable resistor of the dimensions specified (see 3.22). N - Standard ⅜-inch bushing. A - No switch. SK - Slotted shaft 2 ½ inches long. 502 - Nominal total resistance value of 5,000 ohms. A - Linear taper with resistance tolerance of ±10 percent.