Application Notes

Resistor Facts and Factors

A resistor is a device connected into an electrical circuit to introduce a specified resistance. The resistance is measured in ohms. As stated by Ohm’s Law, the current through the resistor will be directly proportional to the voltage across it and inversely proportional to the resistance.

The passage of current through the resistance produces heat. The heat produces a rise in temperature of the resistor above the ambient temperature. The physical ability of the resistor to withstand, without deterioration, the temperature attained, limits the operating temperature which can be permitted. Resistors are rated to dissipate a given wttage without exceeding a specified standard “hot spot” temperature and the physical size is made large enough to accomplish this.

Deviations form the standard conditions (“Free Air Watt Rating”) affect the temperature rise and therefore affect the wattage at which the resistor may be used in a specific application.

Selection Requires 3 Steps

Simple short-cut graphs and charts in this catalog permit rapid determination of electrical parameters. Calculation of each parameter is also explained. To select a resistor for a specific application, the following steps are recommended:

  1. (A) Determine the resistance.
    (B) Determine the Watts to be dissipated by the Resistor.
  2. Determine the proper “Watt Size” (physical size) as controlled by watts, volts, permissible temperatures, mounting conditions and circuit conditions.
  3. Choose the most suitable kind of unit, including type, terminals and mounting.