Choosing The Appropriate Resistor
Resistors are devices connected to a circuit to introduce a specified resistance. The resistance is measured in ohms. As stated in Ohm's Law, the current through the resistor will be directly proportional to the voltage across it and inversely proportional to the resistance.
How Resistors WorkThe 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 that can be permitted. Resistors are rated to dissipate a given wattage without exceeding a specified standard "hot spot" temperature and the physical size is made large enough to accomplish this.
Deviations from 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.
Choosing the appropriate ResistorResistor selection requires three steps:
- Determine the resistance and the watts to be dissipated by the resistor
- Determine the proper "Watt Size" (physical size) as controlled by watts, volts, permissible temperatures, mounting conditions and circuit conditions
- Choose the most suitable kind of unit, including type, terminals and mounting
- Bracket Mount Resistors
- Panel Mount Resistors
- PC Mount Resistors
- Surface Mount Resistors
The Engineering Behind a ResistorOhm's Law, (shown in the figure below) enables determination of the resistance when the required voltage and current are known. When the current and voltage are unknown, or the best values not decided on, at least two of the three terms in Ohm's Law must be measured in a trial circuit. Power in watts, can be determined from the formulas in figure b., which stem from Ohm's Law. R is measured in ohms, E in volts, I in amperes and W in watts.
Stated non-technically, any change in current or voltage produces a much larger change in wattage (heat to be dissipated by the resistor). Therefore, the effect of apparently small increases in current or voltage must be investigated because the increase in wattage maybe large enough to be significant. Mathematically, the wattage varies as the square of the current, or voltage, as stated in figure b. For example, an increase of 20% in current or voltage will increase the wattage 44%. Figure 1 graphically illustrates the square law relation. Hence, the actual current must be used in figuring the wattage and the increase in wattage due to apparently small changes, then determined in order to select the proper size resistor. Allowance should be made for maximum possible line voltage.
To allow for the differences between the actual service conditions and the "Free Air Watt Rating", it is a general engineering practice to operate resistors at more or less than the nominal rating. Most thermal calculations, however, involve so many factors that are usually not accurately known; at best they are only approximations.
The most accurate method of determining or checking the rating is to measure the temperature rise in a trial installation. A thermocouple (made of #30 B & S gage wire) is recommended for the measuring element. Even measurements made with a thermocouple will vary slightly with different samples and techniques. The factors that affect the temperature rise act independently of each other and are as follows:
- Ambient temperature
- Pulse Operation
- Cooling Air
- Limited Temperature Rise
- Other - Including high resistance, high voltage, high frequency and military specifications