Choosing The Appropriate Resistor
How Resistors Work
Choosing the appropriate Resistor
The Engineering behind a Resistor
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.
- 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
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
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