Selecting Proper Free Air Wattage
Follow articleIn order to choose the most suitable resistor meeting the requirements of the application the user would need 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 than or less than the nominal rating. The details by which such ratings can be estimated are given in the following pages. Most thermal calculations, however, involve so many factors which are usually not accurately known, that 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. This more accurate method is not often feasible with complex systems. Since this is the case, it can be appropriate to calculate these temperature rises. The factors which affect the temperature rise act independently of each other and are summarized as follows:
1. Ambient Temperature
As the maximum permissible operating temperature is a set amount, any increase in the ambient temperature subtracts from the permissible temperature rise and therefore reduces the permissible watt load.
2. Enclosure
Enclosure limits the removal of heat by convection currents in the air and by radiation. The walls of the enclosure also introduce a thermal barrier between the air contacting the resistor and the outside cooling air. Hence, size, shape, orientation, amount of ventilating openings, wall thickness, material, and finish all affect the temperature rise of the enclosed resistor.
3. Grouping
When resistors are close to each other they will show an increased hot spot temperature rise for a given wattage because of the heat received by radiation from each other and the increased heat per unit volume of air available for convection cooling.
4. Altitude
The amount of heat which air will absorb varies with the density, and therefore with the altitude above sea level. At altitudes above 100,000 feet, the air is so rare that the resistor loses heat practically only by radiation.
5. Pulse Operation
This is not an environmental condition but a circuit condition. As a pulse of power, when averaged over the total on and off time, results in less heat per unit time than for continuous duty, the temperature rise is affected. This may permit higher power during the pulses. The conditions must be expertly considered for conservative rating. The openwound “PowrRib^{®}” resistor construction is most suitable.
6. Cooling Air
Forced circulation of air over a resistor removes more heat per unit time than natural convection does and therefore permits an increased watt dissipation. Liquid cooling and special conduction mountings also can increase the rating.
7. Limited Temperature Rise
It is sometimes desirable to operate a resistor at a fraction of the Free Air Watt Rating in order to keep the temperature rise low. This may be to protect adjacent heat sensitive apparatus, to hold the resistance value very precisely both with changing load and over long periods of time and to insure maximum life.
With an understanding of the known factors, we can now apply known mathematical values to them. The chart below can be used to assist in choosing the proper Free Air Wattage when certain application conditions exist.
Example:
Four resistors, each dissipating 115 watts, are to be mounted in a group. Spacing is to be 2” surface to surface. Ambient to be 50°C (122°F). Enclosure to be total. Other factors standard. Determine Watt Size required.
Directions:
 For each Condition, locate the relevant value on the scales below and record the corresponding factor (F1 to F7). Note: The Standard Free Air Condition Factor is always 1.
 Multiply the Factors together.
3. Multiply the Watts by the product obtained from 2 above
Operation (1) On Ambient Temperature scale locate 50°C. Note and record F1 = 1.1 as shown. Locate and record the other factors.
F1 
F2 
F3 
F4 
F5 
F6 
F7 

50° 

100% 


Std Conditions 

1.1 
X 
2 
X 
1.2 
X 
1 
X 
1 
X 
1 
X 
1 
Operation (2) Multiply the factors together = 2.64
Operation (3) 115 Watts x 2.64 = 304 Watts Free Air Watt Size Rating required for each resistor.
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