Author: Leonard Krugman
The physical location of the transistor is not critical with respect to its mounting position, and since the heat generated by an individual transistor is small, many may be packed together. However, since the transistor is sensitive to the ambient temperature, hot spot locations near tubes and power resistors should be avoided. In this regard, a word of precaution on collector dissipation ratings is in order. The maximum collector dissipation is specified at some definite temperature (usually 25°C). This value must be derated if the ambient temperature is greater than the specified rating temperature. Usually this amounts to a 10% decrease in power dissipation for each 5°C increase in ambient temperature. As a numerical example, assume that the maximum allowable collector dissipation for a transistor rated at 250 mw at 25°C is required when the ambient temperature is 40°C. The operating temperature represents an increase of 40° - 25° = 15°C. The power handling capacity should be derated 10% for each 5°C increase or 15/5 × (10) = 30%. 30% of 250 mw is 75 mw. Thus the maximum collector dissipation at 40°C is 250 - 75 = 175 mw.
Whenever a transistor is operated near its maximum rating, it is always good insurance to tie it to a metal panel or chassis. This connection provides a large radiating surface which permits the collector dissipation to be maintained at higher levels. In typical cases, this procedure increases the transistor power dissipation rating from 20 to 50%.