How to Derate Temperature-Sensitive Resistors
 Thermistor Power-Derating Curve | Derating Chart: |
Resistor Thermistor ratings are normally specified at +25oC and must be reduced as the thermistor temperature increases. A derating chart is often used, with derating starting at 25oC [see above]. Since parameters are application dependent, power de-rating curves or charts should be considered general rather than absolute, and only used as a guideline. The safest designs use the largest physical size component operating at conservative temperatures and power ratings.
The graph should be used as a guide only. As any particular body style or physical difference in size of Thermistor may have their own derating chart. In addition, each individual manufacturer may have their own derating graph as well, which may differ from what is shown.
In general the government always uses percent of maximum power and degrees centigrade as shown on the graph. Other companies may use something different to indicate safe operating levels. Some companies may not even provide a graph, substituting a simple equation instead.
Related topics; How to Derate; Guideline for Derating Electronic Components
Derating Resistor Networks, and Thermistor Manufacturers, or Resistor Dictionary.
Engineering Hint: It is possible to have a dearting chart that shows a power rating greater than 100 percent. For example if a component can dissipate a higher power below 25 degrees centigrade than the chart would show 100 percent at 25C and might show 125 percent at 0C [for example]. The chart will normally start to derate at 25C out of convention, but if operation is common below 25C, or more power is available than you may see a chart showing greater than 100%. Only because it's common for most of the specifications in a data sheet to be given at 25C. While the chart normally shows data down to 0C, the tabulated characteristics on a data sheet will provide numbers at 25C [and or a maximum operating temperature].
Editor note; It may seem counterintuitive to have to derate a component designed to function based on the temperature of the device. However even though a thermistor is designed to change its resistance over temperature, the internal structure of the device still has a maximum operating temperature. All components, regardless of their function have a permissible range of temperatures that they may operate in. Once the device is operated above or below the recommended temperature the characteristics of the device are no longer guaranteed to follow the data published in the data sheet.
