Derating
Derating (or de-rating or de-tuning) is the operation of a machine at less than its rated maximum power in order to prolong its life. The term is commonly applied to electrical and electronic devices and to internal combustion engines.
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[edit] In electronics
Power electronic devices have a maximum power dissipation rating usually quoted at a case temperature of 25 °C (77 °F). The datasheet for the device also includes a derating curve which indicates how much a device will dissipate without getting damaged at any given case temperature and this must be taken into account while designing a system.
As can be seen from the derating curve image for a hypothetical BJT, the device (rated for 100 W at 25 °C (77 °F)) cannot be expected to dissipate anything more than about 40 W if the ambient temperature is such that the temperature at which the device's case will stabilise (after heat-sinking) is 65 °C (149 °F). This final case temperature is a function of the thermal resistance between the device's case and the heat-sink; and the heat-sink and the ambient (this includes the heat-sink's temp/watt rating - with lower values implying better cooling characteristics).
[edit] In electrical installations
All dimmers rely on heat conduction and convection to keep the electronic components cool. Similarly, power wiring (e.g., house wiring) not surrounded by an air space (e.g., inside a conduit) needs to have its current-limiting device (e.g. circuit breaker or fuse) adjusted so as not to carry as much current through that circuit. Derating is the reduction of the maximum capacity (load) a unit can reliably handle when fins/side sections are removed.
[edit] Internal combustion engines
Internal combustion engines are normally given different ratings for different applications. For example, a diesel engine rated at 1,000 horsepower for locomotive use may be de-rated by 10% to 900 horsepower for marine or stationary use.
[edit] References
Datasheet of power transistor MJL0281A on OnSemi (PDF)
