Derating

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This article is about electronics. For the adjustment of engines for longer engine life see engine tuning.

In electronics, derating (or de-rating or de-tuning) is the operation of a device at less than its rated maximum power in order to prolong its life.

In electronics[edit]

Derating curve of a hypothetical power device.

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 bipolar junction transistor, 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).

In electrical installations[edit]

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.

References[edit]

Datasheet of power transistor MJL0281A on OnSemi (PDF)

See also[edit]