De-rating of Hi- Reliability Thermal Switches
Background:
At the start of space flight, few practices were in place to assure performance in space. Parts designed for aircraft and basic Mil-Spec devices were used much as they would be on earth. This soon proved to be unsatisfactory, and de-rating of all "Electrical, Electronic, and Electro-Mechanical Parts" became a documented practice. MIL-STD-975 was one of the earlier methods imposing requirements for de-rating. This document is now obsolete, however de-rating is still a vital practice. Several documents have superceded MIL-STD-975.
Purpose of de-rating
De-rating is a technique whereby a part is stressed in actual usage at values well below the manufacturer's rating for the part. By decreasing mechanical, thermal, and electrical stresses, the possibility of degradation or catastrophic failure is lessened.
Current Documents:
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MIL-HDBK-1547, Military Specification
D-8545B, JPL
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MF0004-400, THE BOEING COMPANY
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SSP30312, NASA, International Space Station
The first two documents are available on the Web.
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Discussion
These specifications refer thermal switch users to the relay portion of the specifications for de-rating. The tables in each specification vary in amount of de-rating recommended, however they have a great deal in common. The JPL and NASA specification both use formulas which consider temperature, number of cycles per hour and load factor. The MIL-HDBK-1547 uses a table based on the type of circuit elements: resistive, lamp, etc. The Boeing specification provides a table like MIL-HDBK-1547, a reliability calculation, and specific written guidelines as follows:
- In no case (except for non-repetitive transients) shall the de-rating be less than 25 percent(.75 X rating) for all upper and worst-case specification limits.
- It must be emphasized that the user shall evaluate all thermal switches according to the requirements of the application since the user is responsible for assuring adequate de-rating.
- Worst-case circuit design analysis must consider the possibility of a thermal-vacuum on the circuit and the effect of this environment on the operation and thermal dissipation capability of the device.
Use of these methods and guidance has served the Space industry well. Honeywell endorses the practice of de-rating and encourages users to implement de-rating as appropriate for their application in all instances.
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