nuckolls.bob(at)cox.net
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 Posted: Sat Sep 13, 2008 10:16 am Post subject: Switch Ratings versus Root Cause of Failure |
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At 10:38 AM 9/13/2008 -0700, you wrote:
| Quote: | Hi Scott. The landing and taxi light circuits have inrush current
limiters installed, for this very reason. The objective was to save wear
and tear on the switches. Steady state draw is about 5 amps.
As for the strobe supply, there was some debate on my theory of why this
is the worst load in the a/c. My hypothesis is that strobe supplies have
a negative voltage-current relationship (negative resistance). If you
reduce the voltage to a strobe supply (or, in fact many switchmode power
supplies), its current actually increases. Once a switch or terminal
fails, this leads to thermal runaway and the results are what I have seen
(twice).
Not everyone agrees, but I've had this failure twice and Bob has
documented another one-- all in the strobe circuits.
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It is true that the "negative resistance" or more appropriately
"constant power" characteristics of switchmode power supplies may
have contributed to the pace of failure. However, in terms
of having abused the switch's ratings I'll refer the readers to
the photo at:
http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/g.jpg
This is the working contact in a failed strobe supply switch
we examined a few years ago. Note that the contact - the major
life-limiting component is almost pristine in comparison with
the carnage going on elsewhere . . .
http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/e.jpg
There's more to this failure analysis stuff than beating
on the ratings drum. I'll have to sift through the gray matter
but I don't think I have NEVER encountered a failure of
a component aboard an aircraft wherein root cause was a failure
to observe ratings. It's the FIRST question that should be
asked but it's also the easiest concern to put to rest.
Bob . . .
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