nuckolls.bob(at)aeroelect
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 Posted: Sun Apr 10, 2016 8:08 am Post subject: Switch ratings |
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At 09:08 PM 4/9/2016, you wrote:
| Quote: | if I am interpreting the datasheet correctly, that switch will work.
Mount the switch with the keyway down.
Connect the alternator field to teminals 2 & 3.
Connect the master contactor to terminals 5 & 6.
As for the current rating, I do not understand it. But since the maximum current under certain conditions is 20 amps, then most likely the switch will handle 5 amps at 12 vdc. |
Not sure where the 20a figure comes from but
consider this . . .
Catalog 'ratings' for switches and relays are
very loosely applicable to how we use them in
airplanes. For example, consider the el-cheapo,
whisky-barrel battery contactors used on hundreds
of thousands of airplanes for decades . . . it's
'rated' for 70A . . . yet we subject it to engine
cranking currents in the hundreds of amps.
Switch ratings are not about dancing up to the
edge of failure . . . it's all about service life.
The switch/relay catalogs will offer up perfectly
valid laboratory test results suggesting resistive,
inductive and lamp load limits for their products
assuming the user wants it to last for THOUSANDS
of cycles.
Ain't gonna happen in the RV or Lancair . . . or
even a Beechjet. I cannot recall having trouble
shot for a switch that failed for having reached
end of service life. Failures are much more likely
to arise from environmental stress over time . . .
or manufacturing defect.
Graybeards here on the List will recall a rash
of Carling switch failures in an airplane fitted with
a modern strobe system utilizing a constant-power
switch-mode power supply. This device draws significantly
more power during battery only versus normal bus
voltage. The builder 'failed' several switches over
too-short a period of service life. Another builder
suffered a really strange failure wherein a fuse
blew when the accessory was turned OFF (!?)
See: http://tinyurl.com/jpvv8et
http://tinyurl.com/gphkdgz
Turns out that the switches in question were not
only part of the most demanding control situation
in the airplane . . . they suffered from poor fits
that drove up on-resistance and induced progressive
heating scenarios.
The short answer is: Just about any switch you
buy, whether from the local hardware store or a mil-spec
supplier will perform just fine any where in your
airplane but if you desire deeper understanding and
confidence. See
http://tinyurl.com/gv9gdua
and review
http://tinyurl.com/zop5m4r
The bottom line remains . . . if failure of ANY
component in your system places comfortable termination
of flight at-risk, then you need to ASSUME that
failure is going to happen and have a plan-b in
place. Upgrading any one component to space-flight
reliability numbers is not the path to aviation
Nirvana. For example, why put a $100 switch on a
landing light when you KNOW the bulb is going to
burn out?
An excerpt from another article on aeroelectric.com . . .
Nuckolls' first law of airplane systems design sez: "Things break"
The Second: "Systems shall be designed so that when things break,
no immediate hazard is created."
The third: "Things needed for comfortable termination of flight
require backup or special consideration to insure operation and
availability"
The forth: "Upgrading the quality, reliability, longevity,
or capability of a part shall be because you're tired of replacing
it or want some new feature, not because it damned near got you
killed."
If these tenants are observed (not difficult to do) then it makes
no difference where you buy your parts, how much money you spend
on them or which parts you select. You're free to try any selection
with the goal of exploring how long it will last or how well
it will perform.
Bob . . .
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