/It is important to check the DC rating on the switch.
/You will note that many of the RS Switches only have
/AC ratings.
There are about 100 ways for manufacturer's to "rate"
thier switches. I know of at least 20 different
specifications in the military documents alone. The
way switches are used in airplanes (less than 500
operations per year) the mechanical ratings of about
ANY switch will give good service in an airplane.
Generally speaking, a switch that is suited for
10 amps at 125 volts AC will perform well at 10
ams and 14 volts DC. The most important aspect of
switch selection for d.c. operation is how "snappy"
it is.
I'll quote an excerpt from the AeroElectric Connection's
opening for the chapter on switches . . . .
-----
Switches, relays and contactors are a family of basic
devices used to control flow of current in an electrical
system. The generic switch has been around from the
very beginning of electrification. Houses were convert=1F
ed from gas or oil to electrical illumination by literally
fastening lamp fixtures to ceilings, switches to walls and
connecting the whole mess together with wires that
were stapled to the surface. Even in these rudimentary
beginnings, switches were constructed to accomplish
connection or disconnection of electrical circuits with a
snap. As one rotated the handle of an early light
switch, a feeling of winding up a spring was unmistaka=1F
ble. At some point, tension was sufficient to push
internal parts past a detent and the switch would
complete its operation with a "click"; a sudden release
of tension could be felt in the fingers.
The need for snap action was well understood, espe=1F
cially in the early days of domestic power distribution.
Most houses were first supplied with direct current
(d.c.) in contrast with present day systems which supply
alternating current (a.c.). In the section on over voltage
protection I described special design efforts required
to control the fire between spreading relay contacts
which are attempting to bring a failed alternator/regu=1F
lator system under control. When d.c. was routed to
our ancestors homes, the problem of controlling high
voltage had to be addressed. In this instance, the high
d.c. voltage was not developed by a collapsing magnetic
field. The voltage was already high; 100 volts or so as
delivered to the back of the house!
A simpler device known as a knife switch was universal
in industrial applications. It operates in a manner which
is suggested by its name: a blade of conductive material
was moved by an insulated handle so that it was forced
between two spring loaded leaves of conductive materi=1F
al thus making a connection. Breaking connection was
accomplished by simply pulling the blade from between
the leaves.
My earliest recollection of knife switches is from old
black and white movies. The good Doctor Frankenstein,
standing over his patient, is yelling instructions to Igor
who manipulates many switches and knobs on the
laboratory apparatus. Operation of these switches is
always accompanied with flashes of fire and puffs of
smoke. The effects were undoubtedly enhanced for the
benefit of the movie viewers, but then fire and smoke
was not totally out of character for this type of switch.
Indeed, hesitant or sloppy operation of a knife switch in
a high voltage circuit would produce long, hot blue
sparks accompanied by subsequent damage to the switch.
Purveyors of early electrification products and services
knew that consumers would not be favorably impressed
with little fires and puffs of smoke at their fingertips.
Snap action switches were developed in a successful
attempt to keep "lights" off of walls and up on ceilings
where they belonged! Control of high voltage d.c. using
snap action switches required no instruction or special
action on the part of the switch operator. A child could
easily accomplish a making or breaking of connection.
Thus children began to "make" more connections than
they "broke" and the seeds of eternally illuminated
children's bedrooms were planted. They thrive to this
day!
--------------------------
14 volt d.c. circuits DO NOT fall into the "too hard"
pile for controlling arcing at contacts. The battery
contactor should have a diode or MOV across its coil
to corral inductive kickback, the starter contactor too.
Landing lights are the only devices with a
potentially high inrush and this is generally limited
to about 5X due to wire losses; a 50 amp inrush isn't
going to kill a switch with a few operations. I like
to call people's attention to the switches used on thousands
of Cessna 150's. I'll guarantee you those little plastic
rockers cost under $1 when they were originally installed.
The vast majority of them are still in service after 30
years. The C-150 I fly the most has all original switches
and the airplane has over 3000 hours on it.
/They almost certainly have gold contacts on the switches
/and probably would only support low current DC unless
/specifically stated that they can support higher currents.
Gold is used only on very LOW current switches and it
is easily burned away if the switch's current ratings
are exceeded even for an instant. You won't find any
ordinary switches that use gold contacts.
/Putting high current DC through these switches could
/possibly result in arc over and welded contacts.
I've never seen a welded contact in a hand operated switch.
Contactors do it regularly and that's mostly due to contact
wear and metal transfer after many operations. Even the
very best Cuttler-Hammer "aircraft quality" contactors
weld shut and or burn up . . got one on my desk right now
that came out of a KingAir air conditioning system. Sucker
is pure toast! Except for an occasional sticking starter
contactor (you need a starter energized light on panel !!!)
stuck contacts in single engine airplanes are VERY rare.
/Automobile switches would work fine but do check the
/rating of the switch first.
Except for landing light and pitot heat, most airplane
power control circuits are quite light . . . Even if
you OVERLOAD a switch to 200% of its "rating" you may
reduce it's laboratory life from 50,000 cycles down
to 5,000 cycles . . . how long is it going to take you
to "wear out" that switch in your airplane?
Pick switches with substantial mechanical "feel" and
good construction. Wiring terminals should be secure
in the housing and not "wiggle" when pushed on. And yes,
get a switch good for 7-10 amps and it will work fine
about anywhere in your airplane.
/RS do have some miniature toggles with high current
/DC contacts - my catalogue is in the office and I
/am a home wid a code so I can't dig out the numbers.
The miniature toggles are generally limited to 5 amps
and they have pretty close contact spacing when open.
I do not recommend these devices for power control in
airplanes. They're fine for audio and instrumentation
switching or power control under an amp or so.
/Are automobile switches of suitable quality for
/fitting into our Europa? I have looked high and low for
/suitable DC rated units without much joy. Is there a
/common divider to work out a dc rating for an ac rated switch?
The terms "quality" and "rating" are certainly unquanitified
terms and in the context of this discussion, pretty meaningless.
We need to learn failure tolerant design . . ASSUME that
EVERYTHING you put in is going to fail at some point in
time. If you need it for comfortable completion of flight,
have a backup system. If NOT don't worry about it. In any
case, the only reason for upgrading the QUALITY of a part
should be because you're tired of replacing it, not because
it damned near got you killed.
Then it doesn't matter WHAT kind of switches you use. I
hightly recommend the standard toggles that mount in 15/32"
round holes. They are universally produced by hundreds
of manufacturers, they are available with .25" fast-on tabs
for easy installation and replacement. My idea of the
ideal switch is one that can be replaced in 5 minutes for
$5, sitting in the pilot's seat with only a nut-driver in
hand.
Regards,
Bob . . .
AeroElectric Connection
////
(o o)
oOOo(_)oOOo
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