/The jargon got the better of me here, and as I would
/like to understand what you are talking about Graham,
/can you please explain:
/2)What a SPDT and a DPDT is?
At the risk of running afoul of european switch naming conventions,
I'll share the following . . .
"SP" does indeed mean single pole . . . one wire may be switched
to one or more other wires.
"DP" two wires . . .
"4P" four wires . . . . etc.
Number of throws is a little esoteric . . . an SPST switch is
the common, two-position, on-off switch but taken literally,
one would like to describe it as two "throws" one for ON and
one for OFF. To erase all ambiguities, consider adding a
second phrase to the description: "SPST ON-OFF" fairly describes
the most common switch function.
Now, suppose you wanted to have a switch to an either-or type of
task. You can puchase a single pole switch that makes a connection
at both extremes . . . lots folk like to think that SPDT is
enough to describe this switch . . . electricians on this side
of the pond call it a "three way" switch . . . because it has
three connections to it . . . makes a lot of sense doesn't it?
How about calling it SPDT ON-ON? Now the function is clear.
You can buy this same switch with a center off detent where we'll
call it SPDT ON-OFF-ON.
How about a spring loaded return from one side . . . SPDT ON-OFF-(ON).
Parens are widely used in catalogs to describe a spring loaded
momentary position.
We can also buy both extremes spring loaded . . . SPDT (ON)-OFF-(ON).
The same general functions are available in toggle switches having
two, three or even four poles. This variety in mechanisms allows
you to do some interesting things. Suppose you want to extend flaps
with a permanent magnet motor by holding the switch until the desired
position is reached, yet have the flaps retract all the way when the
switch is thrown up. Use a DPDT ON-OFF-(ON) wired for polarity
reversing to the motor and the momentary side for extend flaps.
Placing the switch in the full up position will have them fully
retract with no further attention from the pilot.
There's a very useful variation on the DPDT switch where one
half of the switch transfers when the toggle is moved from one
extreme to the center; the other half transfers as you continue
through center to the opposite extreme and called an
SPDT ON-ON-ON. This guy is handy for having taxi lights and
landing lights on one switch. Far down position is off, mid position
is taxi, upper position is landing light. This same switch can
be wired to fill a SP3T function where any one of three circuits
are selected by positioning the toggle in down, mid or up.
One of my favorite applications for this switch is to wire strobe
and nav lights to the same switch such that down is off, mid
position transfers strobe controlling side of switch and upper
position adds nav lights. This lets you run strobes by themselves
during daytime ops and add nav lights after dark. Each system still
runs from it's own protection on the bus. Saves on panel space.
I use toggles to control both mags plus starter using yet another
variation of the DPDT ON-ON-(ON). Wire the starter contactor
through the two switches so that the right mag switch (not
impulse coupled) must be OFF for the starter to be accessable.
Use the momentary, far up position on the left mag switch to finish
the path that engages the starter. After the engine starts,
the right mag is turned on and the starter is disabled. Again,
saves dollars (two toggles are less expensive than the classic
key switch) and absolutely prevents kickbacks from having a right
mag come alive at just the wrong time during a failed starting
sequence.
By adding the function callout on behind the mechanical
callout, you can quickly describe the operation of any
switch in a very unambiguous manner.
/May I _strongly_ recommend Bob Nuckoll's AeroElectric
/Connection if you are electrically challenged. This book
/covers all the basics of electrical wiring for aircraft
/and assumes that you know zip about wiring.
Don't know that I've got it ALL in there yet but we're
planing a pretty good sized update in the next revision
package. I think I'm finally going to tackle the chapter
I've been putting off for the past ten years . . NOISE.
/I am sure Bob will let you know how to get hold of a copy.
/(Bob - the address for my commission cheque is....... :))
Actually, we do reward our shil . . . uh . . . 'er subscribers
who help promote the work. A good referral gets the benefactor's
own subscription bumped up a year. I've got one guy in South
Carolina who has a life-time subscription. He's promoted us
so well that I gave up keeping track of 'em!
For those of you who have never seen the book, it's a three-ring,
loose-leaf binder format that gets updated in some way every
year. The materials used in the update process are gleaned from
our participation on the various lists and SIGs. It's truly
a joint effort for us and our subscribers. If I were not taking
the time to work with all of you on the net, it would be very
difficult to know what kinds of things need to be in the book!
Check our website for more details.
BTW . . my wife is taking webcrafting lessons and will be overhauling
our website in the near future. Many big changes coming on line
including a whole new bunch of products. We're working three
techniques for low fuel warning (one is LED/fiber optic) and
a new, 12-slot annunciator that is totally customizable with
respect to ledgends, colors, pull-up or pull down activation and
whether or not a warning onset gets a flashing light and master caution
indication. Tiny enough for an RV-4 and sophisticated enough
for a Lancair-IV but about 1/2 the price of contemporary offerings.
All of our new gizmos for homebuilders will become bureaufodder
for my efforts to become an FAA santioned DER (designated engineering
representitive). We'll be getting STC/PMA on most of these items
as they become available. Electronics is wuuuunnnnderrrrfuuulllll.
Regards,
Bob . . .
AeroElectric Connection
////
(o o)
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