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.
/The ever popular RS catalogue from England has two
/major ranges of switches with gold contacts. From their
/size and quality they look ideal for aircraft use and
/it was to these I was referring - specifically look for DC rating.
Hmmm . . . interesting. On a quick check the US Radio Shack catalog
I don't find listings for any sort of gold finish on switches.
I do recall an increasingly popular line of high-power automotive
audio components sold here in the states that refers to "gold
plating" on the contact surfaces. I've ordered in some of these
devices (fuse holders for the medium size cartridge fuses) and
I'm a bit suspicious of the nature of the plating. It's a super
glossy finish . . . not unlike the gloss you see on "chrome" plated
plastics which is indeed an aluminum finish. Knowing what I
pay to have gold added to etched circuit card fingers and comparing
the price of plated versus non-plated fuseholders, I'd sure like
to know what the finish REALLY is . . .
In electronics parts, there are miniature relays that offer
a gold flash on their 2 amp contacts. The gold is useful
for maintaining a very low resistance for what is called
"dry circuit" switching (tens of milliamps or less). If one
never runs the rated 2 amps through these relays, the gold
stays intact and they perform very well at the low currents.
Run a few cycles at 2 amps and the gold is gone . . . the relay
still performs as a power relay but never again as a dry-relay.
Just went through a problem diagnosis with a customer where
their functional test department set up automated test equipment
based on the relay's power rating and without regard to the
application (switching thermocouple leads). Needless to say,
their final test proceedure was killing the product before it
ever got into a box for shipment! I did a design for B&C a couple
of years ago that uses 9 very tiny, relays to switch a fist full
of wires from an ILS/VOR/OBS head between a NAV/COMM and GPS
receivers. I cautioned them to do all final assembly continuity
testing with very low currents lest they trash their product.
/I can send you a couple if you want. I learnt a long time ago that the
/RS gold connectored switches don't like high current DC. 15A DC is not
/the same as 15A AC! Luckily it was only on a home build computer
/(yes, it drew 15A at 5v and still only had 2k of memory!!). Took me some
/time to work out why those switches kept failing though.
Yes, I would like to see what they look like. I might even take my
fuseholders and your switches down to a local lab for a "look-see" as
to plating material. To be sure, there are differences between DC
and AC . . . one of the best examples I can think of is that welders
much prefer a DC welder to an AC one . . . DC welders use stabilizing
inductors to achieve a quasi-constant current mode. This allows much
more latitude in electrode-to-workpiece spacing without having the
arc's "heat" run all over the place . . . much easier to weld with.
Now, if you really wanted to weld some switch or relay contacts, DC
is going to be the prefered weapon . . . AC goes through zero-volts
every few milliseconds . . . the arc fires tend to go out and not
re-light.
/The RS ones are rated to 10A DC but ditto.
I'd like to see one of these too . . . Miniature switches on this side
of the pond tend to be on the class of C&K 7000 series toggles. I use
a lot of these in test equipment, audio panels, instrumentation loads,
etc . . . I am looking at a line of miniature rockers current offered
in the Digi-Key catalog and rated in the 10AMP class . . . they have a
decent "snap" to them. I've been using them in some panel mounted trim
controllers and my customers seem to like their look and feel. They've
asked about using the same switch for power panels. I'll do some further
testing/investigation and see if it makes sense.
NOW, for the FEW heavy current loads (lights, pitot heat, ect) on could
consider using relays to boost the switche's capabilities. Low cost,
30 amp relays with fast-on terminals are readily available. This runs
contrary to my keep-it-simple rule but then complexity goes ONLY to
reliability issues . . . higher parts count, lower reliability. If one
really liked the look and feel of smaller switches, there are ways to
accomodate the interest. If we've done our homework and depend upon no
single system for comfortable completion of flight, then FLIGHT SYSTEM
reliability is not compromised by a slight reduction in the reliability
of a single lighting system, etc.
/Its also a hell of a lot easier making round holes that it is
/making square ones I can tell you!!
You got that right. BTW, are you aware of a cutting tool called a
"spot facer"? These are intended to produce smooth, perpendicular
bolt, nut and washer surfaces on irregular or rough shapes. They
are available in many sizes and use a pilot hole to guide on. Used
in a hand drill they cut VERY nice holes in sheet metal. The don't
snag and rip through like a twist drill. I'll be stocking 1/4" and
15/32" tools when our website catalog gets up and running. Use
one of these once and you'll never go back to the old way.
/Does anyone use solid state switching in light aircraft? That would
/allow the use of smaller panel switches. I use sold state switching b
/with cheap toggles extensively on my model railway now - primarily
/because it was cheaper than buying quality switches. Solid state
/switching is used extensively throughout production control systems
/and within robotics, even to the point of being used in life critical
/equipment but you don't hear of anyone doing it in light aircraft - yet.
Oh you bet! This is a logical, more modern follow-on to the relay-boost
technique I described above. I've been watching the development of
power field-effect transistors for over 10 years. N-channel devices good
for 50-70 amps are quite inexpensive and with VERY LOW on resistance;
under 10 milliohms. This is important because you DON'T want to
use a heat sink to cool the transistor if you don't have to. N-channel
devices are quite practical now but they need a "boostrap" supply to
turn them on solidly . . . gate has to be more positive than bus by
about 6 volts or so.
International Rectifier has brought some photo-voltaic opto-couplers
out that provide a 5-10 volt output as opposed to simply being a switch.
I'm going to look at combining these with power fets to do the boost
you suggest. P-channel devices are looking quite good too . . . some
now below .02 ohms in the higher current ratings. These can be used
"upside-down" with source attached to bus and you pull down on the
gate with your low current switch. . . . . This is a VERY practical
alternative to relays . . . no open contacts to corrode, weld, wearout,
etc. . . . I'm planning some articles on the subject . . probably not
until next fall. Got lots of things on my plate for the summer and through
OSH.
Regards,
Bob . . .
AeroElectric Connection
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