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user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
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 Posted: Wed Oct 06, 2010 6:27 am Post subject: Gold contacts? |
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| Quote: | | Also that I should probably be using the gold contacts. |
Gordon,
Gold contacts offer low resistance for low voltage instrument signals. However, gold contacts can not handle large currents like strobe lights. Mouser.com and Digikey.com offer a large selection of switches along with their ratings. Keep in mind that snap action AC switches rated for 120vac or higher can also handle the same current at 12vdc. The switches that came with my RV-12 kit from Van's Aircraft are AC rated. AC rated switches also have a DC rating, even if it is not published. Rocker switches might look cool, but they take up more room on the panel and cost more than equally rated toggle switches.
Joe
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Joe Gores |
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mdnanwelch7(at)hotmail.co
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| Posted: Wed Oct 06, 2010 7:11 am Post subject: Gold contacts? |
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> Gold contacts offer low resistance for low voltage instrument signals. However, gold contacts can not handle large currents like strobe lights. Mouser.com and Digikey.com offer a large selection of switches along with their ratings. Keep in mind that snap action AC switches rated for 120vac or higher can also handle the same current at 12vdc. The switches that came with my RV-12 kit from Van's Aircraft are AC rated. AC rated switches also have a DC rating, even if it is not published. Rocker switches might look cool, but they take up more room on the panel and cost more than equally rated toggle switches.
Joe, Gordon, list members,
I was going to ask that very question a few days regarding the ability to use an AC rated switch
for a DC operation. Thanks!! I have the Dornan 10A 125v AC switches that I got from Auto Zone.
I am installing them in my panel presently.
Coincidently, I was also building the strobe system for my plane. I read somewhere recently that the
reason switches have a bit of a problem working with strobes is due to arcing. Evidently, if the super
high voltage spark leaves behind a little residual voltage, this will cause arcing in the switch, and the arcing is
what destroys it. Someone suggested the remedy is to fix the tabs on the switch with a capacitor.
Supposedly, the capacitor will absorb this latent spark, and keep the switch in good shape.
I don't know if this is true, but someone said it in a blog I read recently, when I was doing some
research to see if my switches would work (and especially for the strobe). I also don't remember the capacitor
rating they recommended, either. Has anyone heard of this, and know the correct size cap?
Mike Welch
[quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Wed Oct 06, 2010 7:33 am Post subject: Gold contacts? |
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> Also that I should probably be using the gold contacts.
Gordon,
Gold contacts offer low resistance for low voltage instrument signals. However, gold contacts can not handle large currents like strobe lights. Mouser.com and Digikey.com offer a large selection of switches along with their ratings. Keep in mind that snap action AC switches rated for 120vac or higher can also handle the same current at 12vdc. The switches that came with my RV-12 kit from Van's Aircraft are AC rated. AC rated switches also have a DC rating, even if it is not published. Rocker switches might look cool, but they take up more room on the panel and cost more than equally rated toggle switches.
An excellent point Joe. I would elaborate by offering
the notion that the switch failures we've studied
here on the list had little to do with "current ratings".
Every failed device we studied exhibited the effects of
overheating at some location OTHER than mated contacts
within the switches. Combining this fact with a market
history going back 5 decades for these switches suggests
that the failures were precipitated by forces outside
design goals for the switch.
As you've noted, gold contacts are specialty products
useful only in VERY low current applications. Microswitch
suggests 1A as the upper limit for longevity in their
gold contact basic switches. Lower still is better yet.
Years ago at Beech we were buying $high$ crystal can
relays that were rated at 3A but came with gold plated
contacts.
[img]cid:.0[/img]
The idea was that the designer could use ONE device to
populate both 'high' current and 'small signal' systems.
A fresh, out-of-the-box relay could be installed in
an instrumentation system that required consistency
of gold contacts . . . or you could install it in
a 2A service application and let the gold burn away
exposing the more ordinary silver-cadmium contacts.
Somebody in QA decided to do 100% receiving inspection
on these relays and guess what? They tested them
for a few dozen cycles at max rated current, blessed
them as 'functional' and put them back in the box.
It took some time and a LOT of dollars to figure
out why some of our VOR/LOC/GS and thermocouple
signal systems were failing. It then took more time
and dollars to figure out how many airplanes went
to the field with "tested and blessed" relays
installed in small signal locations.
This exercise prompted a change to call out
separate part numbers for the low-level and
high-level systems. All because it was less
expensive and more reliable to control the
configuration by part number than to educate every
individual in the supply chain as to the unique
features of the relay with an expectation that
they would leave them alone.
Bob . . .
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