AVR-Chat

Well, once again this forum is a wealth of valuable information.   You all
have brought up a problem I may have at the other end of the spectrum.  I
have a switch panel, in use now for over 5 years, that can be exposed to the
environment, ie, occasional rain or splashing (but not dunking).   I am
using sealed toggle switches, two brands, NKK series WT, and  Honeywell
1NT1.   The switches were chosen strictly for this sealed aspect and carry
some current when closed, 20 to 300 mA depending on the switch.   The NKK
contacts are rated at 10 A at 30 VDC and the 1NT at 20 A at 28 VDC, way
overkill but that's all I could find in sealed switches.

 

However, I am switching over to a uC system and now want these switches to
just supply a logic signal, 3V level.  After the responses to my original
post I checked the specs on the switches.  The NKK have parts which are
various combinations of copper and silver, but no gold.  The Honeywell data
sheet says "Honeywell MICRO SWITCH does not recommend the use of silver
cadmium oxide switch contacts in non-arcing loads. Non-arcing loads are
generally loads less than 12 volts and/or 0.5 amp. NT switches use silver
cadmium oxide contacts."   This is interesting because even my original use
of them (12V, 20 to 300 mA) does not meet this spec but nonetheless the
switches have worked fine for 5 years, and continue to do so.  Neither data
sheet specifically specifies a minimum, or wetting, current.

 

Anyway, my new use of the switches will be much less current.  As the uC
circuit is right now I am using external 35K pull-ups, a value chosen to be
roughly the same as the internal pull-ups in the Atmel uC.  This implies a
current when closed of about 86 uA.  So the problems you all mention could
now arise.  

 

So what are my options?  They seem pretty limited, perhaps non-existent.  I
have  tried looking for gold contact switches that are also sealed but came
up with zilch.   It looks like the only gold contact switches available are
for protected environments.   I would also have to have a new panel made as
the existing switches use 15/32" holes.

 

Would the fact the switches are sealed prevent residue from forming?   (The
Honeywell data sheet seems to imply the answer is no.)

 

If a problem arises with a switch, ie, it ceases to give a logic signal,
could I open the panel up and "restore" the switch by making it temporarily
make/break a current near its maximum rating?  Would the arcing break up the
residue enough to get the switch working again?

 

What about putting a capacitor across the switch, without any resistor, to
force arcing to occur every time the switch is used?

 

Does someone know a source of low current switches that are sealed and can
be exposed to the environment?   I suppose a mercury type switch would work
but I've only been able to find those as tilt switches, so couldn't be used
in a fixed panel mount.

 

Steve

 

PS.  Incidentally, during my research I found a table on the NKK website,
http://www.nkkswitches.com/pdf/electricalratings.pdf, that allows one to
calculate a rating for conditions other than specified.   The table gives a
factor of 1.25 for 12 VDC relative to a switch rated at 30 VDC.   Thus a 4
A/30 VDC switch could use up to 5 A at 12 VDC.  Not relevant here but is
relevant to my original post.

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Roy E. Burrage
Sent: Tuesday, March 18, 2008 6:23 PM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Re: switch ratings

 

It looks like this application is handled, Steven. You might also want 
to keep the following in mind for future apps:

Silver alloys are fine for "power" applications but they're severely 
limited when you get down in the mud with really low currents because 
the contacts will build up an oxide or a sulfide coating. This is why 
they depend on the small amount of arcing that will occur in a power 
circuit. At low currents, such as driving a logic input, you'll need 
gold inlay or a gold flask over silver contacts. The same holds true 
for relays. Be sure to9 look at the minimum current ratings for 
switching devices. There's also a difference between switching current 
and carrying current capacities.

Voice of experience...don't ever try to drive a logic input with the 
auxiliary contacts of a motor starter.

REB

Steven Hodge wrote:
> Thanks for all your input. Got it. I'll use the 28 VDC rating for 12 v.
> I need to make/break 3.5A so I'll have to switch (pun intended) to a
switch
> rated at 4 A at 28 VDC instead of the 3 A one I was hoping to use (for
> aesthetic compatibility with adjacent switches). Steve
>
> 
>



 



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