AVR-Chat

Thanks for the tip.  I'll give them a try.  Steve

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Roy E. Burrage
Sent: Wednesday, March 19, 2008 3:43 PM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Re: switch ratings

 

Try these ones Steven:

http://www.mouser.com/catalog/633/1598.pdf

With these you can even use a less expensive, non-sealed switch so your 
cost will be a wash. However, compare the cost of one service call to 
the cost of a few seals and you may gain a whole nuther perspective on 
up front dollars. We've been using these for several years with no 
problems.

REB

Steven Hodge wrote:
> 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%40yahoogroups.com>
[mailto:AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> ] On
Behalf
> Of Roy E. Burrage
> Sent: Tuesday, March 18, 2008 6:23 PM
> To: AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.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
>>
>>
>>
>> 
>
>
>
> 
>
>
>
> [Non-text portions of this message have been removed]
>
>
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