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'[EE] switch wetting current'
|This is a follow-up to a previous email with additional information. I'm
still looking for more info on extremely low currents through switches.
I'm aware of the minimum current requirement on most switches to get a
reliable closure. Is anyone aware of any documentation indicating whether
a switch, once closed and conducting, needs this current to stay at a low
resistance (not repeated open and close, just close once and stay there
for years). If it is initially at a low resistance, will it stay there?
I'm thinking of the similarity of the switch to a telephone line splice,
which requires a "sealing current" to remain reliable even though the
splice is not opened and closed.
So, is anyone aware of any documentation on minimum switch current for a
switch that closes (properly), then stays closed for years? If resistance
goes up with time, but the sensing current is still low, perhaps the
switch will still be detected as closed.
The best documentation I've found so far is
http://relays.te.com/schrack/pdf/C0_v4bg_4.pdf . It describes the action
of fritting voltage and current. It does not specifically mention sealing
or wetting current. There's an interesting graph showing contact
resistance of constantly closed contacts where the resistance of 99.9% of
the contacts are 10 ohms or less after 12 months.
In our application, the switch manufacturer rates the switch for something
like 50uA to 10mA. It's interesting that the rating on the switch with
gold contacts has the same minimum current rating while I'd expect it to
be lower. Because this is battery operated about 50% of the time, we need
to keep the current down to 50nA or so. We're considering running a mA or
so when line power is available as a wetting current. I'm wondering if the
added complexity is worth the effort. But, Bellcore research on telephone
cable splices indicates that a "sealing current" maintains the low
resistance of cable splices, which could be considered similar to
constantly closed switch contacts.
While I value opinions, documentation from switch manufacturers or similar
would be even better.
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On 14 May 2012 07:40, Harold Hallikainen <hallikainen.org> wrote: harold
> This is a follow-up to a previous email with additional information. I'm
> still looking for more info on extremely low currents through switches.
This IEEE paper may be of interest.
> Measurements made during and after exposure show that application of 20-mA sealing current approximately limits the contact resistance to less than 21 Ù. Smaller currents allow correspondingly higher contact resistances and fluctuations.
Abstract here <http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=76535&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel1%2F33%2F2551%2F00076535.pdf%3Farnumber%3D76535>
Discussion here. 'Light weight' but suggests that occasional bursts may suffice
IEEE paper - liable to be relevant
... The authors examine the sealing current issue, and discuss the
way in which splices degrade through corrosive attack in the service
environment. They also discuss the mechanism by which sealing current
inhibits this degradation, the use of this understanding in applying
sealing current to a subscriber loop, and a low-energy sealing current
supply implementation for ISDN (integrated services digital network).
While the focus is on the application of sealing current to subscriber
loops, the discussion is equally valid in addressing special services
circuits and interoffice cables
Dave van Horn's comments from a PICList discussion with you 4 years
ago may be relevant:
" ... My first experience with this problem
was a gold-on-gold contact in a hermetically sealed switch. The
current was about 1mA, and after sitting closed for hours and hours,
the contacts would noisly go open for a few seconds, then re close
(usually) Boosting the current by 10x solved the problem, and I fed
that back to the manufacturer, who put that into later production
versions of the product."
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