Post by thread:[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. THANKS! Harold -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available! Not sent from an iPhone.

On 14 May 2012 07:40, Harold Hallikainen <spam_OUTharoldTakeThisOuThallikainen.org> wrote: > > 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. They note > 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 http://www.mccartyinc.com/forums/viewtopic.php?t=377 _______________ IEEE paper - liable to be relevant Includes: ... 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 Abstract http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=22342&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel2%2F769%2F892%2F00022342.pdf%3Farnumber%3D22342 ____________________________ 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." http://dics.voicecontrol.ro/process_mails/arata_discutia/43790/:_Issues_using_high_power_contacts_for_low-current_signals..html ______________________ Russell McMahon

Thanks! Harold {Quote hidden}> On 14 May 2012 07:40, Harold Hallikainen <.....haroldKILLspam@spam@hallikainen.org> wrote: >> >> 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. > They note > >> 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 > > http://www.mccartyinc.com/forums/viewtopic.php?t=377 > > _______________ > > IEEE paper - liable to be relevant > Includes: > > ... 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 > > Abstract > http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=22342&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel2%2F769%2F892%2F00022342.pdf%3Farnumber%3D22342 > > ____________________________ > > 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." > > http://dics.voicecontrol.ro/process_mails/arata_discutia/43790/:_Issues_using_high_power_contacts_for_low-current_signals..html > > ______________________ > > > > Russell McMahon > >