Harold, They NEY Contact manual is available here. (1) Warning: they require you to submit personal information before download. Although it may be possible to circumvent. (1) https://www.deringerney.com/publications/ney_electrical_contact_manual/ It is an interesting read. On Friday, June 7, 2019, Harold Hallikainen wrote: > Can you supply a link to NEY Contact Manual: Electrical Contacts for Low > Energy Uses"? I can't find a copy. > > Below are my notes on dry switching. This is pulled from our Engineering > Wiki, so links have been lost, but I hope content is useful. > > In one of our applications, we have a mercury cell driving a tamper > protect circuit (holding private keys) and tamper switches. The total > current draw is 13 uA, so the tamper switches are maybe 20% of that. The > switches have a minimum current rating that can't be met with current fro= m > the mercury cell. We have a low leakage diode and series resistor to main > power (+3.3V) on each switch so a higher current is run through the switc= h > whenever main power is available. The current then backs down to a few uA > when main power is not present. > > The mercury cell was in a battery holder. The mercury cell and the batter= y > holder had nickle plated contacts. In the field, we kept losing keys with > a flag indicating we'd lost battery power. The cell and holder > manufacturer (the same company) said they'd never had contact problems. > But, we moved to a soldered in cell, and that problem went away. > > Low current switching or just maintaining a contact is tricky! > > Harold > > Dry Switching > > Tyco paper on contact systems - Mostly a discussion of relays, but applie= s > to switches also. Points of interest include: > > pdf page 4 - Contact surface contamination depends on contact material, > ambient atmosphere, temperature, and time. > pdf page 4 - Contacts are cleaned during closure by wiping, mechanical > pressure, and bounce. > pdf page 5 - Contacts can also be electrically cleaned through a process > called "fritting" where the increase in contact resistance results in ver= y > thin conducting areas between contacts and current through these > conducting areas heats the area enough to destroy the insulating film on > the contact. > > "If the layers have not been mechanically destroyed by the closing of the > contacts, or if the contacts have been closed for a long time without > conducting sufficient current, the electrical effect of fritting will > contribute towards establishing a metallic contact, despite layers on the > effective contact area. The term fritting describes the electrical > breakdown of the oxide/foreign layer when a sufficiently high voltage > (fritting voltage) is applied across a closed contact. Due to the applied > voltage and the very short distance (the thickness of the layers) between > the two potentials an extremely high electric field is generated. The low > conductive layer will break down and a small current (a few nA) is forced > through very thin channels in the layer. The resulting local high current > density heats the conducting channels up quickly, destroying the layers, > until finally (within a few ms) a metal to metal bridge is established, > electrically linking the two surfaces. The value of fritting voltage > depends on the contact material, composition and thickness of the layers, > conductivity and composition of the contact surface. Voltages in the rang= e > of only a few volts up to some hundreds of volts may be necessary for > fritting to occur." > > pdf page 7 > > "The term dry circuit describes applications with extremely low loads > (e.g. LED's) or circuits which are switched with the electrical load > having been previously disconnected, e.g. by electronic means.In these > cases the current is too low to establish an electro-thermal cleaning > effect and the voltage is below the fritting voltage. The nonconductive > oxide layers on the contact surface will not therefore be electrically > destroyed. > > The only remaining cleaning effect is the mechanical destruction of the > layers which is sometimes insufficient (e.g. low switching frequency) to > give a reliable contact or to keep the contact resistance within > specification limits. The correct choice of contact materials is critical > in such cases for reliability. > > As described above, the contact resistance increases with the formation o= f > layers on the contact surface. The thickness and speed of growth depends > on contact material, ambient atmosphere and temperature. This is > especially important when the contact resistance is tested after a > prolonged period of storage." > > fig4.9.png > > IEEE Paper - "Measurements made during and after exposure show that > application of 20-mA sealing current approximately limits the contact > resistance to less than 21 ohms. Smaller currents allow correspondingly > higher contact resistances and fluctuations." Abstract here > IEEE Paper - "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 > PICLIST discussion of dry switching: "... 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 th= e > manufacturer, who put that into later production versions of the product.= " > Full Thread > Though no documentation related to switches has been found, an > intermittent "sealing current" of a few mA is expected to keep contact > resistance low over time due to electromigration. > Contact Fretting of Electronic Connectors, IEICE TRANS. ELECTRON., > VOL.E82{C, NO.1 JANUARY 1999. > > > -- > FCC Rules Updated Daily at http://www.hallikainen.com > Not sent from an iPhone. > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 Jason White --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .