Hi I am working on a design where we are monitoring a relay, if it has been energized. That is done with high ohmic resistor and a source voltage, detecting a current when drawn But, contact wetting could be a problem, so have added a small capacitor on the contacts, so when they are closed a relatively high current runs for a short period of time to punch through the oxide layer of the contacts I am having difficulty finding any information on the subject, how much current is needed for punch through and how much time. I need a general input, since we do not know if the relay has gold plated contacts or not Anyone got a clue? Regards Klaus
Wetting current
Started by ●June 17, 2021
Reply by ●June 17, 20212021-06-17
On Fri, 18 Jun 2021 00:56:55 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>Hi > >I am working on a design where we are monitoring a relay, if it has been >energized. That is done with high ohmic resistor and a source voltage, >detecting a current when drawn > >But, contact wetting could be a problem, so have added a small capacitor >on the contacts, so when they are closed a relatively high current runs >for a short period of time to punch through the oxide layer of the contacts > >I am having difficulty finding any information on the subject, how much >current is needed for punch through and how much time. > >I need a general input, since we do not know if the relay has gold >plated contacts or not > >Anyone got a clue?There are books written on electrical contacts and their design for various conditions. Unless the current is very small gold tends to weld. Rhodium is better in such cases. Joe Gwinn
Reply by ●June 17, 20212021-06-17
On Fri, 18 Jun 2021 00:56:55 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>Hi > >I am working on a design where we are monitoring a relay, if it has been >energized. That is done with high ohmic resistor and a source voltage, >detecting a current when drawn > >But, contact wetting could be a problem, so have added a small capacitor >on the contacts, so when they are closed a relatively high current runs >for a short period of time to punch through the oxide layer of the contacts > >I am having difficulty finding any information on the subject, how much >current is needed for punch through and how much time. > >I need a general input, since we do not know if the relay has gold >plated contacts or not > >Anyone got a clue? > >Regards > >KlausWe have a similar issue now. We will sell a general-purpose relay module as part of a modular test system, and we are concerned that once a relay is used to switch power, it may not be good for signals any more. That may be urban legend. It would be awkward to test.
Reply by ●June 18, 20212021-06-18
On Fri, 18 Jun 2021 00:56:55 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>Hi > >I am working on a design where we are monitoring a relay, if it has been >energized. That is done with high ohmic resistor and a source voltage, >detecting a current when drawn > >But, contact wetting could be a problem, so have added a small capacitor >on the contacts, so when they are closed a relatively high current runs >for a short period of time to punch through the oxide layer of the contacts > >I am having difficulty finding any information on the subject, how much >current is needed for punch through and how much time. > >I need a general input, since we do not know if the relay has gold >plated contacts or not > >Anyone got a clue? > >Regards > >KlausAren't you actually interested in the existence of practical contact? Any voltage detected indicates no contact, ie failure, even if it's supposed to be closed. Ensuring contact shouldn't be the job of the contact monitor - it's the job of the application designer. Your measuring method shouldn't degrade the degree of isolation expected when the contacts are open, however. So high z sensor with lots of common-mode range. RL
Reply by ●June 18, 20212021-06-18
On 17/06/2021 23:56, Klaus Vestergaard Kragelund wrote:> > But, contact wetting could be a problem, so have added a small capacitor > on the contacts, so when they are closed a relatively high current runs > for a short period of time to punch through the oxide layer of the contacts >Do not wire the capacitor directly across the contact - add a series resistor else the peak current could weld delicate contacts. Even tens of pF capacitance can weld reeds. 10-50mA peak should be safe. Too small a test current could also be problematic. Keep the DC current above 100uA. also too low an open circuit voltage can be insufficient to break down oxides, should be above 3-5V. Min switching of 5V 100uA are figures often quoted in switch/relay datasheets. Old time telegraph/telephone books from 19th cent-early 20th often had information on wetting as folk were just discovering this stuff then. piglet
Reply by ●June 18, 20212021-06-18
On 18/06/2021 01:46, John Larkin wrote:> On Fri, 18 Jun 2021 00:56:55 +0200, Klaus Vestergaard Kragelund > <klauskvik@hotmail.com> wrote: > >> Hi >> >> I am working on a design where we are monitoring a relay, if it has been >> energized. That is done with high ohmic resistor and a source voltage, >> detecting a current when drawn >> >> But, contact wetting could be a problem, so have added a small capacitor >> on the contacts, so when they are closed a relatively high current runs >> for a short period of time to punch through the oxide layer of the contacts >> >> I am having difficulty finding any information on the subject, how much >> current is needed for punch through and how much time. >> >> I need a general input, since we do not know if the relay has gold >> plated contacts or not >> >> Anyone got a clue? >> >> Regards >> >> Klaus > > We have a similar issue now. We will sell a general-purpose relay > module as part of a modular test system, and we are concerned that > once a relay is used to switch power, it may not be good for signals > any more. > > That may be urban legend. It would be awkward to test. > > >Definitely not an urban legend. I have read a warning about gold plated contacts in some manufacturer's datasheet, possibly Tyco? piglet
Reply by ●June 18, 20212021-06-18
piglet <erichpwagner@hotmail.com> wrote:> On 17/06/2021 23:56, Klaus Vestergaard Kragelund wrote: >> >> But, contact wetting could be a problem, so have added a small >> capacitor on the contacts, so when they are closed a relatively high >> current runs for a short period of time to punch through the oxide >> layer of the contacts >> > > Do not wire the capacitor directly across the contact - add a series > resistor else the peak current could weld delicate contacts. Even tens > of pF capacitance can weld reeds. 10-50mA peak should be safe. > > Too small a test current could also be problematic. Keep the DC current > above 100uA. also too low an open circuit voltage can be insufficient to > break down oxides, should be above 3-5V. Min switching of 5V 100uA are > figures often quoted in switch/relay datasheets. > > Old time telegraph/telephone books from 19th cent-early 20th often had > information on wetting as folk were just discovering this stuff then. > > pigletI wonder if mercury-wetted contacts are still available? Are reed relays available in nitrogen-filled enclodures? This might help avoid contamination and oxidation of the contacts. -- The best designs occur in the theta state. - sw
Reply by ●June 18, 20212021-06-18
On 18/06/2021 12:17, Steve Wilson wrote:> piglet <erichpwagner@hotmail.com> wrote: > >> On 17/06/2021 23:56, Klaus Vestergaard Kragelund wrote: >>> >>> But, contact wetting could be a problem, so have added a small >>> capacitor on the contacts, so when they are closed a relatively high >>> current runs for a short period of time to punch through the oxide >>> layer of the contacts >>> >> >> Do not wire the capacitor directly across the contact - add a series >> resistor else the peak current could weld delicate contacts. Even tens >> of pF capacitance can weld reeds. 10-50mA peak should be safe. >> >> Too small a test current could also be problematic. Keep the DC current >> above 100uA. also too low an open circuit voltage can be insufficient to >> break down oxides, should be above 3-5V. Min switching of 5V 100uA are >> figures often quoted in switch/relay datasheets. >> >> Old time telegraph/telephone books from 19th cent-early 20th often had >> information on wetting as folk were just discovering this stuff then. >> >> piglet > > I wonder if mercury-wetted contacts are still available? > > Are reed relays available in nitrogen-filled enclodures? This might help > avoid contamination and oxidation of the contacts.I'm pretty sure all reeds are either inert gas filled or for HV types, evacuated, so it would be a good solution. -- Cheers Clive
Reply by ●June 18, 20212021-06-18
On Fri, 18 Jun 2021 06:38:55 +0100, piglet <erichpwagner@hotmail.com> wrote:>On 18/06/2021 01:46, John Larkin wrote: >> On Fri, 18 Jun 2021 00:56:55 +0200, Klaus Vestergaard Kragelund >> <klauskvik@hotmail.com> wrote: >> >>> Hi >>> >>> I am working on a design where we are monitoring a relay, if it has been >>> energized. That is done with high ohmic resistor and a source voltage, >>> detecting a current when drawn >>> >>> But, contact wetting could be a problem, so have added a small capacitor >>> on the contacts, so when they are closed a relatively high current runs >>> for a short period of time to punch through the oxide layer of the contacts >>> >>> I am having difficulty finding any information on the subject, how much >>> current is needed for punch through and how much time. >>> >>> I need a general input, since we do not know if the relay has gold >>> plated contacts or not >>> >>> Anyone got a clue? >>> >>> Regards >>> >>> Klaus >> >> We have a similar issue now. We will sell a general-purpose relay >> module as part of a modular test system, and we are concerned that >> once a relay is used to switch power, it may not be good for signals >> any more. >> >> That may be urban legend. It would be awkward to test. >> >> >> > >Definitely not an urban legend. I have read a warning about gold plated >contacts in some manufacturer's datasheet, possibly Tyco? > >pigletGot a link to a data sheet or appnote? It would be nice to find a general-purpose relay contact material. Maybe not down to thermocouples, but amps to mA maybe. -- John Larkin Highland Technology, Inc The best designs are necessarily accidental.
Reply by ●June 18, 20212021-06-18
On Fri, 18 Jun 2021 06:36:05 +0100, piglet <erichpwagner@hotmail.com> wrote:>On 17/06/2021 23:56, Klaus Vestergaard Kragelund wrote: >> >> But, contact wetting could be a problem, so have added a small capacitor >> on the contacts, so when they are closed a relatively high current runs >> for a short period of time to punch through the oxide layer of the contacts >> > >Do not wire the capacitor directly across the contact - add a series >resistor else the peak current could weld delicate contacts. Even tens >of pF capacitance can weld reeds. 10-50mA peak should be safe.Reed relays can weld themselves with zero contact current. They are expensive and flakey.> >Too small a test current could also be problematic. Keep the DC current >above 100uA. also too low an open circuit voltage can be insufficient to >break down oxides, should be above 3-5V. Min switching of 5V 100uA are >figures often quoted in switch/relay datasheets. > >Old time telegraph/telephone books from 19th cent-early 20th often had >information on wetting as folk were just discovering this stuff then. > >piglet-- John Larkin Highland Technology, Inc The best designs are necessarily accidental.
