On 18/06/2021 21.24, Piglet wrote:> On 18/06/2021 15:31, jlarkin@highlandsniptechnology.com wrote: >> >> Got 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. >> >> >> > > I think I saw it SDS Relais-now Panasonic or Omron but am away from my > bookshelf. I think it is also in AoE X-chapters. > > See page 4 of this from Phoenix Contact: > > <https://www.phoenixcontact.com/assets/downloads_ed/global/web_dwl_technical_info/105396_en_00.pdf> > >Nice table, but are the currents representative of wetting keepouts
Wetting current
Started by ●June 17, 2021
Reply by ●June 19, 20212021-06-19
Reply by ●June 19, 20212021-06-19
On 18/06/2021 07.36, piglet 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. >Any idea if the current could be ac? I have the possibility to inject ac current, with more than 1mA, a lot easier than DC current
Reply by ●June 19, 20212021-06-19
John Larkin <jlarkin@highland_atwork_technology.com> wrote:> On Fri, 18 Jun 2021 19:08:37 +0000 (UTC), Cydrome Leader > <presence@MUNGEpanix.com> wrote: > >>John Larkin <jlarkin@highland_atwork_technology.com> wrote: >>> On Fri, 18 Jun 2021 09:18:19 -0700 (PDT), John Walliker >>> <jrwalliker@gmail.com> wrote: >>> >>>>Pickering make a wide range of reed relays and provide a lot of advice on how to choose and use them. >>>> >>>>www.pickeringrelay.com >>>> >>>>John >>> >>> They made an RTD simulator with a heap of reed relays and resistors. >>> It was a horror. We do it electronically, which avoids all sorts of >>> interesting effects. >> >>What's some weirdness you see with reed relay logic? > > In the RTD case, horrible transient values when a value is changed. > like spiking to 500 deg C for some milliseconds, going from 48 to 49. > Engine control computers don't like that. > > Reeds have huge thermoelectrics too. Lots of intermetallic splices > with a heater roughly in the middle. > > I discovered reed twang some years ago. The reed wiggles like a struck > bell when it's energized, very complex waveforms lasting many > milliseconds. In the coil mag field, that makes biggish voltages.you can hear them ring mechanically too when you pull a magnet away from them. maybe you need some lovely mercury wetted relays.
Reply by ●June 19, 20212021-06-19
On Sat, 19 Jun 2021 21:21:56 +0000 (UTC), Cydrome Leader <presence@MUNGEpanix.com> wrote:>John Larkin <jlarkin@highland_atwork_technology.com> wrote: >> On Fri, 18 Jun 2021 19:08:37 +0000 (UTC), Cydrome Leader >> <presence@MUNGEpanix.com> wrote: >> >>>John Larkin <jlarkin@highland_atwork_technology.com> wrote: >>>> On Fri, 18 Jun 2021 09:18:19 -0700 (PDT), John Walliker >>>> <jrwalliker@gmail.com> wrote: >>>> >>>>>Pickering make a wide range of reed relays and provide a lot of advice on how to choose and use them. >>>>> >>>>>www.pickeringrelay.com >>>>> >>>>>John >>>> >>>> They made an RTD simulator with a heap of reed relays and resistors. >>>> It was a horror. We do it electronically, which avoids all sorts of >>>> interesting effects. >>> >>>What's some weirdness you see with reed relay logic? >> >> In the RTD case, horrible transient values when a value is changed. >> like spiking to 500 deg C for some milliseconds, going from 48 to 49. >> Engine control computers don't like that. >> >> Reeds have huge thermoelectrics too. Lots of intermetallic splices >> with a heater roughly in the middle. >> >> I discovered reed twang some years ago. The reed wiggles like a struck >> bell when it's energized, very complex waveforms lasting many >> milliseconds. In the coil mag field, that makes biggish voltages. > >you can hear them ring mechanically too when you pull a magnet away from >them. > >maybe you need some lovely mercury wetted relays.We use lots of Fujitsu FTR-B3G relays... almost 100K so far. DPDT, surface mount, small and reliable. We use the latching version at thermocouple levels. They are good up to 3 GHz too. Reeds are expensive junk. -- John Larkin Highland Technology, Inc trk The cork popped merrily, and Lord Peter rose to his feet. "Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply by ●June 19, 20212021-06-19
On Sunday, June 20, 2021 at 11:26:16 AM UTC+10, John Larkin wrote:> On Sat, 19 Jun 2021 21:21:56 +0000 (UTC), Cydrome Leader > <pres...@MUNGEpanix.com> wrote: > > >John Larkin <jlarkin@highland_atwork_technology.com> wrote: > >> On Fri, 18 Jun 2021 19:08:37 +0000 (UTC), Cydrome Leader > >> <pres...@MUNGEpanix.com> wrote: > >> > >>>John Larkin <jlarkin@highland_atwork_technology.com> wrote: > >>>> On Fri, 18 Jun 2021 09:18:19 -0700 (PDT), John Walliker > >>>> <jrwal...@gmail.com> wrote:<>snip>> >maybe you need some lovely mercury wetted relays.They are nice but you do have to mount them so they stray vertical. Orientation-insensitive mercury wetted reeds did get touted from time to time, but never lasted.> We use lots of Fujitsu FTR-B3G relays... almost 100K so far. > > DPDT, surface mount, small and reliable. We use the latching version at thermocouple levels. They are good up to 3 GHz too. > > Reeds are expensive junk.Of course Bell Labs develped them because they needed something more reliable than traditional relays for the telephone network. They were invented in Russia in 1922, but Bell Labs developed them into something useful in the 1930s - there's a 1941 US patent. If John Larkin doesn't like them, it probably means that he didn't use them as carefully as he should have. https://en.wikipedia.org/wiki/Reed_switch -- Bill Sloman, Sydney
Reply by ●June 20, 20212021-06-20
On 19/06/2021 19:20, Klaus Vestergaard Kragelund wrote:> On 18/06/2021 07.36, piglet 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. >> > > Any idea if the current could be ac? > > I have the possibility to inject ac current, with more than 1mA, a lot > easier than DC current >If your open circuit ac voltage is in the 5-15V range I guess that should work. The worst switch contact oxidation film I saw took about 12V before reliable contact was made. Since you don't know what contacts your circuit may be exposed to I would steer on the high side (10-15V?). I think once the current is above a few hundred micro amps you should be OK. Is there a reason why you cannot see the relay coil voltage instead? piglet
Reply by ●June 21, 20212021-06-21
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 > >KlausFor a reliable bulletproof general solution, I think it would be best to use something like 24V/10mA and pulse the current for the measurement to minimize the wasted power. You can probably get away with less.. -- Best regards, Spehro Pefhany
Reply by ●June 21, 20212021-06-21
John Larkin wrote:> In the RTD case, horrible transient values when a value is changed. > like spiking to 500 deg C for some milliseconds, going from 48 to 49. > Engine control computers don't like that.No Gray-or-compatible encoding? That's too bad. Best regards, Piotr
Reply by ●June 21, 20212021-06-21
On 20/06/2021 12.12, Piglet wrote:> On 19/06/2021 19:20, Klaus Vestergaard Kragelund wrote: >> On 18/06/2021 07.36, piglet 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. >>> >> >> Any idea if the current could be ac? >> >> I have the possibility to inject ac current, with more than 1mA, a lot >> easier than DC current >> > > If your open circuit ac voltage is in the 5-15V range I guess that > should work. The worst switch contact oxidation film I saw took about > 12V before reliable contact was made. Since you don't know what contacts > your circuit may be exposed to I would steer on the high side (10-15V?). > I think once the current is above a few hundred micro amps you should be > OK. > > Is there a reason why you cannot see the relay coil voltage instead? >Yes, the relay is in another product, so we run wires from our box to the relay contacts, to detect the state of the external product Cheers Klaus
Reply by ●June 21, 20212021-06-21
On 20/06/2021 12.12, Piglet wrote:> On 19/06/2021 19:20, Klaus Vestergaard Kragelund wrote: >> On 18/06/2021 07.36, piglet 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. >>> >> >> Any idea if the current could be ac? >> >> I have the possibility to inject ac current, with more than 1mA, a lot >> easier than DC current >> > > If your open circuit ac voltage is in the 5-15V range I guess that > should work. The worst switch contact oxidation film I saw took about > 12V before reliable contact was made. Since you don't know what contacts > your circuit may be exposed to I would steer on the high side (10-15V?). > I think once the current is above a few hundred micro amps you should be > OK.The solution we have now is a capacitor charged to a small voltage. Then when the relay closes, high current runs to discharge said capacitor In this case the voltage is low, but peak current is high (for less than 100ns) Cheers Klaus
