George these relays are typically mercury wetted and sealed in glass in a vaccum, a minimum current in practice is simply not observed until possibly end of life.
One thing to be careful of is one aspect of specsmanship, the max Ib spec on something like the 2SK170 is 3 orders of magnitude above the observed value which would make them useless in several important applications. I would not rule out some relay manu's specing a non-existant minimum current to cover some claim like 10e7 operations at some max voltage and current. In practice I have never observed a single failure at 20fA in an electrometer ATE jig.
If you search LT's site for Jim Williams ultimate settling time tester you will find a reed switch pulser, also TEK has a 7A13 tester again a reed switch.
Last edited:
What do you mean by attaching this datasheet, minimum switched current/voltage indicated are 10uA/100mV
Thanks for that, Scott. I appreciate that the mercury wetted seem to be optimum - years ago when I was looking at this seriously, all the information pointed strongly to these units ... except, the big question mark was whether they were going to be completely phased out, because of environmental concerns. I don't know what the latest thinking is ... if they are essential in some equipment still I can't see them disappearing ...
The ampacity tables for conductors is usually based on the temperature that the insulation will allow. A single guage wire may have a 60, 75, 90, 120, or even 200 C temperature insulation, and the ampacity will be determined by that insulation.
If you do not worry about exceeding the insulation's rating, the current the wire will carry is significantly higher.
jn
Eeh, an example of contactresistance well under 50mΩ after 10e8 operations, by a relay design of 30 years ago (SDS R series)
That is true, bare #30 has a 10sec fusing current of 10A. In a pinch I have used a loop of #12 wire as a tip in my soldering gun.
Vladimir I think you are mis-reading this spec. They are using these values to make an MBTF graph (see graph and conditions) that they can guarantee, as I said above these are by no means some limit below which there is some lack of function.
Yes Scott
Mercury wetted contacts may not have the smallest (overall) contact resistance possible but the one they have they retain for billions operations.
Their consistent repeatability and overall reliability (provided they operate below the specified V limit) is their forte and they achieve this without great manufacturing cost.
With small signal sealed reed relays, mercury bridges the two contact surfaces, wetting achieved through capillary action.
Thus the effective contact area is every time equal to and a bit larger than the physical contact area, the Achilles heel of all the temporary solid metal electrical contacts, with no alien film formation, microcontacts ect.
I’ve read Jim Williams article (2ns rise time if memory serves)
Oh, thanks jn.
I’ve asked Ed to report the temperature the wire reached at 5A steady state current.
Scott, this copper wire is years on my Weller 100W soldering gun but I use it infrequently. The original loop is too expensive.
George
Last edited:
Could be, Scott, but I am not shure, there are many controvercies. Anyway, even if a contact passes-through some averaged current (integrated charge) at pA level, this says nothing about possible distortion of low-level analog signal. Wet reed relays have nothing good among their constituents (highly-magneetic leeds, screened by magnetic metal), and subjective sound effect of any relay is nothing of good. Moreover, contact issues are not only relay contacts, we have also switches and connectors.
My question would be what is the distortion mechanism of switches and contacts. Leaving out capacitance and inductance in any of these contact points what do we have besides resistance? What affect does a resistance have in these circuits, how does this distort the signal besides an attenuation?
Vlad you are correct. As one of the designers of audio equipment using multiple relays (too many) there is little to say to promote them over a silver-on-silver switch, or nothing at all. We have become so used to remote control, that we HAVE to use relays or their inferior equivalent. They are NOT perfect devices AND we can hear them. A real study of relay behavior shows that they make a relatively lousy contact at low currents. Everybody, study HOLM's textbooks and the AMP-Whitley series on contact problems, then something useful can come of this discussion.
Of course, IF you have to use a relay, get the best you can, sealed, gold over silver contacts, and specifically designed for the CURRENT LEVEL that you are using it for. Of course, over 2A, gold will blow away and other relay materials become more practical.
Kindhornman, you don't have to have diodes to have a CHANGE in resistance with different current or voltage, but they are there as well.
Of course, IF you have to use a relay, get the best you can, sealed, gold over silver contacts, and specifically designed for the CURRENT LEVEL that you are using it for. Of course, over 2A, gold will blow away and other relay materials become more practical.
Kindhornman, you don't have to have diodes to have a CHANGE in resistance with different current or voltage, but they are there as well.
Last edited:
They are heading toward reliably working down to Attoampere range.
Is it cold there jn ?
What actually is 1 ampere?
George
Is it cold there jn ?
What actually is 1 ampere?
George
I think Scott makes a very valid point on the relays specifications for max current and number of operations vs low level. I use the Panasonic ASX because they are guaranteed for low level operation but in my latest phono design have eliminated them for input switching because no relay is best.
In the past I have worked with RF switching for military equipment and the relays switched as little as .3uV into 50 Ohms (6nA). The relays were not specified to do this but they did although with the occasional failure(open). We probably wouldn't use relays today though.
In the past I have worked with RF switching for military equipment and the relays switched as little as .3uV into 50 Ohms (6nA). The relays were not specified to do this but they did although with the occasional failure(open). We probably wouldn't use relays today though.
My application was an electrometer op-amp in a gain of 100 with a 1e12 Ohm resistor in the non-inverting input with a parallel relay to alternately sample offset with and without the bias current contribution. This would be 20fA and 0.02V with no recorded failures in 20 stations over several years of service. This is in the days when 100% guaranteed warmed up specs really were.
John,
Thanks for answering my question. At the same time I am still wondering if what we are talking about would be considered audio distortion or just attenuation of a signal, or would the lower current capacity of an oxidized contact actually cause a sonic problem? I guess as I am thinking about this I know what a dirty pot sounds like and I have heard noise from a dirty switch, but what is the solution besides using the best quality parts and hopefully just cycling the switch to clean the contact or dosing the contacts in contact cleaner when need be?
Thanks for answering my question. At the same time I am still wondering if what we are talking about would be considered audio distortion or just attenuation of a signal, or would the lower current capacity of an oxidized contact actually cause a sonic problem? I guess as I am thinking about this I know what a dirty pot sounds like and I have heard noise from a dirty switch, but what is the solution besides using the best quality parts and hopefully just cycling the switch to clean the contact or dosing the contacts in contact cleaner when need be?
The relay problem is difficult. Using the CORRECT relay is first priority. Read the small print. Learn the guidelines for relay contact selection. It goes the same for switches, BUT switches can be self wiping much easier. I would avoid mercury switches as pointed out by Vlad a few pages ago.
He also does (or used to) provide the nV front end module to Keithley for their nV meters.
Really that's interesting, I almost got a customer to give me one to reverse. Same story, big piece of medical instrumentation that needs this module inserted. In their case it was clumsy and expensive and they wanted to replace it.
I find some of their application suggestions a little lacking in the details like going from experiment at 4K to instrument at room temp with simple dotted line between the environments.
- Status
- Not open for further replies.