Making switch contacts for Kelvin-Varley project. - Page 1
Forum section: electronics / projects

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 02, 2021, 09:16:22 pm
content: I have collected a bunch of stuff about metrology and Kelvin-Varley dividers.Now I've gotten a wild idea of making such a divider. I'm thinking of 5 decades or so.I'm aiming for a combination of both a reasonably low price and a decent qualitly.Good quality switches tend to be not very cheap, and fiddling with all those switches is a bit old fashioned too. Binary dividers needs a shitload of relays, which also drives up the price.There is no use in rock-bottom pricing, I'm thinking of using RN55 resistors which are approx 50ct each so including some spares that would be EUR40 of resistors.Decent rotary switches can also easily cost around EUR10 to EUR20 a piece (Goes all the way up to EUR60).Therefore I was thinking of using stepper motors. I don't like the cheap YN28BlaBla things much. But decent Nema 17 (smaller is more expensive) start at around EUR7.5I already designed a PCB with a circular array of reed switches, and bought 200 reed switches for cheap, but I am concerned about the quality of 10ct reed switches. (A single stage needs 20 of them)Another possibility is to design a PCB with brass M4 bolts, and use the bolt heads as switches. But I have not thought about the other side of the the switch. One possibility is to use a brass brush with lots of wires, which can also be used to clean the contacts by rotating the stepper motors a few times (back and forth to prevent wires from getting entangled). Another possibility is to use a stack of 4 o 6 brass strips as contacts.Brass oxidizes, and some silicone based grease can keep the air away, or maybe petroleum jelly or dunk the whole thing in a bath of oil.At the moment I'm just thinking about ideas to good switches with low contact resistance at home.So a simple estimation of costs would be around EUR20 per decade (inclusive motor and resistors) and some EUR30 for a 3D printer PCB with a bunch of steppermotor drivers. It also needs a box, so around EUR150 total in parts.With stepper motors It would be relatively easy to do automated measurements, such as measuring linearity of ADC's or DAC's. But maybe the whole Idea of using Kelvin-Varley is obsolete. All "modern" (After about mid '80-ies or so) voltage standards seem to use a PWM based approach. More about that in another thread...This other Idea of using PWM is more in line with a modern approach and may overall be better and certainly also cheaper:https://www.eevblog.com/forum/projects/making-a-pwm-based-voltage-standard/

author: magic
author_rank: Super Contributor
timestamp: September 03, 2021, 07:13:18 am
content: Quote from: Doctorandus_P on September 02, 2021, 09:16:22 pmAre they any good, though? I think you could buy better guaranteed TCR for that price. And their stress stability ratings are only 0.25%, but that's presumably for 175°C and those MIL tests may be harder than usual, so it's apples to orangesA certain alternative, not sure how attractive, is an R-2R divider. With 11 resistors you can have one KVD decade or a little over 3 bits of R-2R, where each bit has the same worst case error but the latter ones are attenuated /2, /4, ... so the total error from those bits is 1.5~2x of the KVD decade. Switching is trivial - a bunch of SPDT.Things get more tricky when it comes to calibration. It seems that simple Wheatstone bridge methods could be used if the switches are tristate, but then they need to have low leakage in the open position and nobody seems to rate commodity switches for better than 1GΩ isolation.editThose R-2R dividers may not be that bad. As further bits are flipped to make more error, the division ratio at the MSB moves away from 50% and the MSB's contribution decreases. I'm not sure what the worst case is, but I managed to produce slightly more error than a KVD in a 3 bit ladder with alternating bit pattern. Anyone with a worse example?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 04, 2021, 06:51:02 pm
content: Just as with Kelvin Varley, the problem is not with the resistors, but with the switches.Good relays with low contact resistance are not exactly cheap, and you also need quite a lot of them.A Nema 17 stepper motor costs EUR7.5 and can replace 4 relays.When making a custom switch with a relay, you can also design it to your specifications. For K-V for example you want 10 positions, but 12 contacts, and the "counter contacts" are spaced two apart. Apparently such switches were produced in relative big numbers some 30+ years ago, and later implementations used more common 12 position switches with two decks.I'm also toying with the idea to build less decades. If I build only three decades, I still have 1000 steps, but if calibrated to 5 digits it can still be used to characterize the linearity of aDC's and DAC's.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 04, 2021, 07:53:23 pm
content: The switch is such a problem for people that back when I wrote about the things, I chose to use big pin headers and 3-contact shells with the center one empty. Low resistance and dirt cheap, but ugly! It would take very good relay contacts and lots of 'em to do relays. IMO, the cost would be a killer. I've managed to find nice surplus rotary switches over the years, but never when I needed them.

author: magic
author_rank: Super Contributor
timestamp: September 04, 2021, 08:03:20 pm
content: Well, if you want an automated KVD, I could imagine a strip of PTFE with a row of contacts and some sort of two-wheeled "cart" riding it, moved by a linear actuator. The details of leakage and insulation may need some consideration.The thing about R-2R DAC is that for ordinary operation it only needs one SPDT per bit and the requirements are modest: low and/or equal and repeatable contact resistance both ways. It could even be MOSFETs. The only high impedance nodes is the internal chain of 1R resistors, which could be suspended in the air. Problems start when the switches need to be disconnected for calibration; I'm not sure if R-2R can be calibrated with all switches in circuit.And there are other issues, like more variable power dissipation than KVD. So it's not just the switches

author: Badwolf
author_rank: Contributor
timestamp: September 04, 2021, 10:24:11 pm
content: I recently had the same idea (which I haven't put into practice yet), but rather than buying new components, I turned to second-hand ones, more specifically old differential voltmeters. There are many models from Fluke. Some have good quality KVDs (switches and resistors), as well as null-meters and voltage references (you have to look at the diagrams in the service manuals)You have to play with the soldering iron a bit, but if you find them at a good price they are a good source of spare parts.

author: PA4TIM
author_rank: Super Contributor
timestamp: September 16, 2021, 02:10:19 pm
content: A KV divider is made to calibrate a voltage sources against a voltage standard. The output resistance is to high to use it for multimeter calibration. I have two of those. A Fluke and an ESI. I use them to check calibrators after I repaired them. When I got the Fluke it was dead. The plastic that holds the decks apart and some plates that hold the axis were disintegrated due to age. I made new ones to repair the switches. The ESI was ex militairy and also almost unused.When I got them many years ago, I was new to calibration and it took a while to understand why I could not use it on most DMMs. (A dmm with a >10G input resistance is no problem but 10M meters all seem to read to low.I also have a 10V standard with several outputs. I thought it was unstable for the same reason, a 10G 7,5 digit meter showed 10V while a 10M DMM showed a much low voltage. The function of that standard is what you call a transfer standard. With the help of a KV divider you compare it with a higher standard like in those days, standard cells.You put a voltage from a calibrator, let say 100V in the one input of a KVD, dial it to 10V. So the output of the KVD now shows 10V. Now you want to calibrate an other calibrator, lets say 10V. You switch/connect a null meter in between (they got down to 1 uV full scale) and then tweak the KV until the meter is zero. The KVD now shows the exact voltage of the 10V calibrator.They are very hard to make, not only the components but also the construction. After repair of the 720 I needed a lot of time to get the isolation resistance high enough (1x10^12 ohm, or was it 14th ? measured at 1000V) ) The Fluke is very complex because it can do self-calibration, the ESI is much more simple, however the resistors use in KVD's are extreme expensive on both. The switches used are extreme low leakage and resistance.And a KVD is useless without a calibrated voltage standard. So instead of one expensive DMM calibrator you now need an even more expensive calibrated voltage standard, an expensive KVD and a null meter with enough resolution (and then there is the whole cable compensation problem, guarding and shielding and climate of your lab)If you just need something to test DMMs for cheap then buy a new fresh decent DMM, even the cheap ones (like 50 to 100 euro) are nowadays new spot on. My Brymen 869 is now after 2 years daily use still spot on. Even my 10 euro Aneng (I never use) reads the same as when I bought it. I bought a new K2000 like 10 years ago, it still is spot on. Even my ancient K199 is still well within specs.Then use that to calibrate some resistors. Build a few standards in a fix voltage using the best affortable Vrefs you can buy and measure them with your new meter. If you use the Vrefs in their default value you only need a zero drift opamp as a 1x buffer. This saves you expensive and unstable resistors. It does not matter if it is 10,12345 V instead of 10.0000V. As long as you know and note that voltage. It will be a lot more stable then one Vref, all kind of switches and resistors to get several nice round values from 1 Vref. As a test I used TL431's, they are good enough to check 3,5 digit DMMs. I got it stable up to 10's of uV. On a Dutch forum someone experimented with a 7805 and that was pretty usable as long as the room did not change temperature or humidity.Do not forget, the calibrator must be better as your DMMs but there is no use in making it much better. If your DMM reads 10 uV resolution but has a terrible stability for temperature or like very cheap ones, battery voltage, you can try to calibrated it to 10 uV but that is pretty useless because it will read different every calibration depending things like climate.To really calibrate a DMM you need a DC and an AC calibrator for voltage and current And some resistors. Resistors are not very expensive anymore. But those that are stable over time, low voltco and low tempco are very cheap compared to 30 years ago, but it can still cost a lot. For 10 to 30 euros you can find pretty usable resistors and if it is for hand DMM checks you can get pretty usable ones under 5 euro a piece (Not talking about unknown unspecified aliexpress trash) But not every resistor is stable over time.It is nice to experiment and build things like standards, you will learn a lot, get frustrated even more, and loose a lot of cash or loose your mind,to come to the conclusion that it is pretty useless if you want to check the average hand DMMs. They are nowadays a lot more spot-on and stable as 40 years ago when they were stuffed with adjustment points. Now it is for most some DMM IC with build in reference that is "software" calibrated.

author: enut11
author_rank: Super Contributor
timestamp: September 26, 2021, 08:01:12 pm
content: Hi @doctorandus_pHave a look at this old Fluke 803 voltmeter, I believe available in Holland.https://www.ebay.com/itm/174466929144?hash=item289f0a11f8:g:XZsAAOSwB~BffFztOn the right hand side of the 803 is a 5 dial KVD made with quality switches and precision WW Fluke resistors.The box also has lots of other very useful parts including a genuine standard cell voltage reference!The KVD is under the green plastic cover in my photo below.

author: enut11
author_rank: Super Contributor
timestamp: September 26, 2021, 08:16:12 pm
content: There is a price to pay for this very easy KVD, apart from the initial purchase price. The first dial is only 5 position resulting in 20% steps. A way around this for me was to switch in a precision 200K resistor on the input to give 10% steps, still a compromise but useful. The 200K was made from those green precision Fluke resistors.Attached is my home-made 'KVD' using Fluke 803 parts in a custom wooden box.enut11