KVD decade pathing - Page 1
Forum section: electronics / general

author: jorgemef
author_rank: Regular Contributor
timestamp: December 21, 2023, 11:15:34 pm
content: Hello,I am set to build a KVD following the mini metro lab article.I already sorted and assembled first two decades with required tolerance but now I am realising I may have a problem.I selected 10K for first decade and 2k for second. Measuring a pair of first decade I get 19,9956K in average.Measuring a set of 10x2K decade I get an average of 19.9906K, so around 5 ohm less then the desired. After patthing both values need to be equal to make two equal halfs and having the decade devide the voltage by 10 equal parts.I know if value is larger I can path with parallel resistor to second decade but as is, what would be the best strategy?Start again the second decade with larger resistor then 2k? Add 2 x 2,5ohms in series to connection to the second decade? Add 1 ohm resistor to each element of the 2K decade and then path in parallel?A current devider resulting from parallel is a clean way to match the impedance but serial I guess needs to be balanced in both harms to maintain the following decade in middle voltage and not sure if any other downside I am not looking at the moment.Cheers,Jorge

author: Harfner
author_rank: Contributor
timestamp: December 22, 2023, 09:24:27 am
content: I am not sure I understand your problem. Your entire second decade in series has just little bit more resistance than 2 resistors of the first decade. That is exactly what you are aiming for!Now you need someting in the order of 10MOhm in parallel to your entire second decade to shunt it down to the exact value. As your shunting value is much higher than the ones in the second decade, the tempco of that 10Mohm is not relevant.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 22, 2023, 10:35:01 am
content: Quote from: Harfner on December 22, 2023, 09:24:27 amI think the problem is that the combined resistance of the entire 2nd decade is actuallylessthan the average combined resistance of two R in the first.So, either shunt the resistors in the first decade (will be tricky as the shunting resistor would be very large) or -- and that's the approach in commercial products afaiu -- add very small trimming resistors to each of the resistors in the first decade (or every decade, but the last/least).

author: Harfner
author_rank: Contributor
timestamp: December 22, 2023, 11:03:19 am
content: Oh, sorry, I read it the wrong way round. Then yes, adding a small resistor to each resistor of the first decade is the best option. It can be used bring the resistors of the first decade even closer and it's tc is negible.Adding 2 x 2,5ohms in series to connection to the second decade is not an option! That would completely destroy the KVD.

author: jorgemef
author_rank: Regular Contributor
timestamp: December 22, 2023, 06:58:21 pm
content: Thanks, was not thinking of that, to shunt/trim each of the resistors of first decade. Thus I can lower its arms below the 10x"2"K values.How stable in terms of TCR needs to be the pot for trimming? I have normal 15 turns pots and SMD resistors of 1%. So I need to lower each 10K around 2,5 ohms. For that would need 39Mohms resistor. Or use the largest SMD I have which is 10M plus the pots and still use the shunt on the seconde decade which I think I will need to do anyway as last step of assembly requires calibration across decades. With 10M will suffer on shunt needed for second decade in the order of 30Mohms.But if I use 1M as shunt on first decade will lower the first decade resistance around 100 ohms which will require second decade to lower 200 ohms so would need pathing second decade with 2MThat is a shame as both decades where already inside the article tolerance by two folds.Can a KVD be used as precision resistor? Asking as moving first decade away from 10K starts putting down that line as I am running away of the 100K on first decade.Thanks,Jorge

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2023, 07:50:04 pm
content: To make to decades compatable it would need added series resistors of some 5+ ohm to the 2nd decade, not the first. To high a resistance in the second decade can be trimmed with a large resistor in parallel to the whole 2nd decade.For trimming the resistors (mainly sensible in the first 1 or 2 decades) one would normally have a resistor combination, so that the trimmer has a limited effect, like 0.2% of the resistance if 0.1 % resistors are used to start with. According to the weight the TC / drift of the trimmer can be high, like 500 x for 0.2% trim range.Trim pots don't work well for large (> 100 K) or small values ( e.g. < 500), so one usually would need a mix of 2 fixed resistors and the pot. So something like 10 K and than 200 K plus a 5 K variable resistor in series. This would give about a 0.12% trim range. With better matching resistors one may get away with smaller trimmers. Depending on the trimmers available one would also adjust the fiexed resistors (e.g. some 300 K with 10 K trimmers) for the desired range.With the parallel trim one naturally gets a bit lower resistance in the 1st decade.Chances one would lower the 1st decade more than needed to match the frist decade and one may need the parallel trim for the whole 2nd decade.

author: jorgemef
author_rank: Regular Contributor
timestamp: December 22, 2023, 08:14:52 pm
content: I think I will try to use resistors of 49,9 0,1% I have around for the 5th decade and match a set of 11 and add to the 2K decade in series. I bought a set of 50 of each so hopefully will be enough to trim second decade and still have a set for 5th decade which is not so precision demanding.I am using sets of 3216 with lead wires as were the cheaper I could find in this range precision with thin film.Of course I am mathing the resistors after soldering the lead wires.This will reduce trimming amount needed and not require additional lowering the first decade.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 22, 2023, 09:24:36 pm
content: Figure 2 of the KVD article shows how I trimmed each decade. FWIW, Julie Research used short lengths of manganin wire on each resistor of the upper decade(s) to get the values matched. Must have been a PITA to build! You don't want to get in the position of having decades too low to trim to match the previous one as there's no good way to do it.

author: r6502
author_rank: Regular Contributor
timestamp: December 25, 2023, 10:08:41 am
content: Hello all,one short q:is it this article series, that we are talking about:http://www.conradhoffman.com/mini_metro_lab.htmlAnd: Merry x-mes to all.Guido

author: bastl_r
author_rank: Regular Contributor
timestamp: December 25, 2023, 03:06:53 pm
content: Quote from: r6502 on December 25, 2023, 10:08:41 amHe's the maker of the Hoffman-dividerRegards

author: RoadDog
author_rank: Regular Contributor
timestamp: December 27, 2023, 12:28:06 am
content: Quote from: Conrad Hoffman on December 22, 2023, 09:24:36 pmWhen I built 720’s I would get one from the assemblers and it took almost a whole work day to select all those little card resistors for the shunts, bridge and the A and B decades trim circuits. I could usually do that and test it against my reference divider in a single day if everything went smooth.I still want to build one of your dividers. I have piles of resistors and some parts for it, a decent LM399 10V reference and an 845. I just have so many other projects going on right now.

author: ivo
author_rank: Regular Contributor
timestamp: December 29, 2023, 02:11:17 pm
content: I have all the resistors for this design, 'only' have the busywork of selecting them nowdividers 10. 5.6 3.3 1.8 1target parallel 20 11.2 6.6 3.6 2next range total 56 33 18 10 5precise divider 31.11 16.95 10.42 5.63 3.33standard + pot |62|+.2 |33|+1 10+1 5.6+.5 3.3+.1precise divideris the parallel resistor needed to bring the next range down to double the previous',standard + potis the trim scheme used to get that,|62|is two 62k's in parallel. Planning to have a multi-turn pot for a 6th range. Values were selected between 10k to 1k to keep all values well above switch contact resistance, standard values easy to purchase, and even some of values could be re-used.

author: jorgemef
author_rank: Regular Contributor
timestamp: January 27, 2024, 03:34:32 pm
content: After months of planning and many learnings from you guys, I finished my KVD.The switchs are not so great and second decade contacts drive me crazy, but gets along.I made it with 7 decads and is not so bad for the end results. I can see microvolts climbing.I forgot it starts at 0 and not 1 so had to turn the knobs one mark further to not redo front screen.When set to all 0 has 1.1 microvolt offset.Now I need to find out what to do with it.Thanks Conrad for the inspiration.Some pictures bellow.Cheers,Jorge

author: jorgemef
author_rank: Regular Contributor
timestamp: January 27, 2024, 04:45:27 pm
content: It is repeateable, so I am happy.Now I can move on to the my next project.Hopefully will not have to calibrate it many times.The resistors on the decade have TCR between -2 and -7.5 ppm/C depending on the decade (third decade was positive TCR. Green resistor instead of black).The pathing reistors are ten times worst in terms of TCR (cheap 1% 805 SMD), but each 50 of the decade ones where around 4 euro from Ali. 0,1% resistors SMD.I pathed with one R from available 805 sample book in series with the parallel of a smaller R from the sample book and a 15 turn variable resistor as per table.BR,Jorge

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 27, 2024, 05:18:37 pm
content: Very cool! Proof that an individual can do this with fairly normal methods and materials. The switches are always a problem. In the original article I used the goofy header system because it's hard to find good affordable rotary switches, but rotary switches are absolutely the way to go. Somewhere I have a Julie calibration sheet and they checked 111111, 222222, 333333 and so on, and then a few other patterns, rather than checking all 1 or 10 million combinations.

author: donlisms
author_rank: Frequent Contributor
timestamp: January 27, 2024, 08:48:20 pm
content: You can do that? I'm still checking the 1 million combinations. Carpal Tunnel.Actually, as near as I can tell, the higher precision KVD calibration seems to be a pretty big deal with much complication, because of the interaction between the decades. The papers I have found have, like, huge quantities of algebra, and sometimes graphical correction charts, to reduce the 1 or 10 million combinations to some linear interpolation kind of thing. Still seems just a little tedious, but relatively speaking, I guess that's okay.Besides, doesn't metrology really just boil down to "close enough?" I mean, really?Congrats on The Thing, though! Nice.

author: tggzzz
author_rank: Super Contributor
timestamp: January 27, 2024, 11:44:34 pm
content: Quote from: Conrad Hoffman on January 27, 2024, 05:18:37 pmI appear to have triumphantly reinvented that technique with my 7 decade JRL KVD and Solartron 7081 in ratio mode. Even without taking precautions to make decent measurements, the worst error was 7 in the lowest decade, e.g. 5555555 being 5555562.Pleasing

author: tatel
author_rank: Frequent Contributor
timestamp: January 28, 2024, 02:24:07 am
content: Very nice box! Congratulations. Binding posts also look gorgeous.Could you give us any informations about that hardware, including the switches?I just downloaded "Online Potentiometer Handbook" and began to read it.@at old gurus: any recommendations about the trimpots? Cheap cermets are OK? Used as rheostats or as dividers?

author: guenthert
author_rank: Frequent Contributor
timestamp: January 28, 2024, 10:26:00 am
content: Quote from: jorgemef on January 27, 2024, 03:34:32 pmAwesome, well done!Quote from: jorgemef on January 27, 2024, 03:34:32 pmIt was meant to go to 11Quote from: jorgemef on January 27, 2024, 03:34:32 pmThermal EMF or residue of minuscule current over remaining resistance? Turning over the source should tell.Quote from: jorgemef on January 27, 2024, 03:34:32 pmIt's a bit hard to recognize -- what do I see in the first picture?I, for one, would like to know details about used resistors and how thermal and long-time stable the resulting divider became.

author: jorgemef
author_rank: Regular Contributor
timestamp: January 28, 2024, 11:13:49 am
content: I followed Conrad article to build Wethston Bridge. Even did the null detector which I also reused for a miliohmeter but Keythly is more stable and I can log it in windows with testcontroller.I have built some time ago a 10V reference from a LM399 chinese board which I pimped up for stable results. Also added a voltage amplifier from 0-22V which I used instead of batteries as battery voltage keep falling when used in WB and I wanted to have stable results behind the null so I could rank the resistors without worrying that the readings where due to discharging battery.I started by measuring each of the 50 resistors from the set (https://pt.aliexpress.com/item/1005003973215614.html) then picked one from the extremes(max or min R) for one of the arms of the bridge and remaining 49 to rank. I measured the variation for a set of 20 for the ones with closer resistance. Then I would rank it and choose the 11 ones closer to assemble on the rotary switch.The wb voltage detected will drift with time as the resistors "wam-up" so you need to set a time to measure as it keeps floating. If you pass that time bether to start with another one as that one will be "warm" for some time. You want all to go from same starting position(temperature) to keep your process checked. Keep the measuring process rolling to not cool down the bridge components.I took measures after 150 seconds for the average of last 100 samples (testcontroller used for Keythly2000) and after 10m. The values varied but not much. Since was a lenghty process that is why I ranked only 20. If it was now I would rank entire 50(49) with only 150s as the differences between 150s and 10m are stable. See attached charts.Unfortunately I started with the higer decade first so the learnings where used for the lowest decades only. I redone first decade with the refined process to ensure I choosed from a larger set.I stacked the resistors from the R11 with largest variation, the R10 largest from the other extreme until the R1 the one from the middle. Since it goes two by two I figured out this would result in more even devision of the voltage in each decade. This gives me concerns though for the calibration process as one of the arms of the WB as presented by Conrad article will be unballanced by the drift of larger and smaller resistance from the set of 11, but at least the pairs in the set of two dividing for the next decade will be more even.So this was the methode I used for the selection.As Conrad repeated several times in different post, R measured without load means nothing. You really need to use WB to sort out the resistances. See the 120 ohms picture I attach.Also any breeze will affect your readings. I kept the Weathston bridge inside a plastic bag with a zipper while measuring.The binding posts I used where these ones:https://pt.aliexpress.com/item/32849448782.html They seemed alright.I used this kind of rotary with 2x11:https://pt.aliexpress.com/item/1005006157667208.html. You can check the configuration from the picture in the midle of this post. You need to cross the lines from one side to the other to have the selector in between 2. Not sure if 2x10 would be ok. Dont want to set my mind on it now.From this you need to do your math with ohms law, keep the wires short as they add resistance, or if they are long keep the lengths even for all parts.Also use for a same decade the resistances from a same set so they have TCR similar due to the common fabrication process.It will take tones of time to sort out but is fun.Good luck.

author: jorgemef
author_rank: Regular Contributor
timestamp: January 28, 2024, 11:27:27 am
content: Quote from: jorgemef on January 27, 2024, 03:34:32 pmThe first two decades plus the wires to connect the decade to external voltage and null detector as weathesone bridge configuration (see Conrad's article how you calibrate the thing) with rotary set at 0 and to above two wires to the series of the pathing resistors including the variable resistance.I measured the TCR using a cheap mechanic heating plate which has some jawls I used to press the resistors. Measured normal temperature resistance, and 40ºc and 50ºc temperatures.https://pt.aliexpress.com/item/1005006053565260.htmlResults I got for TCR attached. Measured once only, forgot to measure for 24.9 but since is from same vendor figured out is in same range, and lowest decades are not so important.I expect no flutuations in the decade as all resistors from the decade came from same batch, except the fluctuations ones resulting from the pathing resistors variation. Since its contribuitions are not so large as I followed somehow the 5x rules for the next decade, maybe will be more stable. Contribuition from the pathing resistors would be larger if all decades would use same resistors like the ones in the IET labs KVD 700 manual where they use 10k 10k 4k 1k 1k 1k 1k. So pathing resistors have larger contribuition, but at least the leads length doesnt contribute so much for the decade divisions.You win some and you loose some.

author: tatel
author_rank: Frequent Contributor
timestamp: January 28, 2024, 01:37:10 pm
content: Quote from: jorgemef on January 28, 2024, 11:27:27 amI don't know what to think about that manual. Please see attachment. It looks to me as if that schematic doesn't have any decade populated with 8K resistors. Perhaps Murphy was fooling around there when they published that paper?

author: jorgemef
author_rank: Regular Contributor
timestamp: January 28, 2024, 02:54:06 pm
content: You path the previous decade 10R(n-1) to the current decade 2R(n). Maybe they are referring pathing the 10K of the fourth decade to the (4+4) 8K of third decade. Anyway you start calibrating from the last decade, and as you path the seen value for the previous decade will be the parallel with previous decade pathing (maybe will be 8k?), but you can calculate upfront the pathing network by knowing aproximately your values. Try to minimize the variable resistor quota on the pathing as is the weekest point for the thermal stability.I havent read that much the manual. I understood the concept from Conrad paper and did my own formulas in excel for the values I used/measured.But yes, larger resistors also have the benefit of being less dependant on the rotary switch contacts. But then you need to care the pathing network thermal stability more.

author: donlisms
author_rank: Frequent Contributor
timestamp: January 28, 2024, 03:58:13 pm
content: Quote from: tatel on January 28, 2024, 01:37:10 pmI think you do have to sort of get in the KVD frame of mind to follow these things. The important point is that the total resistance of the next decade is the same as two resistors in the current decade. It doesn't matter how you get there.So... if the current decade is 1k steps, the switch contacts span two of them and make a 2k resistance. The next decade must also be 2k, so that when it is placed in parallel, we get a 1k step. So far, so good.We could make the next decade from ten 200 ohm resistors. Or... from ten 400 ohm resistors, with a 4000 ohm resistor in parallel with the whole thing. Or from ten 1k resistors, with a 2.5k resistor in parallel with the whole thing. Or... a 2000 ohm pot.So then in the KVD700, if the current decade has 4k steps, the next decade must be 8k total, in some way. The 8k from two resistors in the current decade, in parallel with 8k from the next decade, give us a 4k step.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2024, 08:42:21 pm
content: There is one more downside of use rather large resistors: it increases the output impedance for the total divider.To limit the heatng effect when used with a relatively high voltage and the loading of the source, one usually wants a certain, not too low resistance for the 1 st decade (e.g. some 20 or 50 K per element).The 2nd decade than should really be lower value, like nearly the factor of 5 one would get without the parallel trim resistor. For practical reasons to allow the trim thus would be a little more than 1/5.One should use the same resistance main with the already low resistor decades. In the worst case setting (e.g. 555555) one would get an output resistance of about 2 x the resistors of all the decades added + a few times from the first decade. So one does not want many decades with a high resistance.

author: KK6IL
author_rank: Regular Contributor
timestamp: February 07, 2024, 12:49:34 pm
content: Rather than trim pots for the first decade, with 11 resistors with an average value of 10K, arrange them in a high, low, high, low, high pattern such that any 2 adjacent resistors total 20K. Of course, starting with a larger resistor pool will make a better fit.