Best option for homebrew 1-10-100V divider - Page 1
Forum section: electronics / beginners

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 11, 2019, 02:54:39 am
content: Still waiting on odd and end Vrefs to arrive and others to burn in so I am thinking about raising the bar on a decent high voltage reference. Researching for a few hours and seen that ref50xx series can be series stacked in shunt mode to apparently an infinite voltage. I have been trolling many EE sites and am nowhere near as equipped knowledge or equipment as I probably should be for this project to be characterized properly but am asking for help from whomever will offer for advice. The plan is for me to build this contraption or a few types to send around as a club type mediocre transfer standard or just to get feedback from others with better equipment as to if this would be a feasable project for homegamers on a budget. This is a second idea building off of my original which was to have multiple 10v references divided down to 1v so I could try to get more digits resolution to characterize drift during burn-in/normal operation. The issue im running into is that I made 2 10 resistor15ppm dividers with a 20ish ppm spread according to 2wire ohm measurement 5.5digit resolution 3478a calibration questionable. So my primary concern is after a few weeks I am seing drifts most less than 5ppm on 4 different references. I don't know if it is the mutimeter/references/or resistors drifting. Now to the fun part, upon trying to get the perfect 10-1 ratio I finally realized that its almost impossible so more reading and learning to come. One of the more knowlegable gents on the forum was trying to feed an adc I believe and mentioned linearity between the resistors are more than getting perfect 1 volt in the end of the chain. From my understanding you should get a perfect 10v/ whatever your ref voltage is by measuring between each resistor than adding all voltages to get total. I do understand that much but also have read to parallel 4 close matched resistors to make it more stable/linear/offset drift, help me out on this one. Now my idea is to make either 10 jumpers or 10 position switch to tap off of each individual vref to get nominal value as well as test taps off of each resistor. My thoughts are by characterizing each vref against each resistor tap you should be able to somewhat see which componet is drifting. Now should I use bank 1, bank 2 or bank 3 decade in the schematic also would there be any benefit in padding bank 2 for even 10k. The 10k2 resistors are 0.1% 15ppm 1/2watt through hole, 505k resistors are 1% 10ppm 1/4watt smd, and the 20 turn trimmer will be standard 100ppm through hole.

author: beanflying
author_rank: Super Contributor
timestamp: February 11, 2019, 03:33:48 am
content: Do a search here for Hamon Divider and in particular read this and reread ithttp://conradhoffman.com/HamonResistor.htmlThis recent thread toohttps://www.eevblog.com/forum/metrology/resistive-divider-calibration-request/

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 11, 2019, 12:56:22 pm
content: Have read Conrads pages and others that replicated his build. I do plan on a building a hamon divider as well maybe put it on this circuit after I have more answers that im looking for. The title isnt really specific but in its current configuration I can measure/ output 1,2 3,4,5,6,7,8,9,10volts as well as 100,90,80,70,60,50,40,30,20,10volts. The 1-10 switch was just to see resistor drift vs reference drift vs all 10 refs. When I get some more knowledge on the best setup for my resistor banks I would incorporate another switch to add in refs 1 stage at a time possibly digital relay type with some help from members familiar with arduino or other mcu. That would theoretically give a range of 1V increments from 1-100 and even better resolution with more resistor bank stages. This is all just a learning experience but in ny head seems viable to be an upgrade on the poor mans kvd seeing as trimming vrefs is easier and simpler more stable than resistors. Im aiming for around 10 ppm or better not 0.1 like the high end kvd. Would this be a decent transfer for up to 6.5 digit meters because thats what the end goal is.

author: beanflying
author_rank: Super Contributor
timestamp: February 12, 2019, 12:24:06 am
content: If you have a look at the string of Resistors I have chosen for my Hamon divider you will see you can hit all of those same points too. It wouldn't be as easy to tap those points using a rotary control or relays but I will be mounting the resistors under 4mm jacks so they will be easy to find with bannana leads.Attached is a slightly corrected (doh) spreadsheet I posted in the linked thread above. Depending on which of the resistors or several of them you go across you can hit all of the 0-10 points on each ratio. If you use the 100:1 on 10V in you can also hit all 10 points between 0-1V too.You need to also look at the impedance of the chain and what it does to the 'actual' voltage measured depending on your meter as per the other thread hence I have selected the resistor values. Go to high and unless you have a GOhm or Null meter your readings will be all over the place. To low and the TempCo comes into play due to heating of the resistors. The Fluke 752A for example will not like 10MOhm meters on the higher ratios.I have a set of 0.005% (2.1 PPM/C or lower) resistors getting made by Edwin Pettis which is really overkill but it will make trimming have a much lower impact due to TempCo of the trimmers being generally horrid.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 28, 2019, 06:28:06 am
content: I was noticing the loading effects when I attached my ut61e in parallel while trying the logging software. The ut61e i beliece is 1meg and 3478a is 10meg using 1V range. I assumed that using a opamp buffer would cancel out loading effect and thought about a buffer in front of the resistor chain as well to unload the vref. Maybe that would be overkill. Currently playing with lm358 as buffer during burn in but will be going to auto-zero when I start on pcb.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2019, 04:35:25 pm
content: An output buffer could be a good idea. However the LM358 is kind of a poor choice - more like an OP07 as the minimum useful OP for the job.The usual handheld DMMs are 10 M. The HP3478 should be able to get > 1 G in the 3 V range (no 1 V range on this meter), but the 30 V range will be 10 MOhms only.The KCD part is a different part: this is for getting from the intermediate values. So like from 10 V to 2,3,4,5,6,7,8,9 V and more in between. It was important in the past, before good resolution DMMs where available.A reference, KVD and Null-meter could be used to measure voltages with good resolution and little loading. This setup was available as a differential voltmeter.For the 1:100 or 1:10 step the Hamon divider is much more suitable, as it can work with much lower accuracy parts - though with some manual adjustment before use. So a DIY Hamon divider is quite reasonable to get to some 10 ppm or even 1 ppm accuracy - a DIY KVD would be hardly better than the resistors used and if needed adjustments are a lengthy procedure. Still there are some traps for young players - the resistors should still be low TC and not so low in value to keep self heating effects small. A poor (TC 100 ppm/K) resistor is just not linear enough at the ppm level.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: March 05, 2019, 01:53:21 pm
content: Yeah I just had th lm358 readily available and wanted a quick buffer to see how horrible the uni-t meter load was. I do have quite a few higher grade parts that I just haven't opened up yet as I want to prototype on breadboard for proof of concept and still have to aquire a few more refs. I am very interested in the Hamon divider method but more for a spot check than a be all end all. I only have a questionable hp3478a to monitor and log my 8 current references which are on breadboard and nothing but home hvac to regulate temperature. Learning things the hard way using a breadboard for sure but now have a better system for measuring each ref with a jumper wire with slight pressure on small copper wire loop on the output as opposed to how I was previously measuring by moving pomona mini grabber lead to random clipped leads stuck into breadboard. While I wait for more supplies I still have been toying with the idea of instrumentation/differential op amps. After getting impatient waiting on a galvanometer to match my resistors I decided to get a handful of INA105kp,LT1050CN,and AD620ARZ. My question to the more knowledgable members is if my attached schematic would be feasable, how much tolerance is there for error supposing you are in differential configuration and one voltage is lets say 0.99993 and another is 0.99993 per resolution of 5.5digit meter. Im trying to figure error budget total using available parts ppm wise. Also attached is ongoing log of various refs I have. I guess its obvious when I figured out a more reliable method of measuring outputs.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: March 26, 2020, 12:08:39 pm
content: Im finally getting to a mock up of the hamon divider and can now understand better the impedance loading effect you mention with >1 gigohm on 3volt and below vs 10 megohmabove 3 volt. The pitfall Im having is using one meter measuring voltage on 10v range while using hamon divider on other meter in lower ranges. Now will be adding a voltage buffer to the output to the hamon divider. Should I add 2 seperate opamps for both the .5 and .1 output and would opa277 be as suitable as ltc1050 for a 5.5 digit dmm?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 26, 2020, 01:19:36 pm
content: The OPA277 is a good BJT based OP. It can be ok for more lower resistance parts, but is not very good for higher impedance (e.g. > 10 K). The bias current can add to the offsets / errors. One can compensate of this, but this takes extra time and may add noise.There is one more downside: the OPA277 only works from about 2 V above the negative supply. So it would also need a negative supply. Depending on how often the offset is adjusted, it may also take extra effort compared to an AZ OP that has a pretty much constant offset.I don't understand the extra .5 output. If this is the extra bridge arm to do the adjustment, than one may not need an extra buffer. It depends on the way the adjustment is done. The auxiliary arm can be lower impedance as it sees a constant voltage and it thus not sensitive to heating effects.