Understand Calculation a Tinsley 3333A KVD - Page 1
Forum section: electronics / general

author: bastl_r
author_rank: Regular Contributor
timestamp: July 30, 2025, 01:51:05 am
content: HiGot a measuring furniture in pretty good shape.The Tinsley 3333A is a fourstage Kelvin-Varley divider with a following Potentiometer.See schematic.The first dekade is easy to calculate.But in the following decades, my math skills are leaving me.The values on top of the resistorchains are measured by myself(R6581T).The 168.9 and 33,69 are failed attempt to calculate them.Can someone here help me to understand the calculation the 2nd, 3rd and fourth decade?Regards bastl_r

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 30, 2025, 02:34:05 am
content: Hi,Link:https://en.wikipedia.org/wiki/Kelvin%E2%80%93Varley_dividerThe individual resistors in each stage are 1/5 the value of the previous stage.So the sequence is 10k, 2K, 400, 80.There are 11 resistor in each string, one pair of resistor is connected in parallel with the next string.Regards,Jay_Diddy_B

author: Vgkid
author_rank: Super Contributor
timestamp: July 30, 2025, 02:51:26 am
content: Conrads site is good reading information. Part 3:https://conradhoffman.com/mini_metro_lab.html

author: trobbins
author_rank: Frequent Contributor
timestamp: July 30, 2025, 05:10:19 am
content: Note that there are no 'balancing' trims across each downstream divider decade, so each separate resistor in each decade string has been matched (to within the string elements, and to the string above, and below) - whereas Conrad's setup uses balancing trims.Perhaps try and confirm the separate resistor values in each divider decade - if the top decade comprises 10k parts, then as Conrad and Jay_Diddy state the other divider decade parts should be comprised of 2k, 400 and 80 ohm parts, and the last divider should be a 20 ohm slidewire. If however the slidewire measures 75.8 ohm, then the x10 decade should comprise 379 ohm parts, and the x100 decade should comprise 1k89, and x1000 comprised of 9475 ohm, and the x10000 of 47,375 ohm.Your schematic indicates some confusion, as you indicate decades are 100kohm, 10kohm... etc, whereas the front panel identifies the divider ratio (x10,000 - x1000 - x100 - x10 - x1), so it is effectively a 5 decade KVD.Here's hoping all the parts have aged gracefully - especially in the top dividers.

author: bastl_r
author_rank: Regular Contributor
timestamp: July 31, 2025, 12:44:17 am
content: HiTherefore, if you start with the highest divider, the third divider should be 2k and not 1.899k. However, I have not seen a correction resistor yet, so I will check again later.As you can see from the R6581T, the voltage divider result is correct, apart from the small error on the slider.Incidentally, the X1 divider you mentioned is the slider on which I was able to measure 75.76 ohms.Regards bastl_r

author: trobbins
author_rank: Frequent Contributor
timestamp: July 31, 2025, 04:02:29 am
content: There are parallel paths to consider when measuring individual resistors, except for the 10,000x divider string (and when the next lower divider decade is not in play).So if you measured the top resistor in the 10,000x string as 10.001k, then the 1000x second divider string should comprise 2k resistors. Measuring a 2k resistor part will have a parallel path of 9x2k+2x10k=38k, and so the part should measure 1.900k.For the 10x fourth divider, any 80 ohm resistor part would measure 76.0 ohm. The slidewire in parallel with two of the 80 ohm resistor parts of the 4th divider should similarly measure 76 ohm due to the 400 ohm string parallel path.Measuring 1.8999k is 50ppm away from 1.9k, so you are starting to get into ppm territory where the ambient temp, and meter, and ageing/drift may all play a part. Also, the KVD is a divider, so resistor values per se are not the final arbiter of accuracy, but may indicate drift amongst individual resistors in a string if you can confidently measure ppm differences.

author: bastl_r
author_rank: Regular Contributor
timestamp: August 02, 2025, 06:28:49 am
content: HiNow i made a simulation with falstad.The measured values are correct.Does someone know why falstad calculate only 1,997V on the first decade?Did they do any calculation with wireresistance? (Falstad2.jpg)Edit: Shorturl for falstad:https://tinyurl.com/262vwsu6bastl_r

author: trobbins
author_rank: Frequent Contributor
timestamp: August 02, 2025, 07:56:11 am
content: Perhaps better to provide more description with your comments. Just a few initial comments:Why are you doing a timed simulation? Even a simulation for initial conditions will need some awareness of accuracy of simulation end-point - what accuracy is set?What 'measured' values are you referring to?One sim uses a battery as voltage source, and the other sim doesn't? Are the models for voltage source and voltmeter 'ideal'?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 02, 2025, 01:28:25 pm
content: bastl_r,This is the answer given by LTspice:Have attached the LTspice file for those playing along.Jay_Diddy_BLTspice KVD.PNG(33.75 kB. 886x781 - viewed 6 times.)