Resistance substitution box (NOT rotary switches) - Page 1
Forum section: electronics / projects

author: ftransform
author_rank: Frequent Contributor
timestamp: January 20, 2013, 10:43:53 pm
content: So I don't recall seeing a thread about making a decade substitution box that did not use thumb wheel switches. Most people on this forum are after a combination lock style substition box, but has anyone looked into a switch box (with like 50 switches on it).http://www.ebay.com/itm/PHIPPS-BIRD-INC-RESISTANCE-SUBSTITUTION-BOX-236A-/150554899472?pt=LH_DefaultDomain_0&hash=item230dc57410It seemed that the weak link in designing a box is the switches, after all the IET labs 500$ one only used 25ppm resistors.Could using a design like this be the solution to home resistance boxes? I assume one could get good quality toggle switches much cheaper then good quality decade switches as they are less complicated and way more commonly used.Thoughts?Through hole resistors would be used with UNCUT & heatsinked leads (dual stage PCB with a seperator) in order to prevent soldering iron from heating the actual resistor and changing value.TCR of resistor can be measured before it is put into the unit.

author: ftransform
author_rank: Frequent Contributor
timestamp: January 20, 2013, 10:54:09 pm
content: A pcb sandwich design could be used to prevent resistor temperature change during soldering and to ensure that the device is free from vibrational problems.http://image.bayimg.com/58d3cb15a8bb2763b8ba0a2c2c59070dcee48ab3.jpg

author: PA4TIM
author_rank: Super Contributor
timestamp: January 20, 2013, 11:29:20 pm
content: Isolate the box for RF and thermal effects. A way to make them thermal more stable ( also caused by current through them) is something I have done once. I took a heatsink with fine ribs. The resistors had a snug fit between the ribs. That kept them all at the same temp. Mechanical stable and the heating caused by current was less.Long leads increase inductance but as long as you use it for DC or audio it does not matter.All depents on your needs. If you are worried about switch contantact resistance ect I think you try to make a transfer standard.If you just want an R-subtitution box it does not matter. You can use a potentiometer with a scale or after you ound the value measure its value.I have done that with a WW precision potentiometer from an old measurement instrument, one with a normal potentiometer and one with a 10K 10 turn pot. This as substitution.I made a box with 10 K 0.1% smd resistors soldred direct on a 20 position switch. ( from contact to contact. One wire to the first, the other wire to the wiper.Also one using a 9 position switch with 0, 10, 100, 1K, 10K, 100K, 1M, 10M, 100M for quick multimeter checks. Instead of tweaking them perfect I just measured them accurate and those values are on the cabinet.Also one in 100M steps upto 1.2GOhm for the same purposeI made a decade box with big ceramic switches and 1% resistors and I trimmed that a bit.This is how General Radio did it for many years. This one is still spot on, and it dates from 1932( besides that I have 5 ESI units ( 10, 100, 1K, 10K and 100K, each unit has 12 resistors and 4 banana busses per resistor. The resistors are all in series so 24 banana busses. So you can make other values. That is the way to go if you want optimal results, no switches and kelvin connections

author: ftransform
author_rank: Frequent Contributor
timestamp: January 21, 2013, 12:37:47 am
content: How accurate did your 0.1% ohm resistor box come out to be?

author: EEVblog
author_rank: Unknown
timestamp: January 21, 2013, 01:57:54 am
content: I've done one using DIP switches and SDM resistors in a PEN style format PCB. Don't have it any more though.Dave.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 21, 2013, 07:32:26 am
content: Quote from: ftransform on January 21, 2013, 12:37:47 amI have not calculated it. Is a lot of work to do for 20 positions. But I think I was lucky they kind a middle out. It deviates a few ohm in some positions but it stays rather close to what is should be. I only use it for quick tests on analog and 2,5 upto 4,5 digit meters and for that it is good enough. ( the 0 to 100M is in the same cabinet together with a 10K 10 turn. A switch lets me chose to put the 10 turn in series with the 10K resistors. Or real calibration I use the ESI transfer standards and the GR decade. ( that is 1% but they must had other % in the 1930's as now ;-) you need a 6,5 digit meter to see the deviation, that thing is amazing, even more considering I found it on a fleemarket tossed in a box without a cabinet.I bought a 1M 0.01% vishay thickfilm precsion resistor to repair a bridge ( it was damned expensive, something like 15-20 euro if I remember well) but i was not lucky it was excact 0.01 % down so I had to put a second resistor in series. But ths is still better then 0.01% to high.Use a good, big longnose plyers, holding the resistor pins to form a heatsinck while soldering.

author: ftransform
author_rank: Frequent Contributor
timestamp: January 22, 2013, 10:29:52 am
content: Hmm perhaps I will leave bare wire instead of switches so I can just hit it with the soldering iron!

author: ftransform
author_rank: Frequent Contributor
timestamp: January 23, 2013, 08:21:46 pm
content: Is it even worth building this type of box?Ok, so assuming you want nice gold bypass switches with a 10mOhm maximum resistance the cost is roughly 2.5-3.5 $ a unit (from mouser).So you are paying from 70-100$ for switches alone.Then you also need to order resistors.The ex-tech box goes for 110$http://www.ebay.com/itm/Extech-380400-Resistance-Decade-Box-/261118095656?pt=LH_DefaultDomain_0&hash=item3ccbd9f128The ex-tech box is specified to have a .3 ohm resistance, which corresponds to 10 miliohms times 28 switches pretty well.The only question is, do you think that the 0.3 ohm resistance stated on the ex-tech box is 0.3 ohms measured or 0.3 ohms "worst case end of life" of the switches.And of course one has to wonder how much care was put into making that box, could those resistors be near the range tolerances...The cost of the switches in this case is discouraging and I'm beginning to feel that buying the ex-tech box might be a better option.Does anyone own one of these? If i'm going to spend 110$ I feel that maybe even the old general radio boxes might be better.It seems that KVD/substitution boxes are something best left to the people that can buy thousands of parts and sell the units, as aluring as the idea is. I think my money will be better spent on ordering 100 value decade resistor sets and perhaps a general radio unit.What lab uses have you found these decade resistance boxes useful for?

author: PA4TIM
author_rank: Super Contributor
timestamp: January 23, 2013, 08:30:30 pm
content: That is a 1% box. My ancient GR decade box is that too, on paper, but in real it is much better, even after > 80 years.

author: ftransform
author_rank: Frequent Contributor
timestamp: January 23, 2013, 09:17:34 pm
content: But do you think that 1% is 1% in extech or is it more like 0.5% or better?I thought extech was generally a haphazard company

author: PA4TIM
author_rank: Super Contributor
timestamp: January 23, 2013, 09:54:10 pm
content: I do not know. Mabey the resistors are 1%, maybe the box. Maybe minimum or maximum, maybe absolute against national standards, maybe in theory because of the design, maybe because the prototype was ect. A GR was guranteed to be 1% at its worst, in the worst conditions, but in lab conditions it will be much better. Extech is not GR, not even close. One switch in that GR will be more expensive as the Extech. I needed such a resistor for my 1608, wrote to a manufacturer who still is able to made them. But that was over 100 dollar for one resistor, then Henry Hall advised me to take the vishay thickfilm precision version. There are about 50 of those WW resistors in that GRBut what about things like tempco, switch resistance and durability ect in the extech. Building your own using so much switches and buying cheap ones, and then cheap resistors and a cabinet will probably be as expensive so I would not expect it to be better then 1%.Funny, they say it is for calibration use, if you select the value and the meter reads different as the box the meter needs calibration, so if I switch it to 1M and my 7,5 digit meter reeds 1,000,100 Ohm my meter has to be calibrated based on a 1% box ? 1M and 1% will be 1M +/- 10K !!Do not forget this is a substitution box, it has nothing to do with calibration or precision. They are just handy tools to find something like a good feedback resistor. I have one that has just the most important values and two wires with crocs on them, nothing fancy or percision. I use it to find a value that comes close while testing or repairing in non critical desisngs. It had also some capacitors. If it must be more precise I use the GR, or other decade or place a potentiometer, measure it and then place a fixed one. If it really comes too it, i measure then caclulate the value. Use that value and some finetuning ( small resistor with big 10 turn parallel or some fee places to place more precision resistors parallel/serie to get the value.So what you want to use it for ?

author: Spawn
author_rank: Frequent Contributor
timestamp: January 23, 2013, 10:11:48 pm
content: ftransform, I ordered the one below in the link, I had the last box he had, seller didn’t relisted again but he is selling all electronics stuff you could mail him and see if he has something.It is 1% box but comes 51,55 USD shipped:http://www.ebay.nl/itm/160950378459?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

author: ftransform
author_rank: Frequent Contributor
timestamp: January 23, 2013, 11:45:10 pm
content: Can you measure it accurately?

author: Spawn
author_rank: Frequent Contributor
timestamp: January 23, 2013, 11:51:07 pm
content: I ordered 3 days ago, so I have to receive it first but I was planning to do some video review of it, since it was so cheap compared to others.EDIT:Look what i found:https://www.eevblog.com/forum/reviews/elenco-resistor-substitution-box-rs-500/It is also widely sold on Amazon

author: ftransform
author_rank: Frequent Contributor
timestamp: January 24, 2013, 01:16:25 am
content: Quote from: Spawn on January 23, 2013, 11:51:07 pmaww shucks no measurements