Multichannel resistor substitution box - Page 1
Forum section: electronics / general

author: sevenTech
author_rank: Contributor
timestamp: November 07, 2017, 10:35:21 am
content: Hi all! I would like to introduce one project I was working on for a pretty long time. Now in late development stage but soon I will present it on some crowd funding site. It is very special resistance substitution box (device) that can be useful for prototyping on breadboard, fast circuit board debugging and teaching. It consists of four potentiometers that can be easily replaced by the user. There are no limitations - the user can mount multi-turn pots, stereo pots, mono pots of different resistance rating. By using mono (single) potentiometers the principle is easy - when user wants to set some resistance, he just flips a switch that will connect the potentiometer to measurement circuit of the device and he is informed about set resistance via LCD display.After the value is set, you can flip the switch again and the potentiometer will be connected to the output connector. The value will be still indicated. Leads are inserted into breadboard where they behave like you inserted resistor instead. It is a matter of seconds to change its resistance by the same procedure.With stereo(dual) potentiometer you have one significant advantage. Due to the existence of the second gang(track), you can have potentiometer resistance connected to your circuit, change its resistance at the same time and still be informed continuously about resistance value that is set (device measures the resistance of the second gang(track) and after compensation you get the resistance value that is set on primary gang(track)). Compensation is necessary, because the resistance measured on second gang is not exactly value that you will measure on primary gang that is substituting resistance in your circuit (so-called gang error - dual pots arent ideal). But with calibration procedure that this device is capable, the value measured is compensated and you get value you can trust.The principle is maybe more obvious on attached picture.Additional featuresPossibility to save all resistance values to one of the 4 profiles. You can then reload the profile values and manually adjust pots so your circuit that you had connected in past will behave the same way.Thus you can use this device for several circuits at the same time and you have information on what resistance was set for the circuits before.Calibration capability for stereo pots (gang error reduction)Capability to remove and mount different potentiometers (multiturn, linear, logarithmic) with different values of maximum resistance. Information about the all potentiometers is then displayed below the displayed resistance.DFU - device firmware update with the PC and micro USB cable.Additional configuration - color schemes, test modes, factory reset etc etcAs an example I've just plugged dual op amp to my breadboard and inserted leads from the device and created relaxation oscillator. Only had to plug capacitor. It was finished after quite a short time and the possibility to change resistance easily of all three resistors in circuit is just awesome.The accuracy of measurement of resistance is also pretty good. Still improving but for now it is definitely below 1% of resistance value. Tested against calibrated HP multimeter.What is your opinion? Fill the poll please :-) Thank you!

author: bd139
author_rank: Super Contributor
timestamp: November 07, 2017, 11:05:45 am
content: Got to agree.When I need a resistance substitution box, I use a couple of trimmers and measure the value with a DMM after I've set it.

author: sevenTech
author_rank: Contributor
timestamp: November 07, 2017, 11:13:17 am
content: Hi Wilfred,thank you for your opinion. Yes - I also have these feelings sometimes...But also I can see many people around that are learning electronics and experimenting with all different types of circuits on breadboard and often it can get pretty uneffective by the need to have stockpile of all different resistor values, removing them and replacing just several times to learn your mistakes. On other side during my study on measurement class we got a box of unsorted resistance and begin told "connect this circuit and measure that and that"... These are two things that motivated me to create this beast. And yes it is very special. But on the other side, there are only one channel resistor decades that can be pretty pricey and is suited for completely different purpose - to get one very precise resistance value. So to sum it up - yes, it is niche but on the other side - there are no such things to replace several resistors in a second and have complete control on set resistance :-) Well...I'll see...I will still have to complete it just because the time it cost me

author: Muttley Snickers
author_rank: Unknown
timestamp: November 07, 2017, 11:28:35 am
content: Just like the others have stated I doubt that I would be able to justify the purchase of your device even though it looks very good, for bread board projects and prototypes I quite often use these basic resistance substitution wheels which we bought for five dollars each from Dick Smith Electronics when they were closing up shop, I have ten of these resistance wheels and will occasionally use a couple together and sometimes in conjunction with a ten turn potentiometer depending on the application. Edit: added picture below.

author: Cerebus
author_rank: Super Contributor
timestamp: November 07, 2017, 02:01:07 pm
content: I suspect that the posters who say "I never use one" don't own a substitution box and never have. Once you have one on the bench it gets more use than you might think.I'm voting "Yes definitely" as a vote for "it's a good idea" with the proviso that I wouldn't actually get one as I'm already quite well sorted in that regard.

author: Ian.M
author_rank: Super Contributor
timestamp: November 07, 2017, 02:36:34 pm
content: I've got a 5 decade switched substitution box that covers the range 1R to 1Meg with 1R resolution, and I *NEVER* use it for breadboarding. Its bulky and the parasitics are horrible. I keep a moderate selection of presets from 100R up to 1Meg, both single turn and multiturn for breadboarding with, but rarely need to pull one out and measure its value - if your circuit is that critically dependent on a resistor value that can't be pre-calculated, the design won't be reproducible due to component tolerances and is likely to have poor stability.

author: Cyberdragon
author_rank: Super Contributor
timestamp: November 07, 2017, 02:44:57 pm
content: Yes, component selectors are very useful when designing circuits, especially analog. However, a digital one? Well, yes and no. What you have, absolutely not, not when you have to manually change pots and all it does is measure resistance.It's just pots stuck to a DMM, I can do that at home. If you want a digital resistance box, you use an electronic load or a digital potiometer. Now, if you were to put several digi-pots in a box with current and voltage measurement and overload protection, that would be great.Quote from: Ian.M on November 07, 2017, 02:36:34 pmThen someone explain laser trimmed resistors.

author: sevenTech
author_rank: Contributor
timestamp: November 07, 2017, 03:28:15 pm
content: Thank you all for opinions. From some point of view I must agree. Most of you told that they either use substitution box once a year or they have the possibility to plug in pot and measure by DMM. It is definitely clear to me but the thing that most of you wont use substitution box or this pot with DMM is because it is horribly nasty solution. I imagine myself having few bulky resistor decade boxes under my bed catching dust. But I must say that with that device if I need just to make sure or try some new circuit I have it ready just after few seconds...Really...Plugging pots and measure them with DMM I would rather kill myselfThis is not thought to be substitution box you will use in special cases. This is something that you can use every time with breadboard and speed up circuit prototyping by several times. This was my intention. I think nobody will use this pot method or decade boxes because of its horrible non-comfort. This was made so that it will be comfortable as possible and you can make any circuit on breadboard in seconds...plug op-amp, plug leads...set resistances...done. If I have a breadboard on my table, I have this device on my tabe too...No mess...no resistors laying around to be measured and sorted to back to the cabinet. Sure...for bigger circuits you still have to plug some fixed value resistors. But for educational purposes it can suffice quite oftenIts purpose is not to find some suitable resistor value because you don't know ohms law. Its purpose is to have complete circuit on breadboard in a fraction of time

author: bd139
author_rank: Super Contributor
timestamp: November 07, 2017, 03:33:26 pm
content: TBH I just solder a socket header into the board if I’m doing that or use a trimmer (read troubleshooting analog circuits for pease’s rant about this sort of thing)Main use case is checking input and output impedances against design.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: November 20, 2017, 02:00:21 pm
content: Interesting project and quite a nice way you build the prototype, I like it.I personally would buy one if I hadn't build something like that for myself quite a while ago, also I have some old high quality decade resistors flying around in the lab.One place I would definitely see a market for that is in education.I am working as a tutor for metrology in germany and many of the students don't even understand the way a decade resistor works - meaning that you multiply the chosen value with the multiplier painted on the caseYour resistor substitution box would make it easier for those people to read and tweak the values in an easy way so that they can focus more on the actual project they are working on.

author: sevenTech
author_rank: Contributor
timestamp: November 20, 2017, 07:05:17 pm
content: Quote from: Ysjoelfir on November 20, 2017, 02:00:21 pmHi Ysjoelfir,Thank you - glad to hear that - i needed that :-) . Yes - education will be targeted as well. I can see that in education schools usually create some circuit according to schematic that will be measured, place it to some plastic box with bananna connectors and give students possibility to tweak one or more critical value of resistance via pot etc.However to give students more free hands for applications like "build amplifier in class A on breadboard - here you have transistor, try to calculate all resistors to keep this bandwith and this gain..." will usually without such device result in some messy solution and lack of time. But without that experience many students can be confused that op-amp part doesn't have 5 pins like they can see on schematic but they are often dual-packaged etc...By using this device, teacher can clearly see the values that are engaged in circuit and tell what is wrongly calculated...And no pre-made circuit in plastic box is needed.I'm working on expansion board so you can have more channels...like 6 or 8 total. This way you can build almost any circuit...Second target is to create possibility to quickly evaluate circuits on breadboard for the others...So you will take out REAL resistors only when you will solder them on REAL pcb...

author: Riotpack
author_rank: Regular Contributor
timestamp: December 16, 2017, 01:33:10 pm
content: What if there was one with 4 x motorised pots that could automatically restore presets and do programmable resistance ramps or track other channels or change resistance based on a few programmable logic inputs

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2017, 02:47:50 pm
content: I keep looking for a series and parallel resistor and capacitor substitution box for tuning feedback networks but will probably end up making my own.

author: bd139
author_rank: Super Contributor
timestamp: December 16, 2017, 02:57:23 pm
content: I think Bob Pease had a recommendation for a combo box in Troubleshooting Analog Circuits. I’d have a look but it’s in the car and my wife has it

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: December 16, 2017, 02:58:50 pm
content: Quote from: Riotpack on December 16, 2017, 01:33:10 pmThat would be a cool idea! I think I will start such a project.

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2017, 03:19:41 pm
content: Quote from: bd139 on December 16, 2017, 02:57:23 pmThat is what I had in mind and Bob recommended the VIZ Model WC-412A but VIZ is long gone.

author: bd139
author_rank: Super Contributor
timestamp: December 16, 2017, 03:26:10 pm
content: Quote from: David Hess on December 16, 2017, 03:19:41 pmThat's the one. Unfortunately so. Found an image. I reckon it'd be pretty easy to build one if you could find suitably ganged rotary switches. Latching changeover ones might be a reasonable modern substitute:Edit: Looks like NSF controls still sell modular rotary switches!

author: bd139
author_rank: Super Contributor
timestamp: December 16, 2017, 03:48:53 pm
content: This is on my "may build" list:http://www.instructables.com/id/Build-a-ResistorCapacitor-Selection-Box/I would like some level of determinism with the resistance rather than multi-turn pots though so would probably use rotary switches there as well (not decades - don't need a decade box)

author: Cyberdragon
author_rank: Super Contributor
timestamp: December 17, 2017, 07:30:19 pm
content: Quote from: bd139 on December 16, 2017, 03:26:10 pmOr you could just go to the land of Ebay and hunt for it there.EDIT:https://www.bmisurplus.com/products/39763-viz-wc-412a-resistance-capacitance-circuit-boxhttps://www.ebay.com/i/122837945604?chn=pshttps://www.ebay.com/i/253287721954?chn=ps

author: bd139
author_rank: Super Contributor
timestamp: December 17, 2017, 07:46:34 pm
content: That’s one of the few things I wouldn’t buy from eBay. Usually “one careless owner”

author: Cyberdragon
author_rank: Super Contributor
timestamp: December 17, 2017, 08:22:41 pm
content: Quote from: bd139 on December 17, 2017, 07:46:34 pmYe of little faith...plus, there's two left from BMISurplus

author: floobydust
author_rank: Super Contributor
timestamp: December 17, 2017, 09:06:23 pm
content: A used VIZ, going for $150-$185 so too pricey. It looks like +/-10% tolerance, no idea of voltage/power ratings. Always a few smoked resistors inside...EIA-6increments, 6 values/decade so 3 decades coverage is nice.But the 18-position rotary switch is hard to find at low cost. Electroswitch D9 series are over $100 each.I've made my own R-C substitution box using lower costC&K 12-positionrotary switches $7, 1kV film-capacitors and 1/4W to 3W resistors on the low end. Maybe easier to use 1-2-4-8 switches.

author: sevenTech
author_rank: Contributor
timestamp: December 19, 2017, 09:57:29 pm
content: Interesting box. Never heard of one. But I won't call +-10% determinismNot when I can get <1% with multiturn pot. But that tolerance is probably for capacitor section I guess.Those motorised pots is interesting idea, but it will be too pricey for purpose of my project. I was mainly on many channels with nice values overview for breadboard use...so price is also a concern...unfortunately :-( this motorised pots idea is probably more for laboratory use. Like those switched relays resistor substitution devices with GPIB, USB etc...I'm working on VCP (virtual com port) implementation for my device, so you will be able to make nice graphs (pc can collect resistance value that are set with values from bench DMM at the same time)...So there is some workaround possible to get circuit characteristics. You will just need to sweep through values manually :-)