A SCPI Programmable (Precision) Resistor - Page 1
Forum section: electronics / projects

author: SebastianH
author_rank: Regular Contributor
timestamp: September 05, 2023, 06:31:41 pm
content: I'd like to share a project with you that I've been working on lately: A programmable resistor.At the time I was looking for a decade resistance box for my small lab. A few days later I decided to build one myself - the goal wasn't to build the most precise and generally best programmable resistor - that task I leave to TME manufacturers. It started more as a quick exercise for two upcoming projects, mostly regarding the machanical design/and software. The project then evolved into something a bit more complicated than I originally anticipated, but most of the early design decisions remained - limiting the performance in some areas, but it's well beyond what I wanted to achieve anyway.Main requirements were:0 Ohm to 1MOhm in 1 Ohm stepsReasonably high accuracy and precisionA usable user interface with an LED display (nostalgia)SCPI ProgrammableThis meant using a bunch of relays for switching in conjunction with relatively cheap 1206 0.1% 10ppm 0.4"W SMD resistors. Having the option to control all decades individually allowed me to not only calibrate the resistor and show the calibrated resistance value in the display, but also optimize the actual resistance selection based on the calibration values to match the desired value (setpoint) as best as possible (available for 2-wire and 4-wire). This was more an experiment with some obvious and less obvious limitations, but it worked surprisingly well.I put some emphasis on the "programmable" part, meaning the SCPI interface offers many options, including a list function, a trigger system with bus, internal, external trigger and more.The first diagram shows the absolute value of the deviation between the setpoint and the value indicated by one of my Agilent 34401A multimeters. The second diagram shows the absolute value of the deviation between the calculated resistance (estimate based on calibration values) and the value indicated by the Agilent 34401A multimeter. Connecting lines between data points for visual purposes only.You can find a very detailed description regarding the design and capabilities as well as some resources below (so I won't repeat everything here...).https://hackaday.io/project/191969-programmable-precision-resistorhttps://github.com/sbstnh/programmable_precision_resistorhttps://sebastianharnisch.de/category/programmable-decade-resistor/If there is anything you're interested in, let me know.Thanks,Sebastian

author: mikerj
author_rank: Super Contributor
timestamp: September 06, 2023, 05:01:24 pm
content: That's an impressive level of finish on the front and rear panels, very professional looking. Can you sweep resistances atomically e.g. no open circuits or spurious resistance changes in the opposite direction whilst switching?

author: SebastianH
author_rank: Regular Contributor
timestamp: September 06, 2023, 08:43:41 pm
content: Thanks!that is a great question. Right now I have implemented two switching modes: "fast", meaning switching all relays (almost) at once, and without any intended order; "break-before-make".So the answer is no. Initially I wanted to have a mode that avoids an "open" during switching, but in the end I didn't bother, becausea) I simply usually don't need itb) Insomeapplications it might be good enough to just connect a capacitor in parallel with the programmable resistor.c) With my topology, "monotonicity" during switching isn't achievable in the first place (not generally anyway).It is certainly possible to improve the switching behavior though.The first thing I'd consider is to switch the highest non-zero decade first, then the second-highest, ..., so that the resistance change would become smaller and smaller ("inter-decade" switching, if that makes sense).Changing the resistance from say 2 Ohms to 4 Ohms with no interruption ("intra-decade" switching) could be achieved by switching on the 4 Ohms-path before opening the 2 Ohm path, resulting in 1.33 Ohm as an intermediate state. Or to 8 Ohm first, then to 4 Ohm (1.6 Ohm, 8 Ohm, 2.67 Ohm, 4 Ohm), not really great either, so for me the simpler first solution wins without a doubt... No matter what you do you'll end up with "spurious" or "non-monotonic" resistance changes.

author: KE5FX
author_rank: Super Contributor
timestamp: September 06, 2023, 09:05:01 pm
content: Looks amazing. Is there a part number for that enclosure, or is it fully custom-designed as well? Edit: never mind,found it!

author: abeyer
author_rank: Frequent Contributor
timestamp: September 07, 2023, 07:26:43 am
content: Quote from: KE5FX on September 06, 2023, 09:05:01 pmIt had better for the prices that enclosure fetches!

author: SebastianH
author_rank: Regular Contributor
timestamp: September 07, 2023, 12:03:28 pm
content: I wouldn't have paid that much and I didn't have to

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 07, 2023, 01:38:49 pm
content: Is that a wooden dowel on the power switch?I have some ideas into this direction myself, and (among others) had a look at the PXI programmable resistors from Pickering, and those easily cost over EUR1500, so you can have a nice DIY budget to make this yourself.For myself, I am more of a cheap and simple guy. I bought a 30x70mm PCB on Ali with 70 jumpers and later discovered that a Sweeny guy had that idea earlier (and probably many others too). The first time I wanted to use the thing, was to simulate a PT100 to set a fixed temperature, and I found the 1 Ohm resolution really lacking. A lot of the Pickering units have 0.1Ohm resolution. And even 0.01Ohm resolution would be useful, even if it does not have that accuracy. Often you just want to measure differences between small input changes.I also noticed you "open sourced" the whole thing, and even made KiCad projects for the front and back panels. I am curious about more details of those. Did you order them as IMS (Insulated Metal Substrate) What is the quality of (up close) surface finish? Is the white as homogenous as it looks on the pictures?Another part I am always interested in is the global build up of cost and time sunk into a project.

author: SebastianH
author_rank: Regular Contributor
timestamp: September 07, 2023, 04:30:25 pm
content: Yeah, it's a simple wooden rod, ~3mm diameter. The initial plan was to design and print a 3d part and this was a quick test. It worked so well, that I didn't bother changing itYou could add one or two decades to the design or just populate the PCB differently (e. g. for 0.1 Ohm ... 100 kOhm). That being said, the big issue is the contact resistance of the switches/relays. It might vary with switching voltage/switching current/age/temperature. The contact resistance of the EC2 relays I used in my design is fairly high compared with a resolution 0.1 Ohm (and even more so for 0.01 Ohm): I measured about 0.025 Ohms for both poles in parallel using my Agilent 34401A @ 1 mA measurement current in the 100 Ohm range. (Edit: This is an average of many relays in series using 4W measurement, determined during the calibration procedure.)Depending on the topology used there might be a varying number of contacts in the signal path. As can be easily seen in the diagram, with the topology I've selected the deviation varies when changing from odd to even resistance values and vice versa. Actually, the number of relays in the signal path of a decade is 2* for even values and 1 for odd values (*in parallel with a resistor; at least for my design with 1 Ohm the improvement caused by the parallel resistor is insignificant). The deviation caused by the changing number of switches in the signal path is so large (i. e. ~0.025 mOhm) that I wouldn't feel comfortable going below 1 Ohm with my design. I would recommend using a different topology (meaning more relays and hence higher cost and other technical differences) and/or relays with lower contact resistance for the decades below 1 Ohm (maybe even for the 1 Ohm .. 9 Ohm decade).Maybe also take a closer look at my documentation (see links above, hackaday or my personal page). There I have described many aspects of the design, including the problem with the contact resistance, which topology I used (and why) etc.The front and rear panels are simple prototyping aluminum PCBs. The white solder mask is reasonably uniform. I ordered the minimum quantity of 5 PCBs and there are slight differences (e. g. visible scratches, silk screen uniformity etc.) The black silkscreen as well as the rectangular cutouts are not 100% perfect, but more than good enough in my opinion, not only, but especially if I consider the pricing of cheap prototyping boards. Again: Not perfect, but I'm more than happy with the result and this obviously depends on the PCB manufacturer.Haven't calculated the cost yet, but I think it's a good idea to do so. I'll give you a number soonDesigning the PCBs was relatively quick & easy, writing the software was much more time consuming and - quite franky - sometimes not that much fun. The project took me many hours, many more the I initially planned - but it's more capable than I initially plannedHow many hours though, I will never know - sorry.

author: macboy
author_rank: Super Contributor
timestamp: September 07, 2023, 07:03:24 pm
content: Looks fantastic. I love the red LED display. Even in 2023, it is easily readable and professional looking.

author: SebastianH
author_rank: Regular Contributor
timestamp: September 11, 2023, 03:09:36 pm
content: Quote from: Doctorandus_P on September 07, 2023, 01:38:49 pmThe following are very rough estimates. I didn't necessarily pay that much for this project specifically, because I had many parts left from other projects. When bought new the case is likely much, much more expensive than what is accounted for. For some of the parts like the PCBs there is a minimum order quantity that will slightly increase the cost in most DIY scenarios where you simply don't build that many units.Mechanical parts (including case, panels, connectors etc.): 80 EURMainboard (including PCB, main controller, relays, precision resistors etc.): 176,50 EURUI board (including PCB, ui controller, LED displays, rotary encoder, switch, buzzer etc.): 78,50 EURPower supply board (including PCB, power switch, AC/DC converter, DC/DC converter): 34,00 EURTotal: 369 EUR. Pretty expensive, actually.

author: Neepa
author_rank: Regular Contributor
timestamp: September 13, 2023, 06:30:48 pm
content: A very impressive project, Sebastian! That front panel wouldn't look out of place in a Rohde&Schwarz catalog during the 80ies or 90ies! R&S liked to use these 17(?) segment Alphanumerical digits as well once upon a time.The one ugly duckling is that dark grey knob in the middle. Would look better in white honestly.Where from did you take the buttons and their caps? An old instrument you broke down?Putting the board on standoff might have enabled you to solder the 4mm jacks directly to the PCB and reduce contact resistances a bit.Certainly raises the bar for my own planned diy instrument over the winter!

author: SebastianH
author_rank: Regular Contributor
timestamp: September 13, 2023, 07:50:36 pm
content: Thank you!Quote from: Neepa on September 13, 2023, 06:30:48 pmI totally agree. This was more a temporary solution. I replaced it with a 3d printed knob that is more suitable for a rotary encoder. But sorry to disappoint you, it's grey as well, see below... Didn't like the white one I tried before that...Quote from: Neepa on September 13, 2023, 06:30:48 pmThese are C&K (now littlefuse) PVA series switches with matching caps (available from mouser/digikey; there might be a significant price difference, so I'd recommend to check both distributors). There are different force ratings for the switches. I accidentally ordered switches with the higher rating (3.5N). Since I also ordered a rotary encoder with high detent force by mistake, the experience is consistent and not bad, but in my opinion not optimal eitherI would likely use the 1.2N variant for the next project.Quote from: Neepa on September 13, 2023, 06:30:48 pmI wanted it to be easy to (dis-)assemble, but you have a valid point there, no question. At least the Molex connector is redundant and at least I could have soldered the cables to the mainboard. That being said, themain"problem" is the contact resistance of the relays, no doubt about that.Quote from: Neepa on September 13, 2023, 06:30:48 pmNow I'm curious... Any specifics on what you plan to build?

author: Neepa
author_rank: Regular Contributor
timestamp: September 14, 2023, 06:12:27 pm
content: Quote from: SebastianH on September 13, 2023, 07:50:36 pmThat looks even more horrid somehow. The grain on the plastic makes me think of marble for some reason.Quote from: SebastianH on September 13, 2023, 07:50:36 pmIatleastplan to build an impedance analyzer.DESIRED MAXIMUM SPECS:Frequency range 10Hz to 1MHz(maybe even 10MHz someday).DC Bias capability up to 100 Volts.Test Signal Amplitudes up to 5-10V.Ultimately in a Computer Terminal like form factor like a Tektronix 370 Curve Tracer but a lot slimmer in depth.Graphic display built in with capability to plot colour Curves in a 3D waterfall including DC Bias, Impedance, and Test Signal Amplitude.Accuracy hopefully better than 1%.Ambitious I know but I plan to start with using an all in one chip solution first and then deintegrate more and more of the functions into discrete analog/digital circuitry. Getting to those maximum specs step by step. For this Winter I will be satisfied if I manage to integrate an Impedance Analyzer Chip eval board together with an RPI Pico, or similar, with a small 2X20 character LCD screen and some buttons into a single box.Then growing from there.More of a roadmap than a singular project I know. Buying a HP4274/75 or 4192A might be cheaper in the end but where is the fun in that? Heck while rare even a HP4284/85 should be but those boat anchors got digital calibration requirements in terms of floppy disk software and 90ies computers I don't wanna deal with.And I simply flinch at the thought of making it a PC based device. I like to touch knobs/panel buttons thank you very much.PS: Holy smokes I just found a chip/eval board that can do almost all I want after browsing AD's site for some minutes.https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/eval-admx2001.html#eb-overview. Fucking expensive but it can do it.

author: Tigerwoods
author_rank: Contributor
timestamp: September 26, 2023, 04:04:45 pm
content: Nice work.As an engineer who's been studing in this field for a while (indeed our team have launched a Kickstarter campaign last year for ourQR10 programmable resistance boxhttps://www.eastwood.tech/en/- which can achieve0.1 ohm stepsand up to±0.01% reading accuracy), you need to solve two major problems if you want to make it more precision:1. Forget about the ideal formula (it's simple if you ignore the relay's contact resistance and choose strict 2^n valuse to achieve 1% or 0.2% overall accuacy)2. Find a smart way for calibration (time-consuming measurements can work, but it may not work so well for high accuracy)At last, you should be careful to select the switches.Good luck!

author: luudee
author_rank: Frequent Contributor
timestamp: September 26, 2023, 05:04:08 pm
content: Quote from: Tigerwoods on September 26, 2023, 04:04:45 pmLooks like Kickstarter is over on that project. Can these unit be bought somewhere ?What is the cost ?!Thanks,luudee

author: luudee
author_rank: Frequent Contributor
timestamp: September 26, 2023, 05:10:07 pm
content: Quote from: SebastianH on September 11, 2023, 03:09:36 pmHi Sebastian,have you considered making these and selling them ?I'd be interested !Cheers,rudi

author: luudee
author_rank: Frequent Contributor
timestamp: September 26, 2023, 05:11:00 pm
content: Quote from: Tigerwoods on September 26, 2023, 04:04:45 pmCan it be controlled via LAN and SCPI ?Thank you,rudi

author: Tigerwoods
author_rank: Contributor
timestamp: September 26, 2023, 05:23:16 pm
content: It has a USB port (actually it's USB-COM that presents as a COM port on your PC). You can use simple command like "AT+SP=123.4" to set it output 123.x Ω

author: SebastianH
author_rank: Regular Contributor
timestamp: September 26, 2023, 05:49:14 pm
content: Quote from: Tigerwoods on September 26, 2023, 04:04:45 pmWhich relays did you choose?Quote from: luudee on September 26, 2023, 05:10:07 pmNo, it's not worth it for me. But if you want to build something like this yourself, most relevant parts like schematics and firmware for the main controller are open source (currently minus the UI board firmware). See my github account.

author: delvo
author_rank: Contributor
timestamp: September 27, 2023, 03:11:33 pm
content: Looks great. Definetly will put this on my long list of things i want to build. Probably needs some modifications to bring the cost down to be more in my price range

author: SebastianH
author_rank: Regular Contributor
timestamp: September 27, 2023, 03:19:23 pm
content: You could start with a different topology. (i. e. (1 Ohm)^n, (2 Ohm)^n, (4 Ohm)^n, (8 Ohm)^n per decade) to reduce the number of relays, use cheaper resistors, simplify the UI (the switches/push buttons are expensive...) etc. There is a lot that can be done to make it cheaper.

author: Chipguy
author_rank: Unknown
timestamp: December 14, 2023, 11:15:24 pm
content: That is a very good looking design.I built two programmable resistance decade boxes so far.However I took a different approach: These are 2 decades in one.One is paralleling resistors for the range of 1 ohms to a obout 3400 ohms.Above that a second serial resistance decade is put in serial that contains resistors from 3400 ohms to about 800 K ohm.So I have a high resolution decade from 1 Ohm to 1.6 M Ohm.I lives from knowing the exact conductance value of each resistor so it can combine them via succesive approximation to the desired value.I can use 1% resistors no problem, main thing is: They need to be low tempco.The unit in the picture is shown in PT100 simulation mode. I use it to simulate all sorts of temperature sensors, conductivity of saline solution and plain resistance of course.For quick access I added 16 pre-programmable favourite buttons.I build this in order to improve my embedded C programming skills which were in desperate need of improvement.Needles to say that the code I wrote is bonkers and eventually got me stuck.Just wanted to share that with you, so you know you are not alone

author: SebastianH
author_rank: Regular Contributor
timestamp: December 15, 2023, 01:46:58 am
content: Very nice! If you don't mind me asking: Are you willing to share the schematics of the resistor networks?

author: zapta
author_rank: Super Contributor
timestamp: December 15, 2023, 05:28:18 pm
content: Looks great. Initially I thought that you repurposed an old HP case.What is the topology of the resistors/relays network? Trying to understand how the resistances of the relays factor in, especially in low settings.

author: shapirus
author_rank: Super Contributor
timestamp: December 15, 2023, 05:38:59 pm
content: I want to hear how this thing sounds in action. It must be really sweet.

author: Chipguy
author_rank: Unknown
timestamp: December 15, 2023, 07:44:28 pm
content: Quote from: SebastianH on December 15, 2023, 01:46:58 amThere they are.The trick is, that each resistor value is just under 2 times greater than the one before. 95% to 97% is the sweet spot.Once the microcontroller knows the exact value of each resistor is is possible to set very fine, actually ridiculously fine values. Beyond the capabilities of the resistors even.Some of the relays are low EMF relays that also have a really constant contact resistance.Have fun!

author: Chipguy
author_rank: Unknown
timestamp: December 15, 2023, 09:40:39 pm
content: Had a look at your design on github.You did some nice things that I regret not having done as well.My design only used one PIC microcontroller for the UI and the relay and one smaller one for serial to USB. There is an additional ARM controller inside the Ethernet but I don't count that.Early on I ran into issues with the PIC18 microcontroller flash size and migrated to a bigger one.Also the choice of the 4x20 Display turned out to be not sufficient for what I wanted.I also like the power supply design you have choosen. I temporarily went with a external DC power supply design but also got a very similar AC/DC converter to yours sitting around in a box.The case you choose looks awesome. The two unit I made use totally different cases but I am not satisfied with either of themMy goal was to make the device the same size as the most common 9 inch wide and 2U high devices on the market, like a 34401 multimeter or something.However I could not find any suitable housing that would really do the trick.

author: SebastianH
author_rank: Regular Contributor
timestamp: December 15, 2023, 09:44:46 pm
content: Quote from: zapta on December 15, 2023, 05:28:18 pmThanks! I thought about doing something like that, but buying old equipment and ripping it apart instead of repairing it doesn't feel right, generally speaking. It can be cheaper than buying a new case though. If I found a faulty HP unit damaged beyond repair, i think I would go for it. Actually, I just ordered an old non-working Agilent DC Mobile Communication power supply. So if I can't repair it...In this series-style toplogy the contact resistance plays a significant role, as you can see in the first performance diagram in my first post: At 1 Ohm the deviation was about +5% .. 6% (4-wire measurement), meaning a total of 50-60mOhm for 2 relays in series + PCB traces etc. In these cases the resistor tolerances (0.1% spec) are negligible. The relative error drops very quickly though. For 2 Ohm it's already well below 2%, for 100 Ohm about 0.1% (all for 4W). With values above 10k it's the other way round - the resistor tolerance of (in my case) +-0.1% dominates the error. Then the calibration/adjustment improves the accuracy to a point where my 34401A's accuracy becomes a concern.I implemented some measures to reduce the effective contact resistance, i. e. using both relay poles in parallel, shorting the upper decades for the three lower decades (i called that "bypass relays"). But at the time I didn't care too much about the accuracy of the really low values anyway. The lower decades are partially there to improve accuracy in the upper ranges...Quote from: shapirus on December 15, 2023, 05:38:59 pmThe signal relays are reasonably quiet. Plus the case doesn't have any cooling vents. I made a short clip (will take a few minutes for the higher resolutions):Quote from: Chipguy on December 15, 2023, 07:44:28 pmThanks, appreciate it! Will take a closer look at it! Did you do any measurements/graphs showing the accuracy? I might wanna know moreWhich low EMF relays did you use?

author: SebastianH
author_rank: Regular Contributor
timestamp: December 15, 2023, 10:19:04 pm
content: Quote from: Chipguy on December 15, 2023, 09:40:39 pmI use the STM32G4 series with 128kB RAM and 32kB Flash as you have probably seen. I didn't really optimize anything (not an embedded dev) and didn't write the firmware with memory efficiency in mind (like at all), except for switching on the "optimize for size" compiler option - and with that in mind and the SCPI library, FreeRTOS included the 128kB of Flash are barely ok. In future project I'll probably use a micro with more Flash. For DIY builds a few bucks aren't worth the trouble imho.Let me tell you, if 4x20 isn't good enough then imagine how difficult it has been to utilize the 1x18 display properlyI'd like to use a 800x480 LCD touchscreen in the next project, but I'm not yet sure how this will work for me. This certainly requires a much more powerful micro, something like the STM32H series.And yes, finding a good looking case that meets the requirements is really hard. I was lucky here, I guess. Unfortunately, my variant is only ~25cm in depth and has no air vents, so for anything related to power it's a challenge. It's available in other form factors and generally fairly customizable, but you really have to pay for it, so this is not an option for me.Using an off-the-shelve AC/DC converter with built-in filter was very convenient indeed.

author: Chipguy
author_rank: Unknown
timestamp: December 15, 2023, 10:46:30 pm
content: Quote from: SebastianH on December 15, 2023, 10:19:04 pmI was also thinking of using a 4" or so touch display.An STM32H or F7 MCU using ToughGFX could handle that.Alternativly there are those Bridgetek HMI processors. But I think they are rather slow.I totally believe you that only 18 characters of display must have been a challengenging.The bad desicion I made on the microcontroller made me write all the low level code myself sind almost nothing from the PIC libs worked.The PIC compiler can't even handle pointers correctly. So that's something that created an extra headache.

author: SebastianH
author_rank: Regular Contributor
timestamp: December 15, 2023, 11:11:18 pm
content: That sounds bad. I never used PICs...Yeah, a few days ago I ordered a STM32H board and will order a 4.3" display w/ capacitive touch soon. The problem with many screens: Either low res/quality, resistive touch or huge footprint relative to the visual area and hence incompatible with the fairly restrictive case I use. LVGL would be an option too. Not sure, whether I should start with TouchGFX or LVGL...Would be really nice to build a second generation of the programmable resistor with a large touch screen. Then again, a variant of the DIY SMU, a DC Load and a 200V power supply are already waiting for attention.

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 10:32:24 am
content: The relays I used are Meder BT05-2A66 for the low resistance values. For the very low ones it was necessary to use two in paralllel.For the higher values I used Pickering 100-1-A-5-2D. Those can be driven directly by many logic ICs without using an extra transistor.

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 10:44:15 am
content: TouchGFX: I got some STM32F and STM32H EVM boards here. STM32H750b-DK. There is now also an Arduino Board with STM32H MCU and a display shield. But for now it seems that the software support for a GUI is rather sparse. i.e. not existent yet. No wonder, since that is a new product IIRC.I also thought about a solution using some Raspberry Pi Module, but that looks pretty oversized to me.Same goes for a System using some sort of Windows. However, I could then write the code in RAD Studio, which I am familiar with.So there are a lot of ways to go, which are equally complex and nothing that could be done just over a few weekends.

author: SebastianH
author_rank: Regular Contributor
timestamp: December 16, 2023, 10:48:21 am
content: Thanks! Yes, in the mean time I looked at the schematics and found the relays you used. It looks like your relays alone cost waaaay more than my whole project. If I look those up on digikey, mouser, farnell then your relays alone cost about 1000 EUR, which means almost three times the total cost of my project. Does this sound right or do you have a cheaper source? Compare this with about 60 EUR for my cheap signal relaysWhich performance would you expect when using cheap signal relays like my EC2-xNU?The only time I bought a low thermal EMF reed relay was for my 34401A repair...

author: SebastianH
author_rank: Regular Contributor
timestamp: December 16, 2023, 10:53:31 am
content: Quote from: Chipguy on December 16, 2023, 10:44:15 amI actually tried that. Linux distros on 800x480 are a pain in my opinion. Out-of-the-box support for on-screen keyboards really poor. I programmed a mock with python/tkinter plus a 4.3" touch screen. Works great, kind of. My main problem with this approach are the packaging and power/thermal considerations. Don't like to have a 5-10W furnace and the power supply to support it right next to precision resistors in a closed box.

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 11:13:52 am
content: Here is a graph that shows the resolution depending on set resistance value.By connecting resistors in parallel it is clear that, the higher the desired value is the lower the resolution becomes.I found the sweet spot to be at around 3400 Ohms. After that, the serial decade kicks in to add resitance in 3400 Ohm steps into the parallel decade.Back then I tested out a lot of relays, cheap and expensive ones.the problem with most cheap ones it, that their contact resistance varies a lot with temperature and each switching occurence.I also evalulated COTO relays that were equally expensive, but they were not as good, by far.Futhermore I aged them with a function generator and measured how the contact resistance changes over time.The Meder ones were the best of them. And I think the second expensive ones. The COTO relays were even dearer.A surprise for me was the cheap TQ2-5V DPDT relay: Their contact resistance was fairly stable as well. No good EMF performance though.However, they don't age well. They change contact resistance over time, so regular re-adjustments are necessary.For a other solutions I went with them and they have not let me down yet.

author: SebastianH
author_rank: Regular Contributor
timestamp: December 16, 2023, 12:08:44 pm
content: That is really fascinating. For me it was just a quick project (hardware-wise), I see you put a lot of effort and money into the design. And thank you for sharing your findings, might come in handy some time. I don't think I have enough applications for it to justify the cost right now. I mean... If I run out of project ideas I circle back to your approachDo you have any pictures of your PCBs? Ah, and what resistors did you use?

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 12:36:00 pm
content: Yes I have some dodgy pictures of the PCBs, but they don't show everythingThat nice case you used. Who makes these and what type is it?This project came to be for a few reasons:1) I needed something that could emulate PT100 sensors at work2) I also needed something that could emulate the conductivity of NaCl solution3) Having done some programming in Assembler it was time to learn some embedded C, so creating a project that would be bonkers to code in assembler was something to practice C.4) I was just interested if that concept even worked.Now the old version of this is at my workplace and became a key test equipment used in service and repair. It also gives me information about the endurance.So far, one of the pickering relays had to be replaced, it did not turn on anymore.

author: SebastianH
author_rank: Regular Contributor
timestamp: December 16, 2023, 01:31:30 pm
content: Very cool, thanks! Looks like metal film resistors. ppms? Is this an ethernet -> serial adapter of some sort?The case is made by ELMA, I think it's the so called "Stylebox 15 Standard".

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: December 16, 2023, 01:42:45 pm
content: Hi Chipguy,nice work too.Did you implement the GPIB-Interface?....or is it just there to confuse your coworkers??Best regards

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 02:00:35 pm
content: Quote from: SebastianH on December 16, 2023, 01:31:30 pmThe resistors are standard 10-15ppm TC 0.1% tolerance resistors. They turned out to be sufficient for the job. However it is totally possible to use S102 0.2ppm Vishay PG resistors, but then you need about five times more relays in parallel for the low values in order to make their influence small enough to justify the expensive 8 EUR resistor.The ehternet adapter is a USRIOT USR-K3https://shop.usriot.com/ethernet-to-uart-modules-usr-k3.htmlIt contains a TI Tiva Ehternet to UART MCU, so no work needed there for me.I can emulate com ports via ethernet then. No LXI though.The GPIB is not functional (yet). I left a few headers there to plug in a little module with GPIB to UART. Never got around to design one.As you can see it was planned as an option anyway.I had the idea to maybe sell them and get self employed with this on the side. But german buerocracy prevented me from that. It is a headache even thinking about it.Gonna have a look at the housing manufacturer you have choosen, it looks really good.

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 04:53:22 pm
content: Quote from: Roehrenonkel on December 16, 2023, 01:42:45 pmMy current coworkers have no clue of what GPIB even is. Maybe one. But we are not really working with any GPIB Interface at work.I made this in preparation, but never got round to design the module to handle GPIB. So it's a DUD just for show

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 04:55:53 pm
content: Quote from: SebastianH on December 16, 2023, 01:31:30 pmJust downloaded some nice STEP files from their site.Seems that housing got a successor called "Stylebox 15 e-motion". The handles look less chunky which is also nice.How much did you pay for that?

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 05:01:39 pm
content: The first one I made looked a bit different.While the 2nd one used a case called "XDream" from a company called "Apra Norm", the first version used a housing from Bopla.I have attached a few pictures of the first tests and housing. Only partially assembled. And the first PCB, before the 2nd serial decade PCB came in.

author: SebastianH
author_rank: Regular Contributor
timestamp: December 16, 2023, 06:01:08 pm
content: The Meder relays look really fancy on your boardGot them pretty cheap on ebay (or something, surplus), my guess would be something like 50 EUR, can't quite remember.Yep. For my 3d models I simply used the step-file of a Stylebox 15 e-motion - close enough...

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 06:33:21 pm
content: Quote from: SebastianH on December 16, 2023, 06:01:08 pmI bought them from Mouser and back in 2017 I paid 26,02 EUR per piece. As you can see in the photos they are silver and not blue like in the picture on the Mouser website.Today that would be 460 EUR worth of Meder Relays on that board. Back in 2017 it was 290 EUR. Even more expensive today.

author: SebastianH
author_rank: Regular Contributor
timestamp: December 16, 2023, 07:13:50 pm
content: (Just to clarify: 50 EUR was for the case not the relays.)Yep, that's is why I'm hesitant (to say the least) to build something like that. Otherwise I'd probably have to try.

author: peter-h
author_rank: Super Contributor
timestamp: December 16, 2023, 08:10:43 pm
content: That is seriously impressive. Beautifully constructed. I've been doing this for 50 years and this is outstanding!A lot of test gear uses reed relays, BTW. They have a longer life, supposedly, although my 3314A contains loads of them and they need replacing periodically.

author: Chipguy
author_rank: Unknown
timestamp: December 16, 2023, 08:55:14 pm
content: Quote from: SebastianH on December 16, 2023, 07:13:50 pmYeah after reading it a second time I figured that muchUsing different relays would be an option as well.How are the relays doing, that you use?I put my first unit into the fridge and the oven a few times to see how everything would behave after being stressed.Funny thing: After a few initial changes it bedded itself in.