Precision Load Cell (Wheatstone Bridge) Simulator - Page 1
Forum section: electronics / projects

author: snarlingsnoyl
author_rank: Newbie
timestamp: July 12, 2019, 03:08:41 pm
content: First Post on EEVBlog, long time follower...The goal of this project is to create a precision load cell simulator for testing/debugging load cell frontends/amplifiers/converters/etc. I am no expert in material science, precision resistors, low themal EMF design, etc; so I thought it best to share with everyone and hopefully get some good advice in return.A load cell simulator is a device that produces a very small ratio-metric output simulating to the output of a strain-gage load cell. It can be used to provide a test signal to an instrument or device that is used to measure load cells.Typically, the output is referenced to an excitation voltage and specified in terms of mV/V, or milli-volts output per volt of excitation. Most industry load cells are of the full-bridge type with a full scale output range around 2 mV/V; for example, if you excite the load cell with 10 VDC, then the full-scale output of the load cell would be +-20mV ( 2mV/V * 10V excitation).The device will have a selection of outputs ranging from 0mV/V to 2mV/V in 0.2mV/V steps. Negative outputs will be accomplished via a polarity switch. My initial calculations show that this can be accomplished using a Wheatstone bridge and switching resistors in and out of series/parallel with each bridge leg in order to achieve the desired change in resistance.Starting with a Wheatstone bridge:We can then add parallel resistors across R1 & R4. However, in order to keep the total bridge resistance constant, it may be necessary to add resistors in series to R2 & R3. I've been double checking my math with a little LT Spice schematic:The math (for a full bridge simulator) follows as such:RB - Bridge Resistance:- Typically 350 ohms, also common are 120ohm, 1000ohmRP - Leg Parallel Resistance:- This resistance is added in parallel to the active leg in order to lower its resistance by a calibrated amount- RP = RB * ~10 / Sensitivity- RP = 350 * 10 / 0.002 (or 2mV/V output) = ~1.75MegOhmRS - Leg Series Resistance:- This resistance must be added to the opposite leg in order to compensate for the lower resistance caused by adding the parallel resistor- RS = RB²/(RB + RS)- RS = (350²)/(350+1750000) = ~80mOhmIt may be possible to add outputs or switches to reconfigure the simulator into half-bridge or quarter-bridge modes, but that could be a later improvement. Best to start simple.Another possible improvement is for the simulator to work with both DC and AC excitation. For AC excitation, resistance need to be replaced with impedance. Fortunately, most industrial load cell systems using AC excitation typically only use 100-1000Hz carrier frequencies. This means that parasitic capacitance/inductance would need to be controlled, as well as balancing any resistor inductance, but should be reasonable easy to accomplish at such low frequencies.As for parts, Caddock's low TC film resistors is the initial choice. However, I have to admit my experience with working with precision equipment is limited, and designing precision equipment is none. So things like temperature coefficients, parasitics, etc are on my radar, but I'm still trying to figure out how much impact on real world design and construction. This things are easy toFinal note, I lacking in the precision test equipment department. This project is going to require some precision resistance testing equipment. I've been looking through eBay and the like for a nice DMM/SMU. Anyone have any suggestions?Be gentle

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: July 12, 2019, 06:24:35 pm
content: You might get more information if you post in Metrology. Everything I am about to say comes from there, so-to-speak.First, the basics are nicely handled by reading Conrad's DIY Metrology Lab articles. The wheatstone bridge is the basics of diy metrology.http://conradhoffman.com/mini_metro_lab.htmlAttached is a photo of a basic setup using a Kelvin-Varley divider measuring the tempco of a Vishay s102k resistor. It looks crude, but lets examine it. The 720A is a 0.1ppm KVD, so quite good. The aluminum box in front is just used to connect the auxiliary wires needed to create the other half of the bridge. Not very visible is another s102k resistor serving as a reference. (This setup is diagrammed on page 78, figure 9 of Conrad's part 3). The usb stick measures temperature. The gray cable on the left goes to a Fluke 731b to supply 10v to the bridge. The output is measured with an HP3456a.For a modern DMM folks seem to recommend the DMM6500 or DMM7510, but I've stuck to the tried and true as noted above.I have not heard anything bad about the Caddock series. For higher precision I have good experiences with the AE z series. e.g.https://www.newark.com/alpha-electronics/hcz10k000t/resistor-metal-foil-10k-0-01-radial/dp/96Y7830In looking at what you have written, the major question is the switching. There is a reason that the my bridge is all hard-wired. Its to accurately fix all of the variables having to do with those pesky interconnections. If you need programmable switching--relays, jfets, analog switches; that's a big step up in complexity and error sources.Hopefully, this is helpful.

author: MosherIV
author_rank: Super Contributor
timestamp: July 12, 2019, 07:00:56 pm
content: Hi.I did not read everything in detail but it seem strang to me that you are trying to use a real bridge to simulate a strain gauge in a bridge.If you want a simulator, all you need to know is the transfer function.Knowing that, you know what output you should get for any input.You then just build a system eg microcontroller with ADC and DAC. The micro then gives the simulated output voltage for a input voltage.Or am I missing something?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 12, 2019, 07:26:13 pm
content: For the resistors I would consider the 350 / 120 and 1 K bridges as separate projects.I would consider some thing like 350 Ohms + a number of around 1 M resistors in parallel for all the resistors. So to keep the sum constant one could start with 2 x 1 M in parallel and add / remove the extra resistors where needed. How large the extra resistors should be depends on the range, but there usually is a limit of some 0.1% of change for the usual DMS. With 1 M resistors switch resistance should not be a major problem, it more like leakage limited. If needed the next step would be using something like 300 Ohm + 50 Ohm in parallel with a few 100 K resistors.DMS bridges may use quite some power, so that self heating can be important.At 350 Ohms I would consider wire wound resistors and possibly bulk metal foil if needed.The larger resistors for signal steps are less critical and could likely be thin film types.For some tests one could consider a real dummy type, that has the bridge part only for a much smaller voltage and just series resistors to get the overall resistance. so more like 170 Ohms resistors at each end and than a bridge with 10 Ohms resistors. This version could get away with much less critical resistors and still get a very stable reading.

author: jbb
author_rank: Super Contributor
timestamp: July 13, 2019, 06:18:55 am
content: I think it's fair to say that if you want acontinuously adjustable, high accuracybridge simulator, you're going to have a difficult time.Maybe you could split this into two sorts of tests: a lower precision adjustable type (maybe simple digital potentiometers would be sufficient) for debugging / exploring, and a number of discrete high-accuracy 'strain references' for precision testing?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 13, 2019, 07:14:47 am
content: For testing strain bridge amplifiers, I don't see a need for a very fine adjustable bridge. More like a few (e.g. 5) well defined settings rather close to balance. Some tests might need a fast / smooth step switch - so more like electronic switching - this could be a little tricky.So I think the circuit with extra resistors at all 4 terminals is a good idea. The extra resistors can reduce the required stability a lot (e.g. some 100 fold) - so no more need for very expensive resistors. A limited number of settings could be generated by just a multi step switch and a string of low resistors (e.g. 1 Ohms). One may not even need to adjust both branches together, but could use separate switches, so that it could even be used for a linearity test or fine and coarse steps.

author: snarlingsnoyl
author_rank: Newbie
timestamp: July 13, 2019, 01:13:52 pm
content: It always seemed like the community here was quick to respond to even the simple project, so thanks for all of the responses so far.Why Not Digital (ADC->MCU->DAC)? - MosherIV- I hadn't really considered it before, but I suppose its possible.- Potentially a continuously adjustable range which may be useful in some testing/debugging situations.- The tricky part being low voltage/low noice DAC section for +-2mV at 0.1% (+-2uV), seems reasonable to use a normal DAC with a precision 1000:1 voltage divider, just have to deal with noise.- However, this would not likely not work with load cell amplifiers that measure excitation current to compensate for lead length and do not have dedicated sense lines.- It may be possible to simulate the proper excitation current with a DAC driven current sink.- Would have to watch power management closely for low excitation voltages.- Trickier to allow AC excitation, but not impossible.- Overall, this is a great idea. However, I personally would like to try and accomplish this with an analog solution first, then maybe try digital after that.Next, lets narrow the scope of the project a little bit. Here is updated goals:- Target 350ohm bridge load (Kleinstein), no need to be able to do all 3 at once. Should be easy to apply same principles to other loads- Limit the number of steps to 5 (Kleinstein/jbb), 10 steps does seem like too many for an initial project, 5 discrete steps seems like plenty for its intended purpose.- Target of precision is 0.1% across 5°C-45°C, not really precision but good enough to help spot issues in the field, may be able in do better in future revisionsThe first post was a little short on construction details, so here's some more details- The original plan was to use a nice (low contact resistance/low thermal emf) multi-position switch. Most likely, it would be salvaged from an old decade box/transfer standard.- Both reed relays and other signal relays seem to have contact resistance specs (~100mOhm) that may make parts of this project more difficult.- Most multi-position switches have 10mOhm max initial contact resistance- Its expected that either solution would gain contact resistance over time, so it seems that starting out lower would lead to less error-Anyone have any information on contact resistance vs cycle count for multi-position switches or relays?- For each step, the switch would be adding a different set of resistors both in parallel or series with the bridge legs in order to achieve the desired change.- This is where the precision resistor stuff comes into play.- It would be easy enough to get resistances close using discrete precision resistances- Fine tuning can be accomplished with low TC trimmers- It also seems like the better approach would be to keep adding resistors in parallel/series such that the power (and heat) are more distrusted, instead of switching in and out- As mentioned, the resistors that are added in parallel are going to be less critical, so it should be easy enough to come up with proper combinationsKleinstein also had a good idea worth exploring, split the bridge into a static section and a active section. For a 350 ohm half bridge to output 2 mv/V, the top and bottom resistance need to change (in opposite signs) by only 1.4 ohm. So it could be achieved using 340 ohm resistance of high precision, and then a set of higher tolerance resistors for each step. In order to remain in spec, the last 10 ohms of each leg would only need ~0.25% tolerance. It would be even better to split it at 345 ohm for the base and 5 ohm for the active, then the last 5 ohm would only need 0.5%.This seems like a much better topology for this project. It also limits the number of expensive components.My next steps are to spend a little time with some uncertainty analysis to determine the actual tolerances needed for my components over the temperature spec range. Then after that, onto laying out the construction details.I am thinking of cross posting to Metrology but maybe later. I think as a project it fits better here, but when it comes time to do adjustments and calibrations, Metrology seems like a better place.Thanks for everyone's replies so far, and please keep them coming!!!

author: Kleinstein
author_rank: Super Contributor
timestamp: July 13, 2019, 02:25:24 pm
content: My verbal description of a possible circuit may not have been clear. Attached is a possible solution with 2 muti stop switches.The resistor values are only rough order of magnitude and likely still too large steps.

author: David Hess
author_rank: Super Contributor
timestamp: July 13, 2019, 06:03:09 pm
content: Load cell simulators do not work the way you have described because of the expense in switching or trimming the leg resistors accurately. Sticking a switch or potentiometer in series with a precision bridge resistor is almost always a bad idea.Instead, the four leg resistors are fixed and the variable output is provided in the same way that a bridge zero circuit is done; essentially a trimmed set of dividers are placed across the excitation which then drive one of the signal outputs through another high value resistor making it possible to have several full scale ranges with several settings each using only two rotary switches and none of them are in series with a low resistance path. The bridge zero is trimmed in the normal matter. This configuration also lends itself to electronic control.We once made one where the bridge resistors were actual strain gauges mounted to a block of aluminum simply because those were the cheapest high precision resistors we had available.I always find it surprising how little documentation is available online on this subject. It is like everything is a trade secret or at least pre-internet which might be more accurate.If it is not on the internet, then it does not exist - Jame Burke

author: snarlingsnoyl
author_rank: Newbie
timestamp: July 14, 2019, 03:35:50 pm
content: Quote from: Kleinstein on July 13, 2019, 02:25:24 pmYour idea is very clear now. Its pretty obvious too, I'm a little ashamed I didn't see it before you had to draw it up for me. This seems to offer a lot of benefits over switching resistors in and out. I have updated my designs to reflect this concept for 0, 0.5, 1, 1.5, 2 mV/V on a 350 ohm bridge. A rough Monte-Carlo error analysis would suggest that 0.1% is achievable with 0.01% tolerance on 350 and 10 ohm resistors and 0.25% tolerance on the RP and RB resistors. These could be some Vishay Bulk Metal Foil potentiometers.As for utilizing one half of the bridge for both +/- signals, it seems like this would leave the circuit vulnerable to input impedance of the measuring device. If it's not prohibitively expensive, it may be best to leave the it as a full bridge. Is this a valid concern or just ignorance?Quote from: David Hess on July 13, 2019, 06:03:09 pmThis is an interesting comment. Is there any further explanation to the issues that arise from this. Of the top of my head, the biggest issues seem to be the contact resistance of the switch or the stability of a potentiometer (both material stability as well as mechanical stability). With the re-arranged circuit, the ~12k and ~250ohm resistor would likely need to be potentiometers and as such may be vulnerable to these problems.Quote from: David Hess on July 13, 2019, 06:03:09 pmThere are designs floating around similar to this. They don't always behave the same as a load cell would though in terms of input impedence. This design may be overkill, but the intent is to simulate as closely as possible to what's actually happening within the load cell.Quote from: David Hess on July 13, 2019, 06:03:09 pmWould you be willing to illustrate your idea? This may become a helpful for those looking for information on this topic in the future.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 14, 2019, 03:51:43 pm
content: The circuit as shown last could also be used as a half bridge, though with 175 Ohms resistance as the 2 branches are in parallel.For just getting the voltages near the center, it would really help to combine the upper und lower 350 Ohms resistors (R1+ R3, R2+R4) and add the extra output resistance to the signal sense lines. The internal circuit is than quite different to a bridge, but the properties measured from the outside (especially with defined drive and sense) are very close. The advantage is that this substitute is nearly balanced and stable by design. So even 1% resistors may be used to get a very stable bridge reading. The simple perfectly (as good as the node is as a 4 wire zero) balanced case would be just 4 resistors of 175 Ohms joint at a hidden node. Even with just carbon resistors it would be more stable than a classical bridge with good (e.g. BMF) resistors.The half bridge would need 2 really stable resistors and maybe a few in the center for steps.

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2019, 06:13:09 pm
content: Quote from: snarlingsnoyl on July 14, 2019, 03:35:50 pmVariations in contact resistance are exactly the problem. The strain gauges only change in resistance by a tiny amount and bridge circuits are used to make the most of this.QuoteLoad cell input resistance is not well controlled unless a shunt resistor is added because of any gain or modulus compensation resistors places in series with the excitation. 350 ohm bridges for instance often have a 40 ohm modulus compensation resistor, often in the form of a gauge, for 390 ohms. Usually there is also a length of resistance wire in series with one element of the bridge for temperature compensation but its value is low so it can be ignored. Various methods are used to trim the zero after all temperature compensation is done.QuoteI thought I described it pretty well. Use one rotary switch to select from a divider placed across the excitation and another rotary switch to select a higher value resistor which goes from the selected divider point to one of the signal output leads.Note that strain gauge bridges really are precision devices. Maintaining that level of precision in a finished load cell or a load cell simulator is not a trivial task.A couple months ago I tried finding the schematics for the load cell simulators I am familiar with but there was nothing available online.

author: snarlingsnoyl
author_rank: Newbie
timestamp: July 16, 2019, 03:37:04 pm
content: Quote from: Kleinstein on July 14, 2019, 03:51:43 pmForgive my ignorance, I just couldn't see it before but this make sense now after going through your design with input/output impedences.Here is an updated schematic based on input from Kleinstein and David Hess:Running through some more Monte Carlo simulations, here are the specs needed to achieve 0.05% total error across 0-40°C, with 0.1% long term stability:NAMERESISTANCE TOLERANCETEMPCO [ppm/°C]RS100.01%2R_BRIDGE1750.1%50R_OUT1750.5%100R_SET1k0.5%100R_SHUNT1k0.5%100R_ADJ100 trimmer5%200These spec for the RS resistors is a little misleading. These must match withing 0.01% and withing +/- 2ppm of tempco. After reading through RandallMcRee linked post (HIGHLY RECOMMENDED -http://conradhoffman.com/mini_metro_lab.html), it seems reasonable to find a matching set of 5 resistors from a large order 0f 0.5% 20ppm metal film resistors.The RS resistors of 10 ohm will work for the 120 ohm simulator too. The key is getting the proper ratio of R_SET/S_SHUNT so that R_ADJ is just making minor adjustments. It looks like the 1000 ohm simulator would be too sensitive with RS of 10 ohm, so it would like need to be 100 ohm.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: July 16, 2019, 08:54:45 pm
content: Output impedance problem in your last schematic : Output impedance = 175 ohm (R_out) in series with 2*175 ohm in parallel (R_bridge) = 262.5 ohm. To obtain the correct input impedance you have to use R_out = 262.5 ohm.Another idea in attached file (Shown for output = 2mV/V. Every new setting requires one additional R_tuning resistor). Lower component count, input impedance is ok, output impedance is ok, the initial offset error is null, calibration can easily be done by tweaking the R_tuning value.R1, R2, R3 can easily be implemented using a resistor network, doing so you'll get a perfectly tracking resistor ratio.

author: gamalot
author_rank: Super Contributor
timestamp: July 16, 2019, 09:21:41 pm
content: I used to make a loadcell emulator using a Maxim 18-bit DAC to test the PCBA of the scale.

author: snarlingsnoyl
author_rank: Newbie
timestamp: July 17, 2019, 01:34:44 pm
content: Quote from: EmmanuelFaure on July 16, 2019, 08:54:45 pmI made the same mistake when I was looking at Kleinsteins earlier post. In fact, the output impedence is just 2 * R_OUT. It goes Vsig+ -> R_OUT_2 -> R_OUT_1 -> Vsig-. Then for different settings, the output impedence rises by ~10-40.The R_BRIDGE resistors don't influence the output impedance at all.Quote from: EmmanuelFaure on July 16, 2019, 08:54:45 pmI believe this is the idea that David Hess was suggesting earlier. I do believe it is a valid design, definitely lower component count and easier to build.The one difference that stands out immediately with this design is that the gain & linearity of the output will depend on the stability of every resistor in the network except for R5. With the current design, the linearity of the outputs is related only to the matching of the 4 RS resistors(which can be done very accurately, see Conrad Hoffman's articles); while the other resistors only affect the gain.A similar analysis will answer this question, but I suspect that this design will require higher performance parts to achieve the same specs which may offset the advantage of the lower component count.Quote from: gamalot on July 16, 2019, 09:21:41 pmThis is certainly another valid idea and one that was suggested already. However, in order to scale this design into the more complex scenarios (like AC excitation), it may require significant complexity (high update rates, filtering for low noise, external power supplies). All of these issues can be overcome of course; however, it would require knowledge beyond my own at this point.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: July 17, 2019, 05:00:44 pm
content: Quote from: snarlingsnoyl on July 17, 2019, 01:34:44 pmNope. Generally speaking the output impedance is measured between the output and the ground. What you're talking about is called the differential output impedance.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2019, 05:47:16 pm
content: For the DMS amplifier it is usually the differential output impedance that matters. This is what sets the noise due to current noise of the amplifier and offset due to bias currents. Also loading due to amplifier input resistance should depend on the differential resistance.So the 4x175 Ohms version should be relatively close to a matched bridge, though not perfect in every aspect. The impedance to ground is slightly different from the real bridge. For most tests I would consider it sufficient.The tests that I would do on an DMS amplifier would be mainly roughly the zero point (to be sure it's reasonably in the center of the adjustment range) zero drift and a coarse scale factor check. Reading noise would be another factor - LF noise and stability are mainly 2 ways to look at the same data. For dynamic use one might want to check the step function (this might need some electronic switching). For the scale factor I would consider something like 1% accuracy sufficient, as calibration is often done together with the DMS / load cell anyway (e.g. using weights).So the main requirement I would see is a really stable 0. This is much easier with the star like configuration - up to the point of having an extra dummy (that could use normal 1%/100 ppm/K resistors) just for this test. If in doubt one could consider to switch between different resistors at the sense outputs. Usually the output impedance should not have a large effect.

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: July 17, 2019, 06:52:59 pm
content: Quote from: Kleinstein on July 17, 2019, 05:47:16 pmIt depends : If the front end of the load cell condtionner is built around an instrumentation amplifier, the current noises at the inverting and non inverting input have 0 correlation. The offset induced by input offset current is less problematic : Although the 2 inputs do not share the same differential pair, they're made on the same chip and are often well matched, so the drifts caused by bias current flowing through the bridge compensate each others for any value of R_out.

author: BrianHG
author_rank: Super Contributor
timestamp: July 17, 2019, 08:19:42 pm
content: Quote from: snarlingsnoyl on July 17, 2019, 01:34:44 pmHave you considered digital potentiometers.Like 2(10 bit total swing, 1 on each side of the bridge) or 4(20 bit with the right dividing resistors) of these with added resistors outside them for the small narrow ranges you will be dealing with:https://www.analog.com/en/ad5293They will support up to +/-16v AC excitation which would be a problem with a normal DAC.

author: snarlingsnoyl
author_rank: Newbie
timestamp: October 18, 2019, 03:49:16 pm
content: Hello All,I've finally had some time to make a progress post for this project. I've jumped down the rabbit hole a little bit so this post may be slightly off topic and a bit long.After my last post on here, I came to the conclusion that I would need to first build a couple of tools to help with matching resistance and tempco's of resistors. Since the performance of the simulator relies heavily on no less than 7 of the resistors, these would have to be either very high quality, or very well matched. So following through CHoffman's fantastic articles (http://conradhoffman.com/mini_metro_lab.html) I decided to build both a Null Detector and a measurement bridge. I know that I could have bought these devices, but this project is about learning and figuring all of this out, so why not try to build these too!So for the null detector; following CHoffman's articles (Part 2). I shamefully copied the schematic and spun up some boards for a pre-fab enclosure. The construction consists of 2 boards; a pcba for the null detector amplifier and components, and a pcba for the front cover that also completes the enclosure's shielding with a ground pour. The null detector runs from a 9V battery inside the enclosure and has worked very well for me. The only deviation from CHoffman's original is the use of the LTC2050 which was a newer part with slightly better performance specs, cheaper and available in a SOT-5 package which helped the pcba layout.And for the bridge, I built this one from some perf board and a switch to adjust the excitation from 9V, 18V, 27V or an adjustable linear regulator capable of 0.05V-7V. I followed CHoffman's designs for the brass clips from his Part 3 article and also tied in some binding posts for conveniently attaching measurements to. All of the binding posts are gold plated copper, and wired through with silver plated copper wire to reduce any potential thermocouple effects.So with these tools in hand, I started the process of matching resistors for tolerance and tempco. I started out by trying to find some thermally stable resistors. This involved making (many) small purchases of generic %1 +-100ppm/°C resistors and then testing each order for a rough scale of tolerance and tempco. The setup changed slightly for the tempco testing by using some test leads to separate the resistor to be tested. Then a hot air rework tool was used to heat the resistor 100°C above ambient temperature and the change in output of the null detector was used to calculate the tempco.There is only a set of 4 resistor in the design that need to be matched to both tolerance and tempco; everything else just needs to have a very low tempco. So after I had matched the resistor tempco, I began to match resistor tolerance using the test clips on the bridge. This involved a multi-stage process where each stage goes higher on the null detector scale.I am not finished with this project however. I have had several setbacks and discoveries that have forced me to alter my designs. I will cover these in an upcoming post along with some interesting discoveries I have made about generic metal film resistor behavior. It will also cover the construction of the load cell amplifier where I encountered some more issues.

author: Thinkster
author_rank: Newbie
timestamp: May 28, 2023, 06:51:52 am
content: Any update on this project, I know it's been a few years...

author: totaltech
author_rank: Newbie
timestamp: June 08, 2023, 04:20:57 am
content: G'day,See attached schematic. Using star arrangement for the bridge eliminates zero offsets. for 175R use 2 x s102j in parallel. For the shunt resistors use low tempco 0.1% resistors with a small value pot in series to trim each resistor/range.

author: hozone
author_rank: Regular Contributor
timestamp: March 25, 2024, 09:03:23 am
content: Thank you for sharing all those information.I would like to build a simulator like this one.Find attached my LTspice simulation file and my schematic.I can find 0.1 for all resistors (175 ore two 350 in parallel) and 0.05 +-10ppm for the 10 one at a reasonable price.Do you think it will work?

author: totaltech
author_rank: Newbie
timestamp: March 25, 2024, 09:39:33 am
content: G'Day,This may or may likely not work. A lot of signal conditioners expect to see the +/- signal levels at half the excitation level not swinging around 0V.CheersRob

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2024, 09:48:03 am
content: The LTspice and Kicad plans don't really match. The Kicad one has the 10 ohm resistor upper end wrong.The circuit does not really need supper stable resistors. As a simulator the circuit is in a way that the resistor drift has rather little effect - much less than the resistance in a real bridge.Normally just 25 ppm/K resistors should be good enough in in most cases. With 10 ohm the layout part can also be relevant.When the pot is used to trim the scale factor, there is no longer the need for low tolerance for most of the resistors (maybe still the 10 ohm). Trimmers also tend to have a rather poor TC, so ideally one would avoid them and maybe have 1 selected resistor for the trim instead.

author: hozone
author_rank: Regular Contributor
timestamp: March 25, 2024, 11:24:34 am
content: Thanks!Fixed the KiCad schematic. Added a Calibration resistor instead of the trimmer.Because 0.1% does not costs a lot more, I think I'll go for 0.1% +25ppm, just to trim lessAll resistor will be 0805 SMD.If all ok I'll move to layout and post there if this schematic seems ok to you.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2024, 11:41:54 am
content: The circuit is still missing the output resistors (175 or 170 ohm). Ideally one would add separate resistors for the different settings (more with the 0 and less with 2 mV/V) to compensate for the extra 10 ohm resistors. I would also expect to have a switch setting also for 0.The scaling should not be fixed mV steps, but more in mV/V of excitation. With the current values one may get it about rigth, though rather large effect for R_cal.

author: hozone
author_rank: Regular Contributor
timestamp: March 25, 2024, 12:46:49 pm
content: Thanks,- added SIG+ as step (gain*VExc)- added output resistor, it should match the 175 + 175 outputAs for the switch settings for 0, what do you mean?

author: hozone
author_rank: Regular Contributor
timestamp: March 29, 2024, 09:34:50 am
content: Find the PCB attached. Note that there is no ground plane. Don't know if it's better to put one.

author: floxia
author_rank: Contributor
timestamp: April 24, 2024, 09:40:43 am
content: How did it perform? I am considering to give it a go!

author: hozone
author_rank: Regular Contributor
timestamp: April 24, 2024, 05:12:56 pm
content: I'll test it the next week.Anyway I've yet made a couple of changes. Like adding a fixed calib SMD resistor of 23R7 insted of the THT. And a toggle DPDT to invert the EXC without disconnecting the cables. This is usefull to simulate the "unload" of a load cell.

author: tmd63
author_rank: Contributor
timestamp: April 24, 2024, 05:33:10 pm
content: I know this is a bit late in the day but have you tried to simulate it onhttps://www.falstad.com/circuit/?I find the visual simulations work really well.

author: hozone
author_rank: Regular Contributor
timestamp: April 25, 2024, 07:29:01 am
content: Thank you for suggestion.I know falstad.com but I've simulated it in LTSpice, that is a little better than falstad.comYou can find the LTSpice simulation file above in my posts (it's the .asc file).

author: floxia
author_rank: Contributor
timestamp: April 28, 2024, 05:33:57 am
content: Please keep me updated because I'm really interested, I have been looking for such project and would be lovely to give it a go!

author: hozone
author_rank: Regular Contributor
timestamp: July 01, 2024, 02:09:08 pm
content: Hello,it works.I can not say more than this cause I've not precision instruments to test it against.Latest version has 0.0mV output, and a dip setting to allow selection of calibration Resistor or calibration Trimmer or a parallel between those.I'll post project files later on my blog and I will link here details.

author: floxia
author_rank: Contributor
timestamp: July 09, 2024, 09:12:31 am
content: Awesome job, please post your blog link when ready!!

author: hozone
author_rank: Regular Contributor
timestamp: July 11, 2024, 10:19:31 am
content: Update.I've notice that the output has a little offset, that depends on Vexc.On 3 out of 5 device I've found an offset of 0.29mV with 10V excitation, and 0.14mV with 5V excitation. That seems related. I can read the offset even on 0.0mV range output.Find attached the Excel with test on 3 devices.LTSpice simulation of course does not have this offset.Where do you think this offset comes from?Measured using AN870. (Even on UT-61e there's this offset).

author: hozone
author_rank: Regular Contributor
timestamp: July 11, 2024, 02:08:08 pm
content: Update 2.It may help.The resistance between SIG- and SIG+ is almost 350ohm when no Excitation is provider. As soon as the Excitation is provided it change from range to range up to 470ohm almost on the 2.0mV/Vexc range.Maybe something related to the PCB design?

author: hozone
author_rank: Regular Contributor
timestamp: August 01, 2024, 03:29:06 pm
content: I haven't find the reason why this happens, maybe@Kleinsteincan help me?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2024, 06:54:38 pm
content: The layout looks not that great and could be the reason the the offset / not to perfect zero. The zero path it not proper from the right point.

author: hozone
author_rank: Regular Contributor
timestamp: August 02, 2024, 07:46:09 am
content: Thank you.Attached the KiCad project.I'm not that sure that it depends on layout. Or better, It also depends on this but I've "big" difference.So i think is something related to other, like the internal resistance of the excitation voltage supply. Note that even using a battery does not make changes.Let's talk about resistance, below the measurements between SIG- and SIG+ in the different position of the switchWithout excitation voltage:- all ranges 350ohm +- 2ohmWith excitation voltage 5V:- 0mV/VExc: 350ohm +-1ohm- 0.5mV/VExc: 350ohm +-29ohm- 1.0mV/VExc: 350ohm +-59ohm- 1.5mV/VExc: 350ohm +-88ohm- 2.0mV/VExc: 350ohm +-116ohmWith excitation voltage 10V:- 0mV/VExc: 350ohm +-2ohm- 0.5mV/VExc: 350ohm +-61ohm- 1.0mV/VExc: 350ohm +-120ohm- 1.5mV/VExc: 350ohm +-179ohm- 2.0mV/VExc: 350ohm +-238ohmConsiderations:- As you see there's a LOT of difference in readings, we are speaking of almost +-30ohm per step on 5V and +-60ohm per step on 10V. So it's linear, and it should be +-6ohm per step at 1V.- Note that there's a +-ohm on ranges. It means that the output resistance depends on the REV switch, so it's like inverting the meter probes. As example on 10V 2mV/VExc i read 588ohm or if i invert the probes i read 112ohmI've try it in a simpler way using a breadboard. The behavior is similar.I miss the knowledge to understand this phenomenon. Have you got any clue with this happens?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2024, 08:03:04 am
content: When measuring the resistance with the excitation still active, the extra voltage will in most cases interfere with the resistance measurement. How much depends on the meter. So different meters can read different things and nothing to worry about. Ideally one should do the resistance test with a 0 V excitation and thus the excitation terminals shorted.Another point can be leakage current between the excitation and meter. This would mainly be an issue with a bench meter and mains powered supply, but also the isolation of some of the cheap banana wires is surprisingly poor (can reach the low Gohm range).P.s.:The layout can cause a small voltage to be read when the switch is in the zero position. So the zero is not absolute accurate. This would be on the order of maybe 1 ppm of the excitation, so one may no notice this very much.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 02, 2024, 12:36:44 pm
content: HelloFor 350 and 120 ohms Micro measurement sale high precision foil resistor as MR2-350-128 with +/- 0.005 % in half bridge configurationI use them for calibrationwiring and connectors as ''banana'' shall be made with Hight quality componentRegardsOS

author: hozone
author_rank: Regular Contributor
timestamp: August 02, 2024, 01:14:16 pm
content: Thank you!So, I will not worry about the resistance measurement, mostly because without excitation applied I read 350ohm +-2ohm between ranges, that is ok to me.Also, thank you for making me notice the zero position drift due to layout.What sounds strange to me is that:- the absolute error is almost the same between rages, and it's twice the mV absolute error in range 0.0- the absolute error is linear and depends on the excitation voltageThis seems something not fully related to component quality/accuracy, but to the design of the device. Am I wrong saying this?Excitation 10.024V readings (expected, read and abserr in mV)range expected read abserr0.0 0.000 0.181 0.1810.5 5.012 5.311 0.2991.0 10.024 10.316 0.2921.5 15.036 15.314 0.2782.0 20.048 20.320 0.272Excitation 4.999V readings (expected, read and abserr in mV)range expected read abserr0.0 0.000 0.087 0.0870.5 2.500 2.647 0.1481.0 4.999 5.146 0.1471.5 7.499 7.643 0.1452.0 9.998 10.142 0.144

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 02, 2024, 07:42:17 pm
content: HelloI dont really understand the purpose of your setup I work with strain gage since 20 yearsYou want to generate a voltage value in mV to simulate a load on a full bridge = OKYou need a mV source as the output of your full bridge is a voltage = OKSOWhy don t use a precision voltage source and a voltage divider set ?Another strange point is the 0.1% resistor you use except if your balance the bridge / sort the resistor as even for a real strain gage bridge that mandatory to match the resistor value 0.1 % on a 350 Ohm resistor is huge value for a strain gage .RegardsOS

author: floobydust
author_rank: Super Contributor
timestamp: August 02, 2024, 08:56:09 pm
content: I think there is confusion about strain gages, they end up being highly imprecise. Their resistance can be quite close but the mechanical mounting and deflection is a mess. There is an offset error, a span error.For this reason, I've always used a multi-turn potentiometer in a simulator (bridge) to dial-in the output voltage I want. It's really only used to check hardware amplifiers and firmware for calibration.Otherwise, how much output you get, you have to measure from the actual load cell anyway instead of expecting a precise number. Temperature also causes huge shifts.

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 03, 2024, 10:29:46 am
content: HelloI agree regarding strain gage that very sensible to mounting / setup . a lot or even most strain bridge have '' zero pot '' system to set the output to zero / zero loadI work with 1/4 or 1/2 bridge and take also in consideration the linearity of the output voltage , in 1/4 bridge I use mostly 3 wire wiring to avoid wire resistance effectto simulate the output of the bridge I use a voltage precision source with a resistor divider built with foil resistorRegardsOS

author: hozone
author_rank: Regular Contributor
timestamp: August 05, 2024, 07:28:32 am
content: Thank you all.The purpose it to build a device as accurate as possible to simulate a load cell in the field, where there will not be a precision voltage source.I should use a multi-turn potentiometer to simulate the bridge and a multimeter in mV range, but i will lose prefixed steps.I know load cell are not that accurate, indeed they could come with a calibration sheet.Of course I can still live with the accuracy of the device I'm using. I'm just trying to understand why it is acting like so.Said this, still I don't understand why the circuit I'm using has an absolute error on readings when excitation is applied.Simulating the internal voltage source resistance does not produce a constant between range absolute error.Maybe I should just consider that this kind of circuit has a .003% VExc absolute error offset on all ranges. So for 10V it will be a 0.0003V, so 0.3mV offset. Like experimental results. And for 5V it will be 0.15mV.. and so on.

author: Henrik_V
author_rank: Regular Contributor
timestamp: August 05, 2024, 09:40:23 am
content: Quote from: Overspeed on August 02, 2024, 07:42:17 pmWell, the trick with a bridge simulator is the mV/V .. and it will run with AC bridge amplifiers too .(And clever bridge amplifier ADC will not measure mV, it will use the bridge voltage as ADC reference to measure the ratio)And a resistive simulator is powered by the amplifier.An alternative simulator is the use of a DAC using the bridge exitation as a reference.See Fig. 5 ofhttps://oar.ptb.de/files/download/310.20150208.pdf

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 05, 2024, 02:16:27 pm
content: HelloThanks for explanation I agree buta strain gage is a not a rubber band , a strain gage have a gage factor so resistance variation is limited and limited also by the fixation / gluing so a simulator will match the reality no need of a wide resistance variation but better to have an very accurate link between Delta resistance and output voltage of the bridgeAC bridge request ( or have in the past as HBM AC bridge ) complex electronic with some advantages but I use DC since years that work easyI link a interesting doc on 1/4 bridge and a symmetrical dual op amp circuitI agree on the output voltage to voltage ref ratio solutionRegardsOS

author: hozone
author_rank: Regular Contributor
timestamp: August 06, 2024, 07:54:27 am
content: Thanks again.The DAC, or any other active component would work better for this design.I think I'll go for a +-0.003% VExc absolute error label for this passive simulator. Even if it's a unknown behavior to me

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 08, 2024, 06:07:53 am
content: Quote from: hozone on August 06, 2024, 07:54:27 amHelloCan you provide the calculation : I'll go for a+-0.003% VExcabsolute error labelThat interestingRegardsOS

author: hozone
author_rank: Regular Contributor
timestamp: August 08, 2024, 09:43:26 am
content: Hi @Overspeed,Don't know if I'm right, and why it's like so, but:Excitation 10.024V readings (expected, read and abserr in mV)range expected read abserr0.0 0.000 0.181 0.1810.5 5.012 5.311 0.2991.0 10.024 10.316 0.2921.5 15.036 15.314 0.2782.0 20.048 20.320 0.272Excitation 4.999V readings (expected, read and abserr in mV)range expected read abserr0.0 0.000 0.087 0.0870.5 2.500 2.647 0.1481.0 4.999 5.146 0.1471.5 7.499 7.643 0.1452.0 9.998 10.142 0.144Locking at the results above (for all ranges expect 0.0):absolute error is almost 0.146(mV) when Vexc is 5V (5000mV), so 0.00292%, let's say 0.003% * VExc => 0.15mVabsolute error is almost 0.285(mV) when Vexc is 10V (10000mV), so 0.00285%, let's say 0.003% * VExc => 0.3mVNote that absolute error for 0.0 range is almost 1/2 (0.003% * VExc)

author: hozone
author_rank: Regular Contributor
timestamp: August 20, 2024, 03:47:59 pm
content: Little updated.I've notice the 0.272mV shift even on simulator on the R1/R7/R5/R8 branch, given a Vexc of 10V. (see image).Is it somehow correlated?