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Precision Load Cell (Wheatstone Bridge) Simulator

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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.

Any update on this project, I know it's been a few years...

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.


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