[chbot] Quad chock fish challenge

[Andrew Dachs]

Hi mark, it’s a bit late and I need a piece of paper but I think the essence is R1 must equal R3, R2 must equal R4 but doesn’t follow that they all have to be the same value. R1 might be different from R2 and R3 different from R4. You’ve done enough measurements to work them all out. 
Andy


Sent from my iPhone

> On 23/07/2021, at 8:37 PM, Mark Atherton <markaren1 at xtra.co.nz> wrote:
> 
> Just acquired a small pile of 250kg load-cells, and everything was going swimmingly until I looked a little too closely.
> 
> A) 5.0V DC excitation voltage is applied between Red and White wires. At rest, and with respect to White, both Black and Yellow are at 2.50V. And yes, I have checked that there is 0V between BLK and YEL. Apply some force to the unit and the bridge unbalances just as it should and a 100kg or so produces a 12mV imbalance which sounds about right from previous experience.
> 
> B) With the unit completely disconnected, the resistance looking between YEL-RED, YEL-WH, BLK-WH and BLK-RED are each 600 ohms ± sod-all.
> 
> C) I was hoping that I wouldn't be pushing my luck too far by assuming that each network leg would be 800 ohms. (600R external = 800R internal in parallel with (3 * 800R internal).
> 
> D) But alas no, BLK-YEL is 700R and RED-WH is 765R. This rather implies that the upper resistor pair are different values to the lower resistor pair. However if that was true, then test A would not indicate a 50% voltage division.
> 
> Also, if each internal limb was 800R, then there should be 800R in both cases of D).
> 
> So clearly applying 5V excitation is different from using an ohm-meter, so I dropped excitation voltage to 260mV and the mid-point voltage was still 50%. I also heated the unit from 17c up to around 27c without any significant change.
> 
> Swapping polarities of just about everything does not affect results, so it looks like a purely resistive network (unless there is something weird about strain-gauges I am unaware of).
> 
> Also tried exciting BLK-YEL which made no difference. BLK-RED and BLK-YEL still measured 2.5V each (per A above).
> 
> I am running out of things to poke at from the outside, and am quite intrigued about the fundamental (and incorrect) assumption I have made with all of these tests.
> 
> The units appear to work as expected, but can't find a datasheet (units are marked 250kg 1420230).
> 
> Suggestions please.
> 
> -Mark
> 
> <load cell.gif>
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