[chbot] Quad chock fish challenge

[Mark Atherton]

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

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