[chbot] Quad chock fish challenge

[Mark Atherton]

Hi Martin,

Interesting. I have seen the 33 ohm pad value mentioned in some load-cell manufacturer data (found this morning).

Wallace,

Evil intent? Most interested to know what that might be !!

I will keep poking when time permits; the network is more complex than anticipated, and I have found some interesting reading on the subject.

At the end of the day, the activity is purely academic; quite an interesting puzzle.

Thanks.

Mark




> On 24 July 2021 at 09:39 Martin Towers <tinytowers at xtra.co.nz> wrote:
> 
> 
> Hi Mark,
> This may not be a full bridge, it may have 1, 2 or 4 active arms and
> precision resistors elsewhere. In the 2 active arm, he dummy arm may
> use different values to the active arm.
> 
> There may also be a trim to bring the reading to zero.
> 
> Try feeding voltage in across black and yellow and see what you get.
> 
> As Andrew says it does look like there may simply be 33 Ohms each in
> series with black and yellow. Maybe???
> 
> Martin
> 
> 
> On Sat, 2021-07-24 at 06:35 +1200, Mark Atherton wrote:
> > Hi Andy,
> > 
> > What you say is true, but then looking into R1 from the outside
> > would 
> > result in a different value than R2.
> > 
> > I am wondering if there are extra components in the network between 
> > RED-WH and BLK-YELL that aren't obvious from black-box testing.
> > 
> > -Mark
> > 
> > On 24/07/2021 12:42 AM, Andrew Dachs wrote:
> > > 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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