[chbot] Molten Media reminder

rudy.kremers@123gaming.com rudy.kremers at racingworld.com
Fri Jul 23 22:24:13 BST 2021

Hi all, just a reminder, we are open now.
Fyi we have a lot of appliances up for grabs. Most need a plug to be fitted to get going again.

Unit 11, 21 Michelle rd. Wigram
Open till 1pm


Sent from my Galaxy

-------- Original message --------
From: Mark Atherton <markaren1 at xtra.co.nz>
Date: 23/07/21 8:37 PM (GMT+12:00)
To: Christchurch Robotics <chchrobotics at lists.linuxnut.co.nz>
Subject: [chbot] Quad chock fish challenge

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.


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