Measuring Load Cell Voltage

[DuncanSteel]

Hi

I have this nice working digital kitchen wage (4 wires) that I took apart & wanted to use the load cell inside for an automatic food dispenser.

I measured resistance & voltage across all wires while applying pressure with finger on the cell. The resistance nor the voltage never changed.

Below are the results of my measurements (second attachment).

From what I searched the google, the voltage between White & green wire should be in mV, after I apply 5V to the Red & Black wire.

The question is, how comes the voltage i measure is in mV is 0 ? I used the 200mV region.

Edit: I added an NE5532 OP amp & now the output of the op amp to GND reads 0.746V, but the value is not changing if I apply pressure on the cell, so weird.

[Andy Watson]

You probably need an amplifier. To achieve a long life, strain gauges are typically operated at 0.1-0.2% strain - maximum - i.e. full scale. It is not clear how the gauges are configured (i.e. how many are active) on that beam, but as a ball-park figure you could be looking at 5-10mV full scale output for 5V excitation. What is the maximum working load for those scales?

[Andy Watson]

Edit: I added an NE5532 OP amp & now the output of the op amp to GND reads 0.746V, but the value is not changing if I apply pressure on the cell, so weird.

Schematic? Or was that included in the above diagram? I think you need to add a few more components to make the NE5532 work correctly. Also, it wil struggle to function with only a +5V supply - you need to be looking at a "rail to rail" op-amp.

[DuncanSteel]

Well I Figured out the problem.

I was using 9V battery voltage divider as a source. So I hooked up the wires to the 5V divided voltage.

As result I was adding more resistance to the circuit making the voltage divider drop the voltage to 1..V, no wonder it did not work.

After I tried this with a proper 5V source & applied pressure to the cell, I did see changes in voltage.

Without an op-amp (0,2mV without load) & (0,2-1,3mV with applied pressure)

With an op-amp 3,6 - 4V (so much fluctuation without load) (with load applied pretty steady 4,4V). I assume this is what you meant by an rail to rail op-amp.

I am still learning how those op amps work, have been watching tutorials for a week now. I kinda understand how they work & kinda dont...Im working on it.

Edit: Don`t what was causing those fluctuations earlier, tried 2 different setups with voltage divider & external 6V supply on the op-amp, got very beautiful & clean results. from 5,4V -1,3V when applied pressure on the cell.

Ill go search, see if I can find a way to supply 5V to the op amp & hope i can configure it to output 5V. I guess its the rail to rail thingie.

Anyway topic can be closed.
Thank you Andy.

[David Hess]

You do not need a rail-to-rail operational amplifier but the load cell has a balanced bridge with two excitation inputs and a differential signal output which has a common mode level about half way between the excitation voltages.  Typical sensitivities are 1 to 3 millivolts per volt of excitation.

A good DC voltmeter will work fine connected between the two output signals to verify proper operation of the load cell.

[tautech]

A recent thread holds some clues:
https://www.eevblog.com/forum/projects/powering-a-load-cell/msg973090/#msg973090