Load Cell

Objective: take the load cell from a 5kg scale, and have a microcontroller reading.

Approach: will tap into the load cell's 4 wires and use an Instrumentation Amplifier to get the signal in the voltage range the microcontroller can read. My microcontroller has a 10 bit analog to digital converter mapped 0-5v.

This is the scale I am using. It is sold as a 5kg max load.

The load cell has a sticker that reads 5kg. It uses two AA batteries.

This is the circuit board inside the scale. Note wire color & labels in the left.

Red : E+

Black: E-

Green: I+

White: I-

I will assume that E stands for excitation and I for Input.

Probing the circuit with an oscilloscope during normal scale operation, I measure an excitation voltage of 2.10v. That is around 200mV below battery voltag. I did not observe an alternating polarity in the excitation voltage.

My Circuit

This is my circuit. I selected a Texas Instruments INA122P as my Instrumentation Amplifier. And I'm using a 5v the excitation voltage.

Results

I performed a "load swipe" with 4 configurations. With a gain of 9761 and 4883 and swapping the polarity of I+/I- going to the op amp IN+/IN-. Convention: POL+ means I'm connecting I+ to opamp IN+ and POL- means that I am swapping I+ and I- .

As you can see in the graph below, I could not find gain & polarity combination that would work in the weight full range. It seems like one polarity works best for higher weights (over 400g) and the opposite polarity works best for less weight (under 300g)

Am I doing something wrong or this is the nature of the load cell I have?

Assumptions/Relevant Info:

* When I take my circuit out and put the original back in scale still works fine. Meaning load cell not damaged.

* I don't see a resistor in the load cell that would let me balance its internal Wheatstone bridge.