How to calculate Rg resistor?

Hi,

How to calculate INA125 Rg resistor? I have 2 half-bridge sensors 2 x 50KG = 100KG
I have constant 4.80V and Arduino 10bit resolution. 100/1023=0.097KG per bit. How to know gain to get 1023 = 100kg? Now I use 100R resitor, but this resistor I think is too big. Need smaller ~10 - ~30ohm.

Thanks for help!!!

Einars

I'd first advise that you put away the INA125 and get an HX711 module instead. Much, much more precision (24 bits) than you can get with an InAmp alone.

What you really need is a variable resistor to set the INA right. Set the resistance high, watch the INA's amplified output with a multimeter, and then keep lowering the resistance until you see the output voltage change. Too high of a resistance will show an output around 2.5V, too low a resistance will be ~4.2V. But you are correct, it's typically somewhere in the 30 - 100ohm range. To get this right, from experience, you have to get it within +/- 2 ohms for decent sensitivity from the load cell.

Is it enough 100ohm Trim Pot?

I’d first advise that you put away the INA125 and get an HX711 module instead

As I recall, the other advantage of the HX711 is it only requires a single ended supply instead of +/-15V.

Einars:
Hi,

How to calculate INA125 Rg resistor? I have 2 half-bridge sensors 2 x 50KG = 100KG
I have constant 4.80V and Arduino 10bit resolution. 100/1023=0.097KG per bit. How to know gain to get 1023 = 100kg? Now I use 100R resitor, but this resistor I think is too big. Need smaller ~10 - ~30ohm.

Thanks for help!!!

Einars

If you have constant 4.8volt, there is something wrong.
Gain should not change the output voltage.
At least not if the bridge is in balance.
Show how you have connected it.
Leo..

Note: The OP didn't state that he is powering the INA125 from a +/- 15V P.S.

@raschemmel.
The resolution with this chip, running on 5volt, and with Arduino's 10-bit A/D, is not that great.
Everything has to be just right, and I think that's where most tinkerers go wrong.
Leo..

My point was that he should be running the INA125 on the specified voltage which is +/-15V, not 5V.

The INA125 should be ok with 5volt.
You're not getting the full resolution of the A/D if you do. Maybe only ~700values.
If you run the chip on a higher voltage, you can improve things only by 40%.
At the cost of extra hardware (dual supply, pin protection etc.)
Not worth the trouble.
Leo..

@ OP
Did you connect the two sensors in opposite phase.
Is your bridge running on Arduino's 5volt rail, 3.3volt. or 2.5volt Vref.
Did you you connect pin 4 to pin 14, 15, or 16.
Show us how you have connected it.
Leo..

Wawa:
If you have constant 4.8volt, there is something wrong.
Gain should not change the output voltage.
At least not if the bridge is in balance.
Show how you have connected it.
Leo..

I powered by 9V battery through Arduino Nano. Arduino Nano 5V output is 4.80V. Between two load cells (red wires) I have -0.00V and resistance 1011 ohms. INA125p OUTPUT is 76.1mV (without load). Rg is 100ohm resistor.

+5V - White|black and -5V Back|white

EDIT

Maybe better is HX711 board? But I do not have full bridge but only half. Is HX711 working with half bridge?

So you did NOT connect them as on this picture, but swapped white and black of ONE load cell.
And pressing down on one cell is increasing the output voltage or digital value.
Pressing down on the other one is also increasing the voltage/value.
If the voltage is not increasing, or going down, swap the two red wires going to pin 6 and 7.
100ohm gain resistor should be ok.
Leo..

I have swapped wires and voltage increases. It is ok. Thanks! So you say 100ohm is also ok.

Zero weight should be ~0.5volt or a digital value of ~100.
You can only change that mechanically by bending, adding/removing weight, etc.

Once you have the zero weight right, put a known load on the scales.
See how many volts increase you have.

Change the gain resistor so that when the maximum weight you want to weigh outputs about 4.5volt.
Too much gain, and you reach that voltage already with a smaller weight.
Leo..

@Einars,
FYI,

You shouldn't use "-5" to indicate GROUND. "+5" is ok for the positive but "-5" means NEGATIVE 5V (NOT GND) and I am pretty sure you are not using a split rail +/- 5V supply with three leads,
-5V , GND, and +5V. The wire you labeled "-5" , should be labeled Gnd. The reason the chip pin is label "-V" is because the chip is SUPPOSED to be powered by a split-rail +/-15V supply. The fact that you chose not do that means that your -V is actually your Ground. Labeling it "-5" could mislead others into thinking you are using a splitrail +/-5V supply.