Hi, I am trying to setup a Wheatstone bridge with just strain gauges (full bridge type 3). I am attaching it to a carbon fibre rod, so the strain is very tiny. When I just measure the output of the bridge the values range from 0.5-0.1mV, therefore I want to amplify the signal. For amplification I use the INA125P instrumental amplifier. I use a gain of RG=10kOhm so the gain should be 10 (4+60kOhm/10kOhm). However in combination with the Wheatstone bridge it does not measure any changes when the rod is bent. The output voltage is constant at 2.5V.

To make sure the amplifier circuit is correct I proceed to use a constant voltage supply instead of the Wheatstone bridge. These are my measurements using a power supply as the input signal (VIN) and to power the system (constant 22V):

RG (kOhm)	10	10	10	10	10
Gain (theoretical)	10	10	10	10	10
VIN (V)	1	0,5	0,25	0,1	0,05
Vo predict	10	5	2,5	1	0,5
Vo measured	5,9	3,2	1,7	0,52	0,4

The output voltages are not what I expect and I have tried with a range of RG’s as well. Does anyone know what the problem could be? I use an oscilloscope to measure, and the readings do fluctuate more at lower input voltages, probably due to too much noise.

This is my circuit:

 

hi eliz,
Welcome to AAC,
This +v & -V is wrong,
This Vin means the bridge output not the supply.
E


e

 

hi eliz,
Check this image.
E

 

and keep in mind that negative supply is not GND. your circuit requires dual supply:

 

and keep in mind that negative supply is not GND. your circuit requires dual supply:
View attachment 335807

It can be used on a single supply but there is an appx 0.1 volt offset on the output and the output cannot swing negative.

 

hi eliz,
Welcome to AAC,
This +v & -V is wrong,
This Vin means the bridge output not the supply.
E


View attachment 335790e

Hi,
Sorry for my miscommunication, I do not power the amplifier at Vin, I just a power supply at low voltage to simulate an input, I power the amplifier at V+ and V- like you've said.

 

It can be used on a single supply but there is an appx 0.1 volt offset on the output and the output cannot swing negative.

What do you mean with dual supply? I cannot use a regular power supply to power the amplifier?

 

hi eiz,
A dual supply has 3 terminals marked..
Positive Volts, 0 Volts and Negative volts,

A simple power supply with only two terminals is not usable for your project.
Do you follow OK?
E

 

hi eiz,
A dual supply has 3 terminals marked..
Positive Volts, 0 Volts and Negative volts,

A simple power supply with only two terminals is not usable for your project.
Do you follow OK?
E

Yes that makes sense, thank you. So to power the amplifier I cannot use a unipolar supply? I need dual?

 

hi eliz,
This option is shown in the datasheet.
But it has limitations

E

 

Hi eliz,
This image shows the dual supply connects, this enables the Vout to swing +/- about zero volts [ ie: no fixed offset]
E

 

Thank you! Any chance you know how to do this for a CPX400D Dual 420 Watt DC power supply?

 

Thank you! Any chance you know how to do this for a CPX400D Dual 420 Watt DC power supply?

Or actually, I just need to connect the V+ and V- terminals and the grnd separately?

 

hi eliz,
Check this image,
E

 

hi eliz,
Check this image,
EView attachment 335872

Cool thanks! But how do you get a negative voltage for V- now? I don't see that

 

hi eliz,
Check this image,
EView attachment 335872

Could I also use the Wheatstone bridge with such a chip instead? Do you recommend it? https://www.sparkfun.com/products/13879

 

Hi.
Look at the PSU terminals.

E

 

Hi,
You can use a HX711 module, but are you able to write the Arduino MCU Code?
E
SparkFun Load Cell Amplifier - HX711

 

Hi,
You can use a HX711 module, but are you able to write the Arduino MCU Code?
E
SparkFun Load Cell Amplifier - HX711

Oke cool. It's been a while since I have... but I can learn! Would you recommend it?

 

hi,
I have used the HX711 many times.
This is a simple example of an Arduino Sketch.
E

#include "HX711.h"

HX711 scale;

void setup() {
  Serial.begin(38400);
  Serial.println("HX711 Demo");

  Serial.println("Initializing the scale");
  // parameter "gain" is ommited; the default value 128 is used by the library
  // HX711.DOUT    - pin #A1
  // HX711.PD_SCK    - pin #A0
  scale.begin(A1, A0);

  Serial.println("Before setting up the scale:");
  Serial.print("read: \t\t");
  Serial.println(scale.read());            // print a raw reading from the ADC

  Serial.print("read average: \t\t");
  Serial.println(scale.read_average(20));      // print the average of 20 readings from the ADC

  Serial.print("get value: \t\t");
  Serial.println(scale.get_value(5));        // print the average of 5 readings from the ADC minus the tare weight (not set yet)

  Serial.print("get units: \t\t");
  Serial.println(scale.get_units(5), 1);    // print the average of 5 readings from the ADC minus tare weight (not set) divided
                        // by the SCALE parameter (not set yet)

  scale.set_scale(2280.f);                      // this value is obtained by calibrating the scale with known weights; see the README for details
  scale.tare();                        // reset the scale to 0

  Serial.println("After setting up the scale:");

  Serial.print("read: \t\t");
  Serial.println(scale.read());                 // print a raw reading from the ADC

  Serial.print("read average: \t\t");
  Serial.println(scale.read_average(20));       // print the average of 20 readings from the ADC

  Serial.print("get value: \t\t");
  Serial.println(scale.get_value(5));        // print the average of 5 readings from the ADC minus the tare weight, set with tare()

  Serial.print("get units: \t\t");
  Serial.println(scale.get_units(5), 1);        // print the average of 5 readings from the ADC minus tare weight, divided
                        // by the SCALE parameter set with set_scale

  Serial.println("Readings:");
}

void loop() {
  Serial.print("A= ");
  Serial.print(scale.get_units(), 1);
  Serial.print("  Avg= ");
  Serial.println(scale.get_units(10), 1);

 // scale.power_down();                    // put the ADC in sleep mode
  delay(1000);
 // scale.power_up();
}