Undergrad The multimeter is not displaying mv of strain gauge in circuit

[Micheal_Leo]

i have made the wheatstone bridge( 3 resistors and 1 strain gauge) circuit but multimiter is not displaying anything , please need guide

 

[Baluncore]

... but multimiter is not displaying anything ...

You have an overload on the mV scale. Switch to DC volts to see what voltage you have.

Please provide the circuit you are using with all resistor values.
What is the resistance of the strain gauge?

 

[Micheal_Leo]

You have an overload on the mV scale. Switch to DC volts to see what voltage you have.
Please provide the circuit you are using with all resistor values.
What is the resistance of the strain gauge?

please find the attached schematic diagram

 

[Baluncore]

The resistor ratios must match.
Is that a pair of 220R = 220 ohm resistors in the top row, making half the supply ?

Below left is another 220R resistor, but it should match the 120 ohm strain gauge, so it also makes about half the supply. Then mV range should work.

Replace the bottom left 220R with a 120R resistor.

 

[Micheal_Leo]

The resistor ratios must match.
Is that a pair of 220R = 220 ohm resistors in the top row, making half the supply ?

Below left is another 220R resistor, but it should match the 120 ohm strain gauge, so it also makes about half the supply. Then mV range should work.

Replace the bottom left 220R with a 120R resistor.

yes half supply,i do not have 120 ohm resistors,.can i use 100+ 22 ohm resistors in series?

 

[berkeman]

please find the attached schematic diagram

Why are you using such a high bias voltage? Maybe switch to a single 1.5V AA battery to avoid wasting so much power...

 

[Micheal_Leo]

Why are you using such a high bias voltage? Maybe switch to a single 1.5V AA battery to avoid wasting so much power...

actually i have limited resources so have to do in limit resource

 

[Baluncore]

yes half supply,i do not have 120 ohm resistors,.can i use 100+ 22 ohm resistors in series?

Yes.

 

[Micheal_Leo]

Yes.

THE Attached results are mv and Voltage , please see this , i will connect it to hx711, these results before force not applied , please guide that should i consider mv or voltage to transfer it to arduino to study stress in that part.the PLA printing material not follow hookes law i have applied force on part for testing so little bending is present in part

 

[Baluncore]

I do not know where you are going with this.
Do you see any change in the mV reading when you flex the strain gauge?

If you see only a small change, you may need to adjust the 22R resistor, to better balance the bridge, to get closer to a zero difference voltage output. Then you can use more gain in the hx711 without the amplified DC offset hitting the voltage limits.

The differential voltage output that now goes to your mV meter, will go to the hx711 differential inputs, INA+, INA-. The bridge drive voltage will probably come from the hx711, AVDD and AGND.

 

[Micheal_Leo]

I do not know where you are going with this.
Do you see any change in the mV reading when you flex the strain gauge?

If you see only a small change, you may need to adjust the 22R resistor, to better balance the bridge, to get closer to a zero difference voltage output. Then you can use more gain in the hx711 without the amplified DC offset hitting the voltage limits.

The differential voltage output that now goes to your mV meter, will go to the hx711 differential inputs, INA+, INA-. The bridge drive voltage will probably come from the hx711, AVDD and AGND.

i see change in mv but also my finger burn accidently comes near to strain gauge , strain gauge get hot during this phenomena mv voltage out of control like it kept increasing ,

should i give 5V , right now i am giving 9V

 

[Baluncore]

... i see change in mv but also my finger burn accidently comes near to strain gauge ...

Too much voltage to the bridge circuit.
Too much bridge current.
Too much heating of the strain gauge resistance.
The gauge resistance is temperature dependent, so balance changes.
Use less voltage, use AVDD and AGND from the hx711.

 

[Micheal_Leo]

Too much voltage to the bridge circuit.
Too much bridge current.
Too much heating of the strain gauge resistance.
The gauge resistance is temperature dependent, so balance changes.
Use less voltage, use AVDD and AGND from the hx711.

thank you very much, hx711 supplying voltage is 5v, i shoud use dc power supply and give 1.5v to both hx711 and wheatstone brdige

 

[Baluncore]

The hx711 can run on between 3 and 5 V. If you use the same voltage as your controller, the digital signal voltages will all match without problems.

The hx711 has a regulator that provides the voltage drive to the bridge. The voltage is set by the ratio of R1 and R2 on the hx711 reference PCB design in the data sheet. Use what it has now, or select resistors that set the bridge voltage to somewhere between 1.2 and 2.5 volts.

 

[Micheal_Leo]

The hx711 can run on between 3 and 5 V. If you use the same voltage as your controller, the digital signal voltages will all match without problems.

The hx711 has a regulator that provides the voltage drive to the bridge. The voltage is set by the ratio of R1 and R2 on the hx711 reference PCB design in the data sheet. Use what it has now, or select resistors that set the bridge voltage to somewhere between 1.2 and 2.5 volts.

i have voltage regulator so have intention to supply 3v to both bridge and hx711

 

[Baluncore]

i have voltage regulator so have intention to supply 3v to both bridge and hx711

If you increase the bridge drive voltage by 1%, you get a 1% increase in the difference voltage from the bridge, for the same strain. That is a problem with accurate 24 bit measurements.

The A-D converter is ratiometric. It uses the bridge drive voltage as a full-scale reference, and so automatically corrects for variations. That is why it has the voltage regulator for the bridge drive on the same chip as the A-D converter. If you do not use that on-chip regulator, you will get scale errors in the strain readings. Read the data sheet, and follow the advice.

 

[Micheal_Leo]

If you increase the bridge drive voltage by 1%, you get a 1% increase in the difference voltage from the bridge, for the same strain. That is a problem with accurate 24 bit measurements.

The A-D converter is ratiometric. It uses the bridge drive voltage as a full-scale reference, and so automatically corrects for variations. That is why it has the voltage regulator for the bridge drive on the same chip as the A-D converter. If you do not use that on-chip regulator, you will get scale errors in the strain readings. Read the data sheet, and follow the advice.

please can you suggest best A-D converter

 

[Baluncore]

please can you suggest best A-D converter

You cannot afford the best.
Use the hx711, because it is available, cheap ($2), and comes mounted on a PCB.

 

[Micheal_Leo]

You cannot afford the best.
Use the hx711, because it is available, cheap ($2), and comes mounted on a PCB.

i have try to do it , somehow it work as i expected , the video attached , secondly the follwoing hx711 amplifier i have , i am very much confused how to attach wheatstone bridge with this hx711 , please need your guidance

video about strain gauge and wheatstone bridge

i have try to draw diagram of connecting , please check

 

[Baluncore]

I think, in the Weixin picture, at the analogue end of the PCB ...
A+ and A- are the bridge output terminals, (your old meter connections).
GND is probably analogue ground to the bridge.
Out+ is the regulated power to the bridge.
You can check those by following the terminals to the chip pins.

Do you have a copy of the hx711 data sheet?
It shows the connections to the bridge on page 1, fig 1. Also page 6, fig 4.
https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf