Hello community, I am stuck on something. I am currently working on a project which requires the output of a strain gauge. Since its deflection is very small, I thought of amplifying the output using a LM358P Op-Amp in Non - Inverting mode. I use a setup which is a simple voltage divider circuit as per attached in the image. The output of the circuit is given to the non - inverting setup whose circuit is also attached in the image. Now the voltage is getting amplified, but since the deflection is very small, the amplified output is of no use. Can anyone help me out with the following:
i) Is the above process correct for the amplification of a strain gauge?
ii) if no, can anyone help me through the correct steps and process to be followed.

This is the first post in this forum, so kindly bare with my errors in asking the question and a BIG Thanks in advance!

 

Hi AF,
Welcome to AAC.
The problem with your circuit, you have a high offset voltage on the input of the OPA.
What is the specification of the gauge. ie: working voltage and mV/V and maximum weight.?
E

 

Often the strain gauge is put in 4-resistor bridge circuit to cancel the DC offset.
This can consist of three resistors plus the strain-gauge, each with a resistance equal to the strain-gauge resistance under no load,
You then differentially amplify the voltage across the bridge (a differential instrumentation IC amp generally works best for that).

 

Don’t think of amplifying anything with an LM358. Buy an op-amp that isn’t 50 years old and doesn’t have a huge input offset voltage.
Microchip do some nice precision low-voltage op amps, with part numbers like MCP6051, MCP6061, MCP6071 etc.

 

I'd suggest having a look at some instrument opamps like the INA121 and use the strain gauge in a wheatstone bridge.

Digikey search: INA121

Hope that helps

 

As far as I could tell, I got away with using TL084 op-amps wired as an instrumentation amplifier for a couple of strain gauge sensor applications. I matched the resistors to the limits of my ohmmeter. But for a more recent strain gauge project, I used an INA instrumentation amp since I didn't need to build it on an existing board that had been laid out for quad op-amps.

 

Hi AF,
Welcome to AAC.
The problem with your circuit, you have a high offset voltage on the input of the OPA.
What is the specification of the gauge. ie: working voltage and mV/V and maximum weight.?
E

Hello gibbs, I bought the strain gauge from a local vendor, I only know its resistance which is 120 ohms.

 

Often the strain gauge is put in 4-resistor bridge circuit to cancel the DC offset.
This can consist of three resistors plus the strain-gauge, each with a resistance equal to the strain-gauge resistance under no load,
You then differentially amplify the voltage across the bridge (a differential instrumentation IC amp generally works best for that).

Hello @crutschow, I was struggling with a voltage divider network, now the wheatstone bridge works perfectly. I just require to select a Op-Amp.

 

Don’t think of amplifying anything with an LM358. Buy an op-amp that isn’t 50 years old and doesn’t have a huge input offset voltage.
Microchip do some nice precision low-voltage op amps, with part numbers like MCP6051, MCP6061, MCP6071 etc.

Hello @Ian0 , I would like to know a website or a software where I can simulate and see the IC's that you have given above.

 

I'd suggest having a look at some instrument opamps like the INA121 and use the strain gauge in a wheatstone bridge.

Digikey search: INA121

Hope that helps

Hello @click_here , can you suggest me a op-amp which is of minimal cost in the same series?

 

I forgot to mention, I use a Labjack U6 to get a accurate reading of the changes. The readings I saw where having large deflections when I used the LM358. Can the offset voltage be the cause of these deflections?

 

As far as I could tell, I got away with using TL084 op-amps wired as an instrumentation amplifier for a couple of strain gauge sensor applications. I matched the resistors to the limits of my ohmmeter. But for a more recent strain gauge project, I used an INA instrumentation amp since I didn't need to build it on an existing board that had been laid out for quad op-amps.

Hello @bassbindevil , can you mention the series of the INA InAmp which you used?

 

I only know its resistance which is 120 ohms.

So how much resistance change with load do you expect?
Need that to determine the required circuit gain.

I would like to know a website or a software where I can simulate and see the IC's that you have given above.

Many, including myself, use the free LTspice simulator from Analog Devices.

 

Hello @Ian0 , I would like to know a website or a software where I can simulate and see the IC's that you have given above.

Just use the LM358 datasheet (available of many manufacturers' websites) and a calculator to work out the maximum offset and noise.

 

Hello @click_here , can you suggest me a op-amp which is of minimal cost in the same series?

I found a few other cheaper instrumentation amplifiers on google, but the real challenge is finding something that you like that is cheap *and* in stock.

 

hi AF, @AtomicFlea69
A very basic Diff OPA with a LM358.
Make up a Wheatstone bridge with the strain gauge and 3 resistors.
E

 

Assuming you don't need much bandwidth - have a look at the Microchip parts MCP60*1 where * can be 2, 5, 6 or 7 depending on how much speed you need.
All are very low offset.

 

So how much resistance change with load do you expect?
Need that to determine the required circuit gain.
Many, including myself, use the free LTspice simulator from Analog Devices.

So according to the circuit I constructed It gives me a change from 120 ohms to 120.3 - 120.4 ohms change which is good enough for me!

 

hi AF, @AtomicFlea69
A very basic Diff OPA with a LM358.
Make up a Wheatstone bridge with the strain gauge and 3 resistors.
E
View attachment 263712

@ericgibbs, if you don't mind could you explain me the circuit?

 

hi,
The LM358 is configured as a Differential amplifier, see attached link.

The 3 resistors and the strain gauge [R2 in that circuit] form a quarter Wheatstone bridge.
The simulation steps the value of R2 [strain gauge] through a small range of resistance values.
E

https://www.electronicshub.org/differential-amplifier/