How to amplify output voltage from load cell?help please:(

[Wayne0809]

Hi, I am doing a subsystem of a project which I have to measure the weight of some food using load cell. However, I don't know how to amplify the output voltage from the load cell. The output voltage of the load cell is only 0.XXmV up to few mV. I tried to use LM358 but it doesn't work. Could anyone help or give me some idea how to amplify the output voltage from load cell as I really tried a lot of method but still failed

 

[DerStrom8]

I tried to use LM358 but it doesn't work.

Can you elaborate? What circuit did you use? How did you connect it?

An op-amp (like the 358) WILL work, if connected properly. My guess is you didn't use the right circuit. There are a wide variety of op-amp circuits out there, where did you get yours?

 

[ericgibbs]

Hi, I am doing a subsystem of a project which I have to measure the weight of some food using load cell. However, I don't know how to amplify the output voltage from the load cell. The output voltage of the load cell is only 0.XXmV up to few mV. I tried to use LM358 but it doesn't work. Could anyone help or give me some idea how to amplify the output voltage from load cell as I really tried a lot of method but still failed

hi,
You really need an Instrumentation amplifier for a Load Cell bridge.

This circuit works for me.

E

 

[Wayne0809]

I simple use a lm358 with 2 feedback resistor to get a non inverting value and the gain I want because I though I only have to design the resistor value to get different gain? What is the criteria to design an amplifier?We cant use only one amplifier and choose different resistor to get different gain?

 

[ericgibbs]

I simple use a lm358 with 2 feedback resistor to get a non inverting value and the gain I want because I though I only have to design the resistor value to get different gain? What is the criteria to design an amplifier?We cant use only one amplifier and choose different resistor to get different gain?

hi,
Please post your circuit diagram for the LM358 also details of your load cell.

 

[Wayne0809]

hi,
You really need an Instrumentation amplifier for a Load Cell bridge.

This circuit works for me.

E

Hi,Thanks for your reply and circuit example. But why should we use so many amplifier instead of just one amplifier?Can we use one amplifier and change the resistance value to get different gain?Can I use any excitatiom voltage for the load cell?

 

[ericgibbs]

Hi,Thanks for your reply and circuit example. But why should we use so many amplifier instead of just one amplifier?Can we use one amplifier and change the resistance value to get different gain?Can I use any excitatiom voltage for the load cell?

hi,
There is only one instrumentation amplifier, an AD623. The dotted line box encloses the internal circuit of the AD623.

The second amplifier is a MCP6002 rail to rail OPA [ operational amplifier]

Please give me details of the load cell specification.

 

[Wayne0809]

hi,
Please post your circuit diagram for the LM358 also details of your load cell.

Hi,this picture is similar to my circuit just that I use 10k for R5 and 10ohm for R6 to get 1001 gain. The non-inverting output of my load cell I connected to non-inverting input of the opamp and inverting output of the load cell I connect it to the inverting input of the opamp.

This is the spec of my load cell.Thanks again
https://www.phidgets.com/products.php?product_id=3133_0

 

[Wayne0809]

hi,
There is only one instrumentation amplifier, an AD623. The dotted line box encloses the internal circuit of the AD623.

The second amplifier is a MCP6002 rail to rail OPA [ operational amplifier]

Please give me details of the load cell specification.

Hi,

Here is the spec of my load cell. Is there any other circuit can amplify the signal because I cant get the MP6002 Ic in my lab and we have limit components for our projects
https://www.phidgets.com/products.php?product_id=3133_0

 

[ericgibbs]

hi Wayne,
Downloaded your cell data.

The load cell is a bridge type requiring a 5Vdc excitation with a output of 1mV/V, 5Kg max so thats 5mV FSD.

So I would recommend that you use a AD623 if possible, it works well on a single 5V supply.

What type of amplifiers do have access too.?

E.

EDIT:
How do you plan to display the weight.??
Also what power supplies do have.?

 

[kubeek]

View attachment 73798
Hi,this picture is similar to my circuit just that...

Why do you post something that is similar to your circuit, but essentially is compeltely different? And of course you most likely have the load cell connected wrong, but no one will ever know beacuse you don´t show how you got it connected.
Even if you had the connections such that it could work, it probably won´t, because the LM358 has input offset voltage of 2-3mV, so your output would shift by somewhere between 3V more and 3V less, which with a 5V supply isn´t too good. That is just a part of why a more complicated circuit is ussually better than a single opamp amplifier.

 

[Wayne0809]

hi Wayne,
Downloaded your cell data.

The load cell is a bridge type requiring a 5Vdc excitation with a output of 1mV/V, 5Kg max so thats 5mV FSD.

So I would recommend that you use a AD623 if possible, it works well on a single 5V supply.

What type of amplifiers do have access too.?

E.

EDIT:
How do you plan to display the weight.??
Also what power supplies do have.?

Hi,basically my lab have only LM311,LM324,LM339,LM358,LM741 and LM747. I tried to get those ic chip u mentioned from some local electronic components shop but I can only get AD620. Can use AD620 and LM358 instead?

My main purpose is actually measure the weight and using ADC of LPC1769 convert the analog value to digital. When the weight of something drop to a preset value, my program will do some action like turn on led or motor.

What you mean what power supplies do I have?Voltage?I can get 3V,5V,9V and 12V I suppose.

Thanks again

 

[Wayne0809]

Why do you post something that is similar to your circuit, but essentially is compeltely different? And of course you most likely have the load cell connected wrong, but no one will ever know beacuse you don´t show how you got it connected.
Even if you had the connections such that it could work, it probably won´t, because the LM358 has input offset voltage of 2-3mV, so your output would shift by somewhere between 3V more and 3V less, which with a 5V supply isn´t too good. That is just a part of why a more complicated circuit is ussually better than a single opamp amplifier.

Hi, attached is my circuit. What criteria I have to consider to design a suitable opamp for my load cell?

Thank you

 

[ericgibbs]

Hi, attached is my circuit. What criteria I have to consider to design a suitable opamp for my load cell?View attachment 73801

Thank you

hi.
I have looked at the LPC1769 data, I see that ideally you want a 3V ADC input voltage from the Load cell amplifier.

You could use a AD620, look at the attached image.

The LM324 could be used to replace the AD705.

To show the problems with your posted LM358 circuit, I will post later.
E.

 

[ericgibbs]

hiWayne,

Look at your amplifier for the LM358, its simulated using LTSpice.

As you can see it will never work as you expect.

E.

 

[kubeek]

Hi, attached is my circuit. What criteria I have to consider to design a suitable opamp for my load cell?View attachment 73801Thank you

How is the excitation voltage related to the opamp supply, do they have a common ground?
Either way, you can´t connect the amplifier like that, the gain would be completely different from what you calculated. One side of the cell needs to go to ground, as that is the reference for the amplifier, and the other side of the cell goes to the +input of the opamp.
Now, if the excitation voltage shares the ground with the opamp, then you need to use a differential amplifier (which is what you should use to measure differential signals in the first place). And the best configuration for that is an instrumentation amp, so either make your own from opamps, or use a chip like eric suggests that will do it all for you.

 

[Wayne0809]

hi.
I have looked at the LPC1769 data, I see that ideally you want a 3V ADC input voltage from the Load cell amplifier.

You could use a AD620, look at the attached image.

The LM324 could be used to replace the AD705.

To show the problems with your posted LM358 circuit, I will post later.
E.

Hi,my lpc is connected as a module and my V ref and AGND is fix I supposed. So can I use only AD620 without AD705? In other words, I think I can only apply an amplified Vin the ADC pin of LPC.Attached is the LPC user manual which I referring. Why I need AD705 to get the AGND?Thank you

 

[ericgibbs]

Hi,my lpc is connected as a module and my V ref and AGND is fix I supposed. So can I use only AD620 without AD705? In other words, I think I can only apply an amplified Vin the ADC pin of LPC.Attached is the LPC user manual which I referring. Why I need AD705 to get the AGND?Thank you

hi,
The AD620 is designed to work from dual power supplies, if you have a -5V supply, use +5V and -5V to power the AD620.

The circuit from the AD620 d/s shows how to use a AD620 with a single power supply, but it requires the AD705 or LM324 to offset the Vout from the AD620.

You do not need to connect the output of the AD705 to your LPC, this would mean that Vout [pin6 of the AD620] would be at +2.5V.

Its is possible to change the AD705 resistors to give an output of ~+1V, so the Vout could swing from +1V thru to +3V, the program software would have to 'remove' the +1V offset.

Do you follow OK. I could post a circuit if you want to try this way.>

I use a AD623 which is designed to work from a single power supply. [nice and simple!]

E.

 

[ericgibbs]

hi wayne,

This is one option for the AD620 with a single +5V supply.

Note the test ramp is from -0.5mV thru +5mV, its IMPORTANT that a load cell bridge amp system can go below 'zero weight' on the display.

The below zero display is required so that the operator can be confident the weight on the scale is truly zero and not just the AD620/amp in a saturated state, showing 00.0kg

The program software will have to remove the small positive offset weight.

I also allow a 10% over weight on the Vout to the ADC input, again you want to display a weight over the 5Kg on some occasions. [ your load cell will also be rated for 10% over weight]

OK.?

E.

EDIT:
I should also point out the load cell will need some metal work to make a base on which you place the weights.

The 'weight' of this metal base will move the load cell mV output above zero, so the ZERO pot will need to be adjusted,

 

[Wayne0809]

hi wayne,

This is one option for the AD620 with a single +5V supply.

Note the test ramp is from -0.5mV thru +5mV, its IMPORTANT that a load cell bridge amp system can go below 'zero weight' on the display.

The below zero display is required so that the operator can be confident the weight on the scale is truly zero and not just the AD620/amp in a saturated state, showing 00.0kg

The program software will have to remove the small positive offset weight.

I also allow a 10% over weight on the Vout to the ADC input, again you want to display a weight over the 5Kg on some occasions. [ your load cell will also be rated for 10% over weight]

OK.?

E.

EDIT:
I should also point out the load cell will need some metal work to make a base on which you place the weights.

The 'weight' of this metal base will move the load cell mV output above zero, so the ZERO pot will need to be adjusted,

Hi,Thank you for your reply and is really helpful but I am still a bit confusing. All LM324 used in your circuit is to offset the output voltage?What is the output voltage range of your circuit?The Rg of your AD620 is adjustable depend what gain we want?I don't really need to show the weight, I just have to set a preset value and measure whether the object is under a preset value, so can I connect my circuit without LM324? If I connect the circuit shown in attached picture, my output voltage from AD620 is 1.67mV when there is nothing on the cell,is this the offset voltage?