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Load Cell Amplifier Needed

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asimzia

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Hi i am Electronics Engineering student. I am using 10Kg load cells in my final year project. But the problem is that the Load cells give very small output. My load cell gives just 0.5mV/Kg change at its output and i need to give it to microcontroller. So it needs to be amplified first.

I need a suitable amplifier for my Load Cell.

Can any one help ?????
 
hi,
Try the AD623 or another IA OPA type.

Look at this option.
 

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Will the load cell be used for tension and compression or just one or the other?

Ron
 
Thanks ericgibbs

I have tried AD624 it gives me good amplification but with noise of about 10mV at input and also there is a resolution problem

Actually i want to us my 10Kg load cell for upto 1 to 2 Kg only so i want almost 1mV signal to be amplified to 5V with a good resolution

Resolution means that i want almost 1g accuracy at output but after amplification it changes after almost 100g i-e 100g accuracy
 
OK, used only in tension. Generally the output of a load cell is specified as mV/V based on the applied excitation voltage. Typically numbers like 2 mV/V or 3 mV/V are popular with a maximum excitation voltage called out. What this means is if I have a 10 Kg load cell with an output of 2 mV/V and I apply 10 Volts excitation that with 10 Kg of force (full scale) my output will be 20 mV. That is 2 mV per Kg.

Years ago I built my strain gauge amplifiers but today I buy them. I did have good luck using the AD524 which can be found here.

As to resolution and accuracy don't confuse them. A very large factor of your resolution will be the A/D process. If your final amplified voltage runs into you u-Controller A/D you will have only so many quantization levels to resolve the voltage. I understand where you are going with only using maybe the bottom 1/5th of your load cell, however even if you scale your amp so 0 to 2 Kg = 0 to 5 volts you still face the A/D process and depending on bits the number of quantization levels.

Your accuracy will be a function of the load cell along with the overall amplification and the overall linearity.

The chip I linked to isn't inexpensive but works quite well for these applications. It also allows for a load cell bi-polar input. Few external parts and easily setup.

Hope all that helps
Ron
 
Thanks Reloadron

But what if i not use A/D converter

Actually my project is a bit simpler I need not to make weigh scale

I need some of desired weigh lets say i want just to know that if 700g weight is reached and then compare it with comparator and give microcontroller the signal 1 or 5V

So i need to know just output of load cell at say 700g and just to compare it

Any solution without ADC ?
 
Ah, OK ....

So you amplify and signal condition your signal from your sensor (load cell) and that gives us a voltage. If you have a set of precision weights to hang from the load cell that would be nice. So let's say in the tension mode you hang (weigh the hanger) enough weight to equal your 700g example. Measure your voltage. Next build a comparator circuit around the LM339 (though you don't need a quad) around what you see here. Allow for a variable reference like a pot. The comparator will give you a High or Low depending on if your input voltage (load cell) is above or below your reference voltage set by the pot. This is sort of a yes/no approach. My choice would still be the AD724 instrumentation amplifier.

That may help
Ron
 
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Thanx Reloadron

You got my approach :)

As we know that Load Cells have very small output voltage as said earlier so it has noise issues also. Do i need any capacitors or something else or i can directly interface it with AD524 ??
 
If you review the data sheet see figure 41. Personally I never used it but set things up like figure 52 for a typical bridge input. Just make sure you use shielded cable and note how they ground to prevent ground loops. That worked for me as I recall. Keep leads as short as possible and things should go fine. Mostly a matter of applying good practices and it worked fine for me. Typically with signals of 20 to 30 mV full scale I amplified X 1000. Then firther amplified to 5 or 10 volts FS.

Ron
 
load Cell amp

Here's what I use for amplifying the output from a strain gauge, I needed about 1 volt and had to amplify my signal by about 3000. Don't seem to have much problem with it. It's a Texas Instrument INA128.
Kinarfi
 

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Thanks ericgibbs

I have tried AD624 it gives me good amplification but with noise of about 10mV at input and also there is a resolution problem

Actually i want to us my 10Kg load cell for upto 1 to 2 Kg only so i want almost 1mV signal to be amplified to 5V with a good resolution

Resolution means that i want almost 1g accuracy at output but after amplification it changes after almost 100g i-e 100g accuracy

hi,
You say that there is 10mV of noise at the input of the AD623 and you want to amplify a load cell 1mV signal by 5000.?

Where is the 10mV noise coming from ? is it common mode or differential noise at the input.??
 
Hey i have amplified my Load cell output according to my desire and its working

Just tell me does it need Buffering before i give amplified signal to the comparator ???
 
Hey i have amplified my Load cell output according to my desire and its working

Just tell me does it need Buffering before i give amplified signal to the comparator ???

hi,
It depends upon your circuit output.??
Do you have a diagram to post.?
 
Nopes its just simple

The Amplified output of AD624 is given to Op-amp 741 just to compare i-e 741 is being used as comparator.

Problem is that when i give this signal to comparator then the output remains constant even if i apply force on load cell
 
Nopes its just simple

The Amplified output of AD624 is given to Op-amp 741 just to compare i-e 741 is being used as comparator.

Problem is that when i give this signal to comparator then the output remains constant even if i apply force on load cell

How is the output connected to the 741.?? Please post a simple diagram..
 
Comparator Design Needed

I am using 2 stage amplified load cell signal. First amplification from AD624 and then from non-inverting op amp. Now i need to compare it with a threshold.
But the problem is that when i give amplified output into the comparator, the change in weight on load cell does not affect the output of amplifier rather it remains constant.
Why it happens ?
 
Can you post the schematic (drawing)? Really hard to see the problem without a schematic.

Ron
 
here is the circuit diagram attached in attacment

The schematic is available in the attachment
 

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