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

Wayne0809 said:

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?

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ericgibbs said:

hi,
Rg is adjustable for the Gain.The output range of my circuit is upto +3V
If you do not need to show the weight, why do you want to use an ADC/MCU.??

If you want a 'Go, no GO' indicator you could use a much simpler circuit.?

If you use your circuit , you must connect AD620 pin 5 to 0V.
Also you will not get good results for very light weights.

Many load cells and OPA's will give you an offset voltage.
Have you made the load cell base and fixed to the load cell.??
this will give you a small +mV signal.

E.

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Wayne0809 said:

Hi,

I use ADC because I still have to measure the weight of the object and convert it to digital value. However, my project don't have to show the weight of the object, just that I have to do some action when the weight on the scale fall to certain value. If I want a good result and make my measurement more sensitive, I have to use LM324?

Yes,I have fixed a base to my load cell but even I remove the base, the output voltage amplified by AD620 is still around the same. The voltage change of my circuit is very small even the weight is changing. How to increase the change of the voltage or increase the sensitivity of the load cell? Why even I remove everything and base from the cell, there is still a 1.67 V from the AD620 output?

Thank you

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ericgibbs said:

Hi,
Using the LM324 circuit will give you the best results.

Are you sure you have the Load cell connections to the AD620 inputs the correct way round, if they are wrong any weight will try to drive the amp output below zero.!

Do this test... put a 5Kg weight on your load cell [ making sure that all the leads are connected].

The output of the load cell will be only 5mV, so the AD620 will have to amplify it by 600 times to make it +3Vout.

So make RG=80 ohms, using your circuit, dont forget to connect pin 5 to 0v.

Let me know what your voltmeter reads on pin 6.

E.

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Wayne0809 said:

Hi,I cant really find a 5kg object but I realize 1 thing, when connected my Rref to GND, my voltage become smaller and the sensitivity become lower, is it because the gain change? Can I increase the sensitivity by increase the gain?What is the minimum RG can I use?Thank you

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ericgibbs said:

hi,
Get an old 5 litre milk or fruit juice container and fill it with water, that will be close to 5Kg for testing.
To do a precise Calibration of the scale you should use a known 5Kg weight.

The Rref pin #5 MUST be connected to 0V or a offset reference voltage [ as shown in the LM324 option], if you leave it disconnected the AD620 will not work as you expect.

The Gain range is shown in the datasheet for the AD620, I have posted an image of the page,

As I said earlier use a 80R for the Rg that will give a gain of approx 600 which will raise the 5mV FSD output of the load cell to +3V.

E

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Wayne0809 said:

View attachment 73877

Hi,If I use the circuit you posted last time, the output voltage range from the AD620 is 0V to 3V?The 10K pot of both LM324 is set to 6K4 and 4K5 respectively?If I use the circuit you have posted last time as shown in attached picture, the output for light weight will also be more sensitive?

Thank you

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ericgibbs said:

hi,

You have to adjust the 10K pots a little to set the Zero and Offset, also the RG pot will need a small adjustment to the required Span accuracy.

The light weights will be more 'linear'

You still have not answered my question...

Did you do the 5Kg weight test and what was the AD620 Vout.??

E.

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Wayne0809 said:

Hi,

I really cant find 5L milk or juice but I tried use 2L juice and the output is around 1.65V something. I use 82ohm for my Rg. If I use 82 ohm I cant really detect a small change of the weight. My load cell with the base and without any object will produce a output voltage 0.646V. Because I need to measure only around 500g to 1 kg in my project, so I have to have a very sensitive output voltage for my amplifier circuit. So is it better to use LM324?

Thank you

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ericgibbs said:

hi,
Which circuit are you using for these tests, is it just the AD620.??

The LM324 should work as you require.

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Wayne0809 said:

Hi,

I using only AD620 to test. Actually now I facing some problem when I convert the analog value using ADC of LPC. Do you familiar with LPC1769?

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ericgibbs said:

I dont use the LPC..

For your load cell a 5Kg bridge is not the best choice for a 0 to 1Kg scale, you will only get 1mV signal for 1Kg.

So if you amplify by 600 that will only be 0.6V, if your LPC has say a 10bit ADC at 3V input you would get 1023 counts, but for 0.6V you get 1023 *[0.6/3] =204 counts.!!

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Wayne0809 said:

I got no choice because I cant find another scale to replace my current load cell before my project due,so I can just try to get as sensitive as possible. The circuit you posted last time have 3 pot resistor. I have to adjust the resistor to get 0V when there is nothing on my cell? Can I use 80ohm as my gain instead of using a pot resistor for my Rg for AD620?Thanks again:

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ericgibbs said:

hi,
I have modified the circuit, this option will give you ~0v thru +3V for a 1Kg weight.

Look it over and ask if you have a query.

E.

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Wayne0809 said:

Hi,what you mean 0-3V for 1 KG?If the weight more than 1kg,the voltage will exceed 3V?So this circuit will change the overall gain dramatically and also the sensitivity?

If there is voltage when there are no object on the scale, how to reset the voltage to 0?

Thank you

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ericgibbs said:

You said you only wanted to weigh from 0 to 1Kg in an earlier post, so what weight range do you want to weigh.???????????

Use the Zero and Offset pots to set the Vout to zero when there is no weight on the scale.

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ericgibbs said:

Hi
Look at this image, you MUST use the values I have marked on my drawing.

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Wayne0809 said:

Hi,so is short the middle pin to one of the pin and use it as 2 pin resistor?I use the same value from your circuit but still not able to get 0V and the output of LM324. The minimum I can get is around 0.6++.

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ericgibbs said:

When you place a 2K or 1Kg weight on the scale what is Vout.?

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