INA122P amp gain

Thread Starter

niterida

Joined Dec 27, 2019
16
I have a loadcell being amplified by an INA122P but at the initial weight being applied to the loadcell the amp is not outputting any voltage.
What is the best way to increase this so it registers just the slightest force ? Should I change the resistor or capacitor value and if so up or down ?
This is the circuit I am using : INA122 loadcell circuit.jpg

I have a 500ohm pot to change the sensitivity but if I put it down as low as possible (highest sensitivity) then it still doesn't register slight pressure and then it goes to full ouput with just a little more pressure. It is a 50kg loadcell and I need it to register full output at 50kg force but still register with just 1kg or less force. Is this even possible ?
The capacitor I have is marked .2A224J
Thanks :)
 

LowQCab

Joined Nov 6, 2012
558
Have you tried looking at the Data-Sheet ????? , All the info you need is on the first page.
What about the Load-Cell..... Part-Number, Data-Sheet ?????
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Thread Starter

niterida

Joined Dec 27, 2019
16
Thanks

I posted a diagram already
I know nothing about electronics - I simply followed the above diagram
It all works ok so there is no issue - I just hoped there would be a simple fix to give a higher output at low inputs.
 

ericgibbs

Joined Jan 29, 2010
12,932
hi niter,
I did request a circuit drawing, I know you posted an image of the PCB, but that is not enough information.
What is the operating voltage.?
The Input and output voltages.?
E
 

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Thread Starter

niterida

Joined Dec 27, 2019
16
But the circuit is not in question is it as it works fine ?
All I know is the 4 wires from the loadcell go into the bottom holes (GND, +, -, +5) and the the 3 output wires go into the top holes (+, S, -).
It has a 0.22uf capacitor and 0-500ohm variable pot resistor but there is no output until the input voltage is too high. I thought it would be a simple fix to increase the amplification ?? But I still need to keep the max input load (50kg) to a max output of 5v
Will a different cap make any difference ?
 

Thread Starter

niterida

Joined Dec 27, 2019
16
Input is from a Wii Fit board - I can't find any info about them so I don't know the input voltage
Supply/output is 5v
Capacitor marked in yellow :
INA122 loadcell circuit.jpg
 

Thread Starter

niterida

Joined Dec 27, 2019
16
Resistor is 0-500ohm variable. The problem is that is I make it low enough to register 1kg then it goes to 5v at say 20kg, but I don't want 5v until 50kg. If I set the resistance higher so that 50kg is 5v then it doesn't output enough voltage until there is say 5kg load.
Maybe I should use a different loadcell - the wii fit is a bending type and the way I am applying force may not be the best. It is actually being squeezed by a brake caliper (effectively the loadcell is replacing the disc) - do you think a button type loadcell would be more sensitive and better suited ?
 

ericgibbs

Joined Jan 29, 2010
12,932
hi,
Many load bridge are either 1mV/Volt or 2mV/Volts, where Volts is the excitation voltage of the load cell.
[Vext is approx 5v thru 10v DC]

Do you have a photo or type number of your load cell to post.?

E
 

Thread Starter

niterida

Joined Dec 27, 2019
16
There are no markings on the wii fit loadcell and there is no info on the net :(
But I just realised it is not 5v like I thought it was - it is 6v - not sure if that would make a difference since I am only supplying 5v ??
 

Thread Starter

niterida

Joined Dec 27, 2019
16
Wii Fit is rated for 140kg and there are 4 cells - so that is 35kg each and since they always build a safety factor in I am guessing it is 50kg - but that doesn't really change my issue does it ??
It is working in its intended purpose - its just the initial load that I want to be more sensitive. Would a 6v supply help ?
 

ericgibbs

Joined Jan 29, 2010
12,932
hi,
If you assumed say 5V excitation for a single 35kG cell and the load cell was rated for 10mV at FSD [ full scale deflection] and you had a 5kG test weight on the cell, you would get only (5kg/35kg) * 10mV output =~1.5mV

So for a group of 4 off 35kG cells , wired to measure up to 140kG, thats only 1.5mv/4 =0.37mV for 5kG.

So to weigh a 50kG maximum load thats 10* 0.37mV = 3.7mV, the amplifier would require a Gain of 5v/0.0037 = 1351

The problem is that if the INA122 powered from say 5v, the output will limit to just under 5Volts.

Try powering the INA and the Load cell bridge from say +10v.
Post what you measure.

E
 

sghioto

Joined Dec 31, 2017
2,363
Load cells are linear devices. If you want 5 volts out at 50kg then the output at 1kg is .1 volt. The load cell consists of those four internal components which are strain resistors that are configured in a bridge arrangement.
 

sghioto

Joined Dec 31, 2017
2,363
Another problem is the low voltage supply. As seen from the data sheet the 5 volt supply only allows an output from .1 volt to 4.9 volts. Question, are you sure the loadcell is connected correctly?
1620834172530.png
 

LowQCab

Joined Nov 6, 2012
558
This is all rather silly.
If there is no Data-Sheet for the Load-Cell,
Buy a different Load-Cell that does have a Data-Sheet, and trash the old one.
"Guessing" just creates a bunch of useless confusion.

Here is an excellent example ........
This unit is also available in an active version, with a calibrated Amp built-in.
https://www.digikey.com/en/products...JwCWAOwDmIAL5EAtJSQhUkAQFdCJciGosJUqhrTJK9bUA

However, since you are trying to measure Brake Caliper Clamping Force,
a much better way of going about this is to simply measure
the total Area of the Brake-Caliper-Pistons,
and then install a Fluid-Pressure-Sensor in the Brake-Line.

Simply multiplying the Fluid-Pressure times the Piston Area
will tell you exactly how much Pressure is being applied to the Brake-Pads.
This is a far more reliable method.
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Thread Starter

niterida

Joined Dec 27, 2019
16
This is for a sim racing brake setup, so I am not actually measuring it - just sending the output to the game, which expects 0-5v max.
It was done because it was cheap and should give the most realistic feeling. Don't want to spend much more on it, so expensive loadcells or amps or changing anything else is out of the question.
I will try giving the loadcell 6v and if that doesn't work I will try a disc type loadcell and if still no good I will buy a manufactured pedal set.
Thanks for all your help and input :)
 

Thread Starter

niterida

Joined Dec 27, 2019
16
OK looks like I may have found the real issue - the software doesn't start recognising the brake input until it sees 2v.
Is there anyway of making the output from the setup start at 2v ?
Would an arduino be able to do this ?
 
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