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Topic: Load cell without ina125p (Read 6273 times) previous topic - next topic

i_dont_know

Hello,

I want to use a load cell in arduino Leonardo, the load cell is from a kitchen scale and the range is 10 Kg, I saw in other topic that some people already do this, but they used INA125p for amplificate the signal.

I didn't find the INA125p to sell in my city and I would like to use another component.

Anybody knows how to do this?

Ps: My load cell have 4 wires, E-,E+,S-,S+

Thanks
=D

jremington

#1
Nov 20, 2013, 05:26 pm Last Edit: Nov 20, 2013, 05:28 pm by jremington Reason: 1
You do need an instrumentation amplifier to use the load cell properly. If you can't buy any of the INAxxx series you can make an instrumentation amplifier from 3 operational amplifiers. Those are very common and even come as four to a single IC package. It is easiest if you choose a "single supply" op amp.

Google "instrumentation amplifier circuit" for lots of information, but here is a start: https://en.wikipedia.org/wiki/Instrumentation_amplifier

Unfortunately, for a home-made instrumentation amp to work well you will need precision resistors, so consider ordering an INA amplifier for mail delivery.

i_dont_know

Do you know any different model of this Instrumentation amplifier circuits?Are there different from INA****?

I am trying do my onw with 3 amplifier, but it's not working...=/

retrolefty


Do you know any different model of this Instrumentation amplifier circuits?Are there different from INA****?

I am trying do my onw with 3 amplifier, but it's not working...=/


Can you post a schematic drawing of how you are wiring up the 3 amp configuration?

First thing you have to know is what voltage value your bridge load cell is outputting for 0 weight and it's maximum weight. Only then will you know how much gain your amp will have to have to output a useful voltage range for the arduino.

i_dont_know

#4
Nov 20, 2013, 06:35 pm Last Edit: Nov 20, 2013, 06:50 pm by i_dont_know Reason: 1
I did like the circuit of INA128, but didn't work, like i sad.

My kitchen scale works with 3 V and I try to put 3V and 5V in my load cell. In the end of my circuit, the voltage still constant no matter what I do with the load cell.

jremington

If you could measure and tell us the S+ and S- output voltages of the load cell with respect to common ground, with and without a mass on the scale, that would help.

MarkT

The first thing you need to realise is that for load cells a precision amp is needed because
the voltages are truly tiny.  An offset of 10mV will get amplified to saturation.  0.3mV or
lower offset voltage is more what you are looking for.  Non-precision opamps will be useless.
[ I won't respond to messages, use the forum please ]

jremington

#7
Nov 22, 2013, 06:30 am Last Edit: Nov 22, 2013, 11:21 pm by jremington Reason: 1
At least under some circumstances, Texas Instruments gives out free samples of their products, which includes several of their INA series of instrumentation amplifiers. Registration with TI is required, so you might check to see if you are eligible. I am pretty sure that students qualify for samples but haven't checked the details. You can always buy from the TI store directly, if that doesn't work.

The INA128, or the lower offset version INA129 would probably work well for your project: http://www.ti.com/product/INA128 Those are intended for dual-supply, so also consider the single supply INA122 http://www.ti.com/product/ina122

Chagrin

You really need to fiddle with the resistance that sets the gain to start to see any voltage change. You'll be in the ~50 to 200ohm range and being off by as little as 5 ohms can be too much.

Assuming nothing is connected to the Ref pin, if your gain is too low the amp will output ~2.5V (assuming you're using 5V excitation voltage). If the gain is too high it will read ~4V. Without pushing on the load cell or anything, keep adjusting the resistance that sets the gain until you see an in-between voltage, and then adjust the resistance down 1 or 2 ohms at a time until you almost hit that minimum ~2.5V you saw earlier. Then do your testing by pushing on the load cell.

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