I am trying to amplify strain gauge with INA 125. I connected everything like in this tutorial http://cerulean.dk/words/?page_id=42 . When i am trying to set up gain, and reading analog values on arduino, only 1k resistor shows some changes. When i try any other ( 220R, 120R 2K ...) it only stays at almost constant 862 analog value in serial monitor. I thought that the chip was bad, but i tried other one, and it does the same. I also tried different strain gauges, but it didn’t made any difference.
I check wiring 5 times ant 3 times reassembled everything.. Please help!
Hi, have you measured the output voltage at pin 10 of the instrumentation amp with a meter, it seems you are relying on the arduino analog input and software to see if the amp is working.
Don't worry about the arduino just yet, aim to get readings out of the amp first.
The specs for the amplifier also state that the max output from 5volt supply is 5-0.8=4.2Volts. So don't try and get any more than that out of it.
If I read the amp spec correctly with the single 5V supply, zero differential will equal zero volts.
I tried using meter. Getting max 4.13 volts. But still, works only whit 1k resistor set up as gain. When I put 220R it doesn’t work. Whit 1k resistor I have readings from 2.8 to 3V when I apply force to strain gauge. Using smaller resistor should mean more amplification, but it settles at 4.13 volts
In my experience you have to be pretty precise with the resistance you use to set the gain, and to get it set right you need to be testing with at least 5 ohm increments. You should use a low value potentiometer (100 or 500 ohm) so you can narrow it down.
The output voltage of the INA125P is typical .8V lower than the excitation voltage so your 4.13V maximum would be normal. That specification is stated in the datasheet.
The output will always be 4.13V, as you've experienced, when you have too much gain.
When you do get your potentiometer and start trying to get the gain set right, beware that you'll never get it to exactly 0 volt output when the gauge is at rest. This is because every strain gauge is imperfect and the resistances will be slightly out of balance. You can try to put it back into balance but save that problem for a later time.
I both 500ohm potentiometer, attached few resistors to reach 1k ohms and tried to sea what will happen when I will lower the resistance. Strangle, the gain didn’t change much, but the output voltage, from the bridge changed a lot. It looked like I’ve attached potentiometer directly to arduino…. I don’t think it should work that way.
hai Marmalas,,how about ur project with ina125p and strain gauge,,, ?can it work with the potensiometer to change the gain??
Thank You very much marmalas and all..God bless you all
The problem that Marmalas was having is that the strain gauge and/or the INA125 had a high offset voltage, and when this offset voltage was multiplied by the gain he was trying to use, the amplifier saturated. He/she needed to either use a lower gain, or trim out the offset at the amplifier input.
If you are using this chip in a full bridge configuration in a beam bending application where the voltage across the bridge could be either negative or positive depending on tension or compression. Does the wiring to the chip have to be compensated as far as Vin+ and Vin- to not destroy the chip or does it not matter?
Yettimania:
If you are using this chip in a full bridge configuration in a beam bending application where the voltage across the bridge could be either negative or positive depending on tension or compression. Does the wiring to the chip have to be compensated as far as Vin+ and Vin- to not destroy the chip or does it not matter?
Anyone's input is appreciated!!
The bridge configuration means the the + or - variation of measurement will be centered at the supplied bridge excitation voltage divided by two. So if you power the bridge with with +5vdc the measurement will be based on a 0 vdc output as measured between the two sense points of the bridge. More strain in one direction will drive the measurement towards +5vdc and strain in the opposite direction will drive the measurement towards 0vdc, there is no 'negative' voltage possible if the bridge is powered/wired to +5vdc and ground.
Basically one side of the bridge will he higher than the other but that depends if the beam is in tension or compression. When it is hooked up to the INA125, does the higher voltage have to also be hooked up to the Vin+? For instance if you had a beam with positive and negative displacement your high and low voltage will be switching sides depending on whether or not the beam is in tension or compression. Do you have to compensate the connection to the chip for this?
For example, if the beam is displaced positively you may have 2.7 V on one leg connected to V+In on the chip and 2.3 on the other. But if it is displaced negativley, the leg originally hooked to the V+in will be at a lower voltage. If I am understanding this correctly.
Yes, you are correct. If you want to measure both polarities of differential input voltage, then you will need to connect the IAref pin of the INA125 to a stable voltage of about 2 to 2.5V. The voltage you apply to this pin is the output voltage you get for zero differential input (ignoring the input offset voltage of the amplifier), so by setting it nonzero, you can measure deviations in either direction.
put the ina on 12 volt as it is not a rail to rail opamp, meaning it can not go blow 1 and above 4
then use two resistors to limit the input voltage on the arduino.
the diagram works but not perfect.
there is no zero in your plan,so short inputs to get zero, then put max weight and adjust input to max with gain potmeter
