ina125 amp with SGT-3F/350-TY41 strain gauge

jeep26

Hi I am trying to build a circuit on my BOE so I can measure strain I did some searching and found that the ina125 is a good am to use. I also bought a SGT-3F/350-TY41 strain gauge. However I find multiple different circuits and im not sure what to use.

I know I need 3 350 Ohm resistors for the wheatstone bride

I also know I need to attach the bridge to pins 7 4 6 and 16 to the ina 125

i am not sure what size resistor i need to use for the gain or where to connect the ina 125 to the stamp

ANy ideas



Here is a link to the INA data sheet

http://www.ti.com/lit/ds/symlink/ina125.pdf

dre

Hi,

First off, normal strain gages have a very low output [not their fault :-)] and must be highly amplified. So you need well matched 350 ohm resistors/gages on all 4 arms of the bridge in order to be balanced without the applied strain. So either use 2 gages and 2 1% or better 350 ohm resistors, or, better yet, use 4 gages.

The connections to the INA 125 instrument amplifier are: pins 4 & 16 supply the bridge with 10 volts. [n.b. You must use the TIP transistor per the figure in the datasheet as the gages take more current than pin 4 supplies.] Pin 5 is ground, pin 6 inputs the + output of the bridge and pin 7 inputs the (-) output of the bridge.

You will probably need a gain on the order of 100 to, say, 500. You will have to experiment if you can't estimate the strain the gages will be under. The best resistor value is one that gives a gain enough to give a meaningful reading into the A-to-D converter.

In summary you will probably need to research the Wheatstone bridge circuit and some knowledge of strain gages [including proper installation] in order to have a successful outcome. I will be happy to help with any further questions or links to more information.

cheers, David

jeep26

Thanks for the fast response

I took your advice and ordered the 1% tolerance resistors


ok I am assuming I need to attach pin 1 to VDD on the BOE

pin 5 to vss
vss to the wheat stone bride
i added a 306 ohm resistor to pins 8 and 9

I attached a picture of what I have

now I am confused on how read the voltage with the stamp
do i need a analog to digital converter?
if how to I wire it in to the BOE?
do I need to build a circuit like the one in the link below

http://www.emesystems.com/BS2rct.htm

if so how do i connect it to the ina125 I am guessing with pin 10 on the ina 125 to p1 on the stamp?

stamptrol

Not to detract you from your current quest, but you might be interested in some application notes from Windmill Software Ltd. in the UK. They offer newsletters on various monitoring/measuring topics. This month's newsletter deals with measuring strain. Just Google Windmill Software.

Cheers,

dre

Hi,

No, you should not supply the bridge from the Vdd pins on the BOE. Vdd only supplies some 5 volts and you have shown to be desiring 10 volts. If your supply to the BOE is somewhat above 10 volts [Vin] then you can use this to supply pin 1 on the INA125. Also, in my experience, using 350 ohm gages/resistors, it is best to use the TIP29C to supply the bridge; as per figure 4 of the INA125 datasheet. I assume that your power supply to the BOE is somewhat greater than 10 volts and has the current capacity to also supply the 350 ohm bridge. If it has the capacity but a lower voltage then you have to use a lower voltage value reference, say pin 15 of the INA125, which gives the bridge 5 volts. However the output will then be 1/2 of that using 10 volts.

The output of a wheatstone bridge is ratiometric. That is to say that the output is directly proportional to the supply voltage; generally expressed in millivolts output per volt of bridge supply. Commercial load cells generally have output at full scale load of 3mV/V. If they use 10 volts to supply the bridge then the full load output is 30 millivolts. Thus even designed for the maximum strain gage output, having gages on all 4 arms results in this low an output and the bridge output has to be amplified [or use a high resolution [ie. 24 bit] A-to-D converter].

I'm glad you found Dr. Allen's excellent literature on using the Stamp. However I don't have any experience with using the 'RC' input to the Stamp. I use a TLC2543 A-to-D converter, which is very well documented in Dr. Allen's literature.

One last recommendation: You will probably need a bridge balancing circuit as even the 1% bridge completion resistors will result in a large initial unbalance [ie. significant output from the amplifier without any load [strain] input from the gage]. Let me know if such imbalance occurs. It is easily taken care of.

Enough for now? Cheers, David