Hi. I have a wheatstone bridge, one arm has strain gauge of 320 ohm resistance.
I studied that the bridge balance happens when ratio of resistance in one side equals another side.
Some webpages recommend using all 4 resistors of same value, whereas some pages had greater value of resistance like 5k ohms so that the ratio is still 1 on either side. I have attached a drawing, I need to select resistors on the left side of the bridge. Should it be same 320 Ohms, 320 ohms? Or higher values like 5k Ohms, 5k ohms?
I wish to know how do I set it up to get maximum sensitivity. Thanks.
It dosn't matter what those resistors are, all they are doing is providing a fixed referance voltage for a differential measurement. If you want to read it on an arduino or othe single ended input then get rid of them altogether.
Since the signals from strain guages are so small you will need precision resistors on
the other arm, 0.1% with low tempco is a good start. Mounting them very close
together will provide thermal matching so the tempco isn't so critical.
You don't need the actual precision just to guarantee matching of temperature response,
so that the resistor ratio is very stable. This can be done with a 2nd strain guage that's
not being strained - or even at 90 degrees to the other one to increase the differential
signal. [you will need to know Poisson's ratio to combine such signals]
Grumpy_Mike, thanks for your idea, but I'd like to use wheatstone bridge since that's the method recommended by the company that made my gauges, and it's the one in my textbook on strain guages.
Mark, thank you, I understand your method, several other webpages recommend the same too, I will try your method, but what I still don't know how I could fix resistor values that are on left side of the bridge? Is there a method to fix those resistance values when straingauge resistance is known?
Grumpy_Mike, thanks for your idea, but I'd like to use wheatstone bridge since that's the method recommended by the company that made my gauges, and it's the one in my textbook on strain guages.
Fine but do you understand why it is recomended and what it is doing and why there is no point or indeed advantage in using one with a single ended input like the arduino. I suspect not because if you did you would not be asking about calculating the other two resistors in the bridge.
In order to use a bridge you need a differential input, you can only get that by using an op-amp ahead of any arduino.
GrumpyMike : Yes I have already bought AD620 amplifier and I will use it, to set gain with external resistor, so that the output can be from 0-5V before giving it to Arduino's analog input. And I understand from studying on Op-amps, it works in this way (Difference of two voltages) * Gain. But I couldn't find suitable method to get value of resistors on other side of bridge, whether they can be same as gauge's resistance or more.
Like you said, If I did know better, I would not be asking it here, it would be helpful even if you just pointed me to a link for a webpage that has details on this.
like I said in reply 1 - It dosn't matter what those resistors are, all they are doing is providing a fixed referance voltage for a differential measurement.
If you use an other strain gauge in their place then any temperature changes will track.
I have already bought AD620 amplifier
It would have helped to know that in the original post.
Thank you, it's my mistake when I didn't give full details in question.
I am trying to build a weigh scale with a straingauge as my first electronics DIY project (so I didn't buy many strain gauges, which are not cheap, so I can't build a full bridge Wheatstone with 4 gauges). I bought an arduino board since almost all DIY pages use arduino. I couldnt buy INA125 as these pages recommend,due to. Its non-availability here, so I bought AD620.
Thanks for your replies, I will mark this thread solved.
