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# Wheatstone Bridge

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#### Squintz

##### New Member
I have a wheat stone bridge circuit with 10 volts applied its resistance values are:

350.0008260633880Ohms 350.0017337273550Ohms
350.0033000591410Ohms 350.0041953085910Ohms

Can somenone calculate the voltage across S+ S- for me. I want to check my math. I think im doing it right but im trying to prove a point to someone else and just want to make sure

the Vout i get is 0.000012878v

Umm, havn't done the math, but at a glance:

All 4 resistors very close to 350ohms, so VOUT should equal very close to 0V, as bridge is nearly balanced.

Squintz said:
I have a wheat stone bridge circuit with 10 volts applied its resistance values are:

350.0008260633880Ohms 350.0017337273550Ohms
350.0033000591410Ohms 350.0041953085910Ohms

Can somenone calculate the voltage across S+ S- for me. I want to check my math. I think im doing it right but im trying to prove a point to someone else and just want to make sure

the Vout i get is 0.000012878v
Your result is more than 100 times what I get.

How is that? Being the the bridge is nearly balanced my voltage should be about 0 and it is about that.

Squintz said:
How is that? Being the the bridge is nearly balanced my voltage should be about 0 and it is about that.
Yeah, well, my answer is closer to zero than yours is. I'm not going to give you my answer, because this sounds like a homework problem, and we generally don't do homework here. If you want to show your calculations, we can go from there.

trust me its not a homework problem. Its 4 Strain Gauges(transducers) Wired into a full bridge and they are all active.

Actualy that is the real circuit and im trying to prove it by using 4 wheatstone bridges with only one active leg and by back tracking from strain to resistance i am then add each of those resistances back into a virtual circuit on paper and calculating the output voltage as if each wheat stone bridge were one single bridge.

Its to prove a point to our development team who is placing these gauges along a 180meter beam all facing in the same direction and wiring 4 active resister together when they are changing in resitance almost at the same rate in the same direction. hince why the values are so close to zero and so precise because of the high tech equipment we are using. although scientifically the answer is only goo up to 2digits because the voltage is only as acurate as 10v and it could be 9.9999992992939

Is this the circuit we're talking about? My calculations and subsequent simulation yields diff=93nV (.000000093V).
I have never used strain gauges, but it seems pretty obvious that you will theoretically get no change in output voltage if all 4 resistors are active and facing the same direction. For most sensitivity, mount diagonally opposite resistors (R1 and R4, for instance) longitudinally, and, assuming there is little or no lateral strain, mount the other two nearby, oriented at 90 degrees to the longitudinal ones. This gives you temperature tracking.

Sorry about the homework comment. The 3 zillion digit resistor values made me think you were a student with no concept of significant digits.

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What program did you use to create that simulation.

I once used Electronic Work Bench and didnt like it. My pc had to be rebooted and now they want me to pay for another licence. Not gonna happen

im looking at the orcad software but im not sure if work will put out the money for it so im looking for a cheap temporary solution.

Now im getting off topic tho

I use Linear Technology's SwitcherCAD III, which is totally free, with no restrictions. You need to have a pretty good knowledge of Spice to be able to use it, but I really like it.
You didn't answer my question. Does the schematic represent what you are doing?

Yes..

Normally we use 3 350ohm resistor and one strain gauge which is a peice of thin plastic with a variable resistor and we glue the plastic to aluminum and measure how much the aluminum bends by the change in the resistance.

Simple really

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