# problem with bridge circuit

Discussion in 'Homework Help' started by arivalagan13, Feb 4, 2010.

1. ### arivalagan13 Thread Starter Member

Apr 30, 2009
29
0
Hi group,
I've problem with a bridge circuit with a strain gauge as one of its arm...I'm using a +5V power supply and limited the current entering the bridge to 2mA..
The bridge is almost showing a zero volt under balanced condition..but even after pressing the gauge with my thump finger I'm not getting any output voltage...

1.Is there any procedure to give force to the bridge circuit?
I mean the procedure to get things done..

2.Also,I suspected the voltage change may be very lo and designed an instrumentation amplifier with gain 100 and even after I was not getting anything significant...at the InAmp output I'm just getting some voltage as 0.124(not changing from this value before and after applying the force)....

3.We know that the strain gauge (in bridge) will produce some voltage proportional to the force..Is there any formula to find the force(in any units) applied(I mean force in numbers)...

The real problem,which I already shared with the group,is I don't know have the data sheet of the gauge which I'm using...it is 350ohms...approximate dimensions are 6 x 4mm...It is one of gauge that belongs to SR-4 series of Vishay Micromeasurements..

This one is making me a lot of problem...Once I solved this I'm almost done...So kindly help me fix this problem

Thanks & Regards,
Arivu

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
You may have a misconception about how strain gauges work. They measure force applied to some fixture by responding to the strain induced in the material. The applied stress causes the fixture to bend. That bending causes the strain gauge foil to either lengthen or compress, depending on the foil placement.

When a foil gauge compresses in length, it becomes less resistive. When it streches, it becomes more resistive. They do not respond to a pressure applied to the surface of the gauge, unless that also causes a strain in the fixture.

www.omega.com has just about all the information there is on strain gauges and their application.

3. ### arivalagan13 Thread Starter Member

Apr 30, 2009
29
0
Hi beenthere,

Thanks a lot..I got ur point

Thanks & regards,
Arivu