Strain gage amplifier

Discussion in 'The Projects Forum' started by unborn, Mar 19, 2009.

  1. unborn

    Thread Starter New Member

    Mar 19, 2009

    I have done a strain amplifier using the circuit that i found on the internet. The problem here is at the output i cant get the -ve value for the strain gage. The output value will always give +ve value either i stretch or compress the strain gage. With this i attached the circuit diagram.

  2. SgtWookie


    Jul 17, 2007
    I don't know how you can expect to get a negative output, as you don't have a -v.

    You'll need a zero reference. You could make one by using a 10k 21-turn trimpot between +5v and ground. Use a small cap, maybe 10nF, from the wiper to ground to suppress noise.

    With no load on the strain gauges, adjust the pot until you read 0v between Vout and the pot's wiper. Then you should get + and - readings.
  3. unborn

    Thread Starter New Member

    Mar 19, 2009
    Thank you SgtWookie. Is it like this? beacuse i have tried before this to give -v supply but the strain gauges got burn.

  4. thatoneguy


    Feb 19, 2009
    Do you have a link to the datasheet for the strain gauge you are using?
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    Could this be because you have connected R4 and R5 in the wrong place? As shown you have conneced a large variable supply directly across the gauge.

    Attached is a quick sketch of a revised layout, without the capacitors. This is drawn to bring out the 'shape' of the circuit more clearly.

    You need to balance the bridge with the pot, before you strain the gauge.
    Normally there are at least two gauges in this arrangement for temperature compensation and one is left unstrained. If you use this the second goes in position R2.
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    There is an additional constraint on such a bridge - the maximum voltage that can be continuously applied. Most often, that is 10 volts DC. That is most often +10 VDC applied and the bottom elements to circuit ground.

    It is also conventional to use an instrumentation amplifier like an INA111 to take the differential signal from the bridge. The ability to reject the 5 volt common mode voltage is quite important.
  7. unborn

    Thread Starter New Member

    Mar 19, 2009
    Thank you very much everybody. I got it.