Strain Gauge Sensitivity

Discussion in 'The Projects Forum' started by ntwhee0, Jun 5, 2014.

  1. ntwhee0

    Thread Starter New Member

    Jun 4, 2014
    I'm a prosthetist (no EE background)
    I have a Health O Meter bathroom scale, which has four strain gauges that have 3 wires each.
    I'm trying to hack it to send S+ and S- to AD623an for amplification.
    I'm told that S+,S- voltage needs to be around 2mV.
    I'm getting about 355 mV at S+,S- and I don't understand why.

    I've sending about 5 volts (4.99) into E+ ,E-

    Resistance between E+ and S+ is 1048 Ohms, between E+ and S- is 1048 Ohms, between E- and S+ is 1048 Ohms, and. Between E- and S- is 1045 Ohms.

    Here's my math to figure S+ and S-:
    4.99*1048/(1048+1048)=2.495 V,
    the voltage between S+ and E- should be
    4,99*1045/(1048+1045)=2.4914 V,
    the voltage between S+ and S- should be
    2.495-2.4914=0.0036 V=3.6 mV

    But i'm not getting 3.6 mV. I'm getting 355mV, and I don't know why. I want to get about 3.6 mV

    Where should I begin looking to figure this out??

    ps. (testing the voltage between each gauge's red and black wire (or red and white wire), I get about 640 ohms at each gauge.)
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    You should measure 3.583mV with those bridge resistance values.
    [see image]

    The value you measure 'I'm getting 355mV,' is numerically very close to 3.58mV expected, are you sure that you are reading and measuring the voltage correctly.?

  3. celticlord

    New Member

    Jun 10, 2014
    Could someone please give me a pinout and and help to hook this up to a Basic stamp. I dont need to know how to connect it just how this pinsout. All I could gather is it uses a diff amp for the strain gauges.
  4. ntwhee0

    Thread Starter New Member

    Jun 4, 2014
    Hi ericgibbs,

    Thanks for checking the math.

    I finally got the correct sense voltage.
    I had not properly probed the OEM pcb to know the correct scheme for E+,E-,S-,S+.
    I figured it out by randomly trying different schemes.
  5. ntwhee0

    Thread Starter New Member

    Jun 4, 2014
    He Celticlord,

    As per your request, You'll notice the Health O Meter's circuit board has labels of S1, S2, S3, and S4 in very small text at the boarder of the board where each gauge's wires connect to the board. I soldered to the RED wire connection points only; Excite positive to S1, Sense positive to S2, Sense negative to S3, and Excite negative to S4. I used AD623an to amplify the signal. Then the signal goes to an STF4 discovery board.