problem in simple digital Scale...

Discussion in 'The Projects Forum' started by eric_s88, Aug 10, 2011.

  1. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    hi everyone , I have made a digital scale via 7017 ADC
    but I have problem with this, the display is 4digit 7segments,
    when I turn on the circuit the display is starting to show some numbers that are not depended on the pressure applied on the load cell..

    whats the problem?? is there any problem in the circuit or there is something wrong with the load cell?? :confused:


    [​IMG]
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,693
    2,765
    I actually design electronic scales for a living....so maybe I can help you!

    What is the nature of the numbers? Is it a static offset? Is it noise (i.e. changing randomly)? What is the magnitude?

    Have you confirmed a linear response wrt load?

    Give me as much info as you can.
     
  3. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    Hi , I'm glad to see you here :)
    the numbers are being changed in decimal and unit digits, and I can't affect them in anyway.. I'm not sure that the load cell is intact

    what do you think ? :)
     
  4. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    So, the numbers do not change when you change the load?

    What is the differential Vin (attach your voltmeter between pins 30 & 31) at no load, and what is it at full load?

    What is the differential Vref (attach you voltmeter between pins 35 & 36)?

    What is the output specification of the load cell? It should be somewhere around 2 to 10 mV/V, I think.

    What is the voltage at pin 3 (with respect to ground) of IC2?

    What is the voltage at pin 10 (with respect to ground) of IC2?

    Also, do you understand the theory of operation of the 7106?
     
  5. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    Yes the numbers change in a disorder way and they are not depended on the load
    diff. Vin at no load and at full load are the same and is = - 0.289~0.3 V
    diff. V ref between pins 35 & 36 is = 4.4V
    load cell full scale output is = 2.017 mV/V
    voltage at pin 3 of IC2 is = -0.7 ~ -0.8 V
    voltage at pin 10 of IC2 is = -0.7 ~ -0.8 V

    I better say I've used 7107 as a ADC and I've applied necessary changes in the circuit. yes I think I understand the theory of the operation , 7106 & 7107 are ADC converters that converts applied voltage from load cell to logical digit ...
    I'm really confused with this problem :(
     
  6. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    I've just turned the circuit on and the display show the following numbers
    -128 or -127 or -129... they don't change in a wide range anymore.. :-?? I really don't understand why...
     
  7. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,693
    2,765
    Did some back of the napkin analysis. The numbers you are seeing don't make much sense, so I am almost sure you have things wired incorrectly.

    1st, the differential Vref should be about 232mV in the oz. mode, and between 686 and 899 mV in the gm mode, depending on the setting of pot R19. You are showing 4.4V, way off!

    2nd, the inputs 3 and 10 of IC2 should be close to about 5 V (assuming a symmetrical bridge in the load cell, which is usually the case). Either you are missing a power connection, or your load cell is damaged. Do you know what the 4 leg resistances are supposed to be? Measure them and confirm.

    Also, confirm that your load cell is capable of being operated at 10V. If you can post the datasheet for the load cell, that would be *very* helpful.

    For now, don't concern yourself with what you see on the display. The analog inputs to the 7106 have to make sense, and right now they don't.
     
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