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Power used by a weight sensor

  1. Aug 22, 2015 #1
    I am making a science fair project, I need to know how much power a general weight censor needs, google doesn't seem to be helping.

    And when a weight censor starts consuming energy?? when a certain weight is applied on it and it switches a circuit on, or as long as the weight censor remains active in the circuit??

    Please help, I used google, but didn't get much benefit so looking for answers from you as my last option.
     
  2. jcsd
  3. Aug 22, 2015 #2

    Baluncore

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    What is the application? What sort of thing is it weighing? How many load cells does it use?
     
  4. Aug 22, 2015 #3

    SteamKing

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    There's a difference between a "censor" and a "sensor".

    A weight sensor, like a bathroom scale, doesn't require any power to function.

    Engineering load cells produce a voltage which is proportional to the load placed on the cell.
     
  5. Aug 22, 2015 #4

    Bandit127

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    Sorry to be picky, but it would be more correct to say that engineering load cells produce a resistance that is proportional to the load that is placed on the cell. An applied voltage will then be varied according to that resistance and can then be measured.
     
  6. Aug 22, 2015 #5
    When a 500 gram or so load will be placed on a certain point, it will let the circuit start, when the load will be removed, the circuit will switch off.
     
  7. Aug 22, 2015 #6

    SteamKing

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    What you say is true, for some cells, but other types produce their own voltage.

    https://en.wikipedia.org/wiki/Load_cell

    Specifically, the piezoelectric type is one such cell which produces its own voltage output.
     
  8. Aug 22, 2015 #7

    Baluncore

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    A +/–1kg load cell will be made as a bridge configuration of 4 resistors. It will have 4 wires. A ratiometric ADC will normally read the differential output voltage and display it as weight.
    But for your simple system you do not need the digital converter, only the sensor, an amplifier and a comparator will be required.

    There will be a current flowing through the sensor while the sensor is receiving power. That will dominate the power consumption. A 120 ohm bridge with 2V excitation will need 2/120 = 16.7mA. The sensor power will therefore be 33.3mW.

    http://www.hbm.com/en/menu/products/strain-gauges/stress-analysis/full-bridge-vy/
    “The right resistance: The selection of the resistance depends on the constraints of the measurement task. Strain gauges of 120 ohms are relatively insensitive to fluctuations in the insulation resistance; for example, due to the effects of moisture.
    The advantage of higher-impedance strain gauges is that they produce less specific heat due to the lower measuring current. In addition, they are less sensitive to resistances in the connecting cables to the measuring amplifier. There is also a disadvantage that high- impedance strain gauges may be more sensitive if noise pulses are received.”
     
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