Wheatstone bridges used for?

Discussion in 'General Electronics Chat' started by watsongrey, Nov 9, 2014.

  1. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    What circuits have you guys seen wheatstone bridges used for?
    I have only seen them used with sensors and strain gauges, but why does a sensor or strain gauge need a wheatstone bridge?
     
  2. joeyd999

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    Have you tried this?
     
  3. LDC3

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    Apr 27, 2013
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    Temperature sensing, light sensing, there are others, but I can't think of them.
    Anywhere where a physical attribute can change and you want to measure it is suitable for a wheatstone bridge. I remember seeing a circuit diagram for a phonograph needle pickup that uses a wheatstone bridge before the amplifier.
     
  4. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    But what does the wheatstone bridge do with the pickup, sensor, strain gauge?
    The Wheatstone bridge is used for what reason?
     
  5. LDC3

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    Apr 27, 2013
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    Did you even bother to read any information on the web?
    From Wikipedia:
    The Wheatstone bridge illustrates the concept of a difference measurement, which can be extremely accurate. Variations on the Wheatstone bridge can be used to measure capacitance, inductance, impedance and other quantities, such as the amount of combustible gases in a sample, with an explosimeter. The Kelvin bridge was specially adapted from the Wheatstone bridge for measuring very low resistances. In many cases, the significance of measuring the unknown resistance is related to measuring the impact of some physical phenomenon (such as force, temperature, pressure, etc.) which thereby allows the use of Wheatstone bridge in measuring those elements indirectly.​
     
  6. watsongrey

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    Oct 31, 2014
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    yes and it doesn't make sense to me
    If i remove the wheatstone bridge it will do what to the sensor or strain gauge? the wheatstone bridge is going what to the sensor or strain gauge, i don't get what the wheatstone bridge does or is needed for

    Yes i know a wheatstone bridge outputs a differential voltage or difference measurement
     
  7. WBahn

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    Mar 31, 2012
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    Let's say that you have a sensor (think of a strain gage) that has a normal resistance of 1815 Ω. When loaded to it's maximum, say with 400 lb, the resistance increases by 2% to 1851 Ω. You want to use this to measure loads between 0 lb and 400 lb to the nearest lb. So when a 1 lb object placed on the scale, the strain gage resistance will change to about 1815.09 Ω.

    How would YOU propose to measure it?
     
  8. #12

    Expert

    Nov 30, 2010
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    Think of it this way: If you have a sensor that is going to change resistance, you can send a current through it and get a voltage change. Need it amplified? You can arrange an op-amp to amplify the voltage on the sensor, compared to ground.

    What if that's inconvenient? What if there is only a tiny change in the sensor resistance in the range you want to detect?
    A Wheatstone Bridge is merely a pair of voltage dividers. One to create a different voltage comparison point than ground, and on the other side, a resistor and a sensor create a slightly different voltage that is easier to amplify.
     
  9. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    How would YOU propose to measure it?

    Just put your DVM meter set to ohms and measure across the strain gauge when you have XXX weight, why can't I do that when I have XXX weight on the strain gauge? i need a wheatstone bridge because of why?

    the wheatstone bridge outputs a voltage
    Yes i know a sensor or strain gauge changes resistance, just put your ohm meter across it and measure the resistance change , i don't get what the wheatstone bridge is there for or doing
     
  10. MrChips

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    You cannot use a single element strain gauge on its own because it is sensitive to temperature.
    You use a Wheatstone bridge to compensate for changing temperatures. That is only one reason for using a Wheatstone bridge.
     
  11. LDC3

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    Apr 27, 2013
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    The bridge only has resistors in it; how can resistors compensate for the temperature variations when the resistors have a different temperature dependence than the strain gauge?

    As #12 indicated, you have a voltage divider to bring the reference voltage close the the nominal voltage level on one side and the sensor on the other side.

    Using the sensor voltage to the ground (or negative voltage) will not allow for the signal to be amplified very much.

    Using the voltage difference allows for much more amplification (and better resolution).
     
  12. MrChips

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    That is not the only concept of a Wheatstone bridge.
    In a balanced Wheatstone bridge there are four equal elements R1 - R4.
    In a four-element strain gauge, all elements R1-R4 are identical. Hence they respond to temperature in the same manner and the voltage across the bridge is zero volts independent of temperature.
     
  13. watsongrey

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    Oct 31, 2014
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    You want to use this to measure loads between 0 lb and 400 lb to the nearest lb. So when a 1 lb object placed on the scale, the strain gage resistance will change to about 1815.09 Ω.

    Yes you have to calibrate the strain gauges software problem
    You set the zero point with no weight zero lb on the scale
    Then you place XXX Lb on the scale and set the software program to calibrate the strain gauge
    The microcontroller is hooked up to the wheatstone bridge, the wheatstone bridge is hooked up to a differential amplifier to amplifier very small voltage changes from very small resistance changes from the strain gauges. The ADC chips are on the differential amplifiers output and the ADC chips go to the microcontroller. The Microcontroller has a software program that does the weight scale but you have to calibrate the strain gauge and weight scale.
    When you're calibrating the weight scale/strain gauge , how does this work? you put a weight on the scale/strain gauge and you Default that voltage measurement to the software program?

    Example:
    So weight 50lbs is a voltage measurement XXX saved to the software program? you're just using memory locations of voltages measurements

    When calibrating a strain gauge you have to do weights from 0 lbs to 100 lbs depending on the scales max. You have to do 5 lbs or 10 lbs weight and save the voltage measurements in the software. The Software program gets loaded into an Eprom chip or RAM on the scale circuit

    Why does it need this software program and to be calibrated?
     
  14. WBahn

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    So your ohm meter is good enough to distinguish, reliably, between 1815.00 Ω and 1815.09 Ω. Really? And how expensive is this ohm meter? And notice that we haven't even talked about temperature effects yet.
     
  15. WBahn

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    There are a number of different ways to do it. One of the best is to use four strain gages mounted on identical materials. One is placed in tension and the other is placed in compression as part of the load cell. The other two do not get loaded at all but are there for temperature compensation. These are the four resistors that are in the bridge.
     
  16. bertus

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  17. crutschow

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    One reason to use a bridge is that the bridge null (balance) point is insensitive to the bridge excitation voltage, thus small deviations of this voltage has little effect on the differential voltage across the bridge near null.
     
  18. mbohuntr

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  19. MrChips

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    The Wheatstone bridge concept is not for measuring resistances only.

    The bridge configuration is used for measuring resistance, capacitance, inductance, current, voltage, frequency and for many other sensor applications.

    A balanced bridge has some useful characteristics, an important one being that all arms of the bridge can be subjected to the same environmental variables such as temperature, pressure, humidity, radiation, chemistry, etc., while only one arm, the active sensor, is subjected to the desired variable of interest.

    A balanced bridge configuration has one significant property when used at the NULL point and that is, zero current flows through the measuring device. Hence the internal impedance of the measuring device has no detrimental effect on the measurement.

    Thorough knowledge of bridge configuration, characteristics and applications is an essential part of any EE study program.
     
  20. #12

    Expert

    Nov 30, 2010
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    When you make 200,000 of your finished products, is each one going to contain a strain gauge amplifier or are you going to go buy (200,000) 6 digit DVMs and install one in each of your products? That's what a Wheatstone Bridge is for; good accuracy, really cheap compared to any other method.
     
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