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Understanding the (point of) the Wheatstone Bridge

Discussion in 'Electronic Basics' started by Jamie Jackson, Sep 21, 2007.

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  1. My son and I are working through some circuits from a Forrest M.
    Mims / Radio Shack learning lab. We got to a Wheatstone Bridge
    circuit, but I'm trying to understand the usefulness of it.

    Let's use this diagram, for reference: http://en.wikipedia.org/wiki/Wheatstone_bridge

    If you need to adjust R2 in order to get the value of Rx, how do you
    even know what R2 is anymore (since you've adjusted it)? Is R2 some
    fancy high-accuracy *graduated* variable resistor that I've never
    heard of?

    After seeing the diagram, I'm thinking: Well, if I have to measure my
    R2 with my multimeter, I might as well measure Rx while I'm at it,
    which blows the point.

    Is the point just that they're perfectly balanced, and the point is
    not what the actual values are?

    Thanks,
    Jamie
     
  2. Dave

    Dave Guest

    Hey Jamie,

    Well, so far as usefulness of this circuit is concerned, I used to work on
    high-speed printers (mainframe environment like banks and oil companies)
    that used the Wheatstone Bridge (they called the actual 'box' that housed
    the circuit an H-switch) to drive the servo motor that moved the paper at
    incredible speed and stop on an instantaneous command. The servo was so
    strong it would literally rip your arm off if you happened to be holding the
    shaft in your hand when it took off. You couldn't let go fast enough... I
    know it has applications in testing unknown components, but that was what
    that company used it for. For unknown components, a built in readout can be
    used to tell you what is plugged into it, without running the readout
    current through the device under test. Can make a useful piece of test
    equipment...

    All I can think of.

    Dave
     
  3. Thanks Dave,

    I think I'm getting it now, actually. I reread the wiki article. It
    seem that a variable resistor at R2 is just a nifty way to test for
    balance (and not necessarily for finding the value of Rx).

    Rather, to find the resistance of Rx, you work with three resistors of
    known value, and use Kirchhoff's rules based on those values and the
    readout of the meter.

    Sometimes it just helps to post, and then the mind clears on its
    own. ;-)

    Thanks,
    Jamie
     
  4. Nobody

    Nobody Guest

    Yes, it's typically a variable resistor with a graduated knob on it.

    Or it might be a multi-way switch and a ladder of fixed resistors. Or
    several of them (e.g. one switch with 10 1kOhm resistors and one with
    10 100Ohm resistors, giving 0-9900 Ohms in 100-Ohm steps).
    You need to know the value of R2.
     
  5. Nobody

    Nobody Guest

    That's an H-bridge, not a Wheatstone bridge.

    A bridge is any circuit with that "shape", i.e. two "legs" joined at the
    top and bottom, an input applied between the top and bottom, and an output
    between the centres.

    A Wheatstone bridge has resistors, a bridge rectifier has diodes, an
    H-bridge typically has MOSFETs (although they could be BJTs or even
    relays).

    An H-bridge allows you to vary the magnitude and direction of a current
    through a load, typically a motor. The name arises from the fact that the
    circuit is normally drawn with the legs vertical, rather than the diamond
    shape which is commonly used for the Wheatstone bridge or a bridge
    rectifier.
     
  6. Don Bowey

    Don Bowey Guest

    In expensive bridges, such as the ones the telcos use to measure cable
    faults, the R2 in the diagram is not a variable resistor. It is a large
    group of fixed, precision, resistors on rotary switches than can be switched
    in, in a sequence, to balance the bridge. The Rx resistance is then read
    from the positions of the switches.

    To be balanced R2 must equal Rx. The ones I used in the Toll Testroom could
    measure to within .01 Ohm.
     
  7. Rich Grise

    Rich Grise Guest

    I saw an app where there were, in effect, three fixed, calibrated
    resistors and the fourth was a magnetron filament. When a magnetron
    is in use, some of the electrons fall back to the cathode (back-
    bombardment), and help to heat it. The bridge was used to monitor the
    resistance of the filament, and the loop was calibrated to the tempco of
    whatever the filament was- probably thoriated tungsten. Anyway, the
    filament current went through the bridge, and a diff amp picked up the
    balance point; the niftiest part was that the whole thing was biased at
    about -1500V. Because of the bridge arrangement, there were two advantages -
    one, that it could float, like I said, but that it doesn't really care
    how much current is flowing in the load, since it's comparing resistance,
    to make a temperature controller.

    Hope This Helps!
    Rich
     
  8. Rich Grise

    Rich Grise Guest

    Actually, R1/R2 has to be equal to R3/Rx. But both sides don't necessarily
    have to be the same. (i.e. 100/20 == 5/1, for example.)

    Cheers!
    Rich
     
  9. Don Bowey

    Don Bowey Guest


    That, of course, was a given, and was likely also explained in the Wiki
    blog.
     
  10. Jamie

    Jamie Guest

    It then must have a Kelvin set up?
     
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