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Kelvin Sensing?

Discussion in 'Electronic Basics' started by Bill Gray, Aug 10, 2005.

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  1. Bill Gray

    Bill Gray Guest

    What is Kelvin Sensing?

    Bill
     
  2. JeffM

    JeffM Guest

    What is Kelvin Sensing?
    View with monospaced font (Courier).

    Wire resistance
    +--------+
    ¦ OUTPUT¦=======/\/\=======+
    ¦ ¦ /¦
    ¦ High ¦----------------/ ¦
    ¦ Sense¦ Kelvin lead /
    ¦ ¦ \ LOAD
    ¦ Low ¦ Kelvin lead /
    ¦ Sense¦---------------- \
    ¦ ¦ \ ¦
    ¦ ¦ \¦
    ¦ RETURN¦=======/\/\=======+
    +--------+

    When you pump current through a resistance,
    you get a voltage drop across that resistance.

    In a power system, you want to sense the voltage at the load
    and adjust the supply's output level
    to get the voltage that is actually delivered TO THE LOAD correct.
    ..
    ..
    How this relates to your original question:

    When a car starter pulls >100 amps,
    the voltage drop across the internal resistance of the battery is
    significant.

    When a LED pulls 0.020 amps out of the battery, the drop is nil.
    To get an idea of what a condition a battery will be in
    to deliver starting current,
    you have to pull significant current out of the battery.

    As DBLEXPOSURE has pointed out,
    to do so continuously will, of course, run down the battery.
     
  3. JeffM

    JeffM Guest

    What is Kelvin Sensing?
    D'oh. Forgot I was working in extended ASCII.

    View with monospaced font (Courier).

    Wire resistance
    ,--------.
    | OUTPUT|=======/\/\=======+
    | | /|
    | High |----------------/ |
    | Sense| Kelvin lead /
    | | \ LOAD
    | Low | Kelvin lead /
    | Sense|---------------- \
    | | \ |
    | | \|
    | RETURN|=======/\/\=======+
    `--------'

    When you pump current through a resistance,
    you get a voltage drop across that resistance.

    In a power system, you want to sense the voltage
    that is actually delivered TO THE LOAD.
    ..
    ..
    How this relates to your original question:

    When a car starter pulls >100 amps,
    the voltage drop across the internal resistance of the battery is
    significant.

    When a LED pulls 0.020 amps out of the battery, the drop is nil.
    To get an idea of what a condition a battery will be in
    to deliver starting current,
    you have to pull significant current out of the battery.
    To do so continuously will, of course, run down the battery.
     
  4. John Larkin

    John Larkin Guest


    1. Measuring a resistance by running 4 wires from the measuring
    instrument to the resistor under test. 2 wires apply current, 2 wires
    sense the voltage drop. This makes the resistance of the lead wires
    not matter. Vital for measuring very low resistances.

    2. Applying a voltage from a power supply to a remote load, with two
    wires driving the load and two more sensing the voltage *at the load*
    and delivering feedback to the power supply. Again, this eliminates
    voltage drop errors in the wires. Also known as "remote sensing."

    After this guy:

    http://scienceworld.wolfram.com/biography/Kelvin.html

    John
     
  5. Jamie

    Jamie Guest

    something like a bridge input only the
    leads normally extend out to the test
    fixture as a loop/part of this bridge
    input. etc..
    the idea is to include the leads as
    part of the balancing bridge on both
    the + - sides, the actual test item
    will actually me measured with out influence
    of the test leads.
    you will see the test leads being connected
    together at the test clip. this is the return
    loop.
    ect..
    Kelvin test set is normally used in performing
    very low ohm readings. readings that normally way
    below the value of your normal DMM test leads would
    give you when they are shorted together.
     
  6. PeteS

    PeteS Guest

    As noted, Kelvin sensing is used in high current sense (low resistance
    sensor) applications. You can find some specifically made sense
    resistors at this link
    http://www.isotekcorp.com/productDisplay.asp?CatID=1&SubCatID=
    where some have 4-wire (Kelvin) sensing, some don't.

    You can get Kelvin type sensing from an ordinary sense resistor (well,
    a decent approximation of it) by doing this: (View with monospace -
    courier)

    -----------------------
    | | | |
    -------------------| | Sense | ------------- main current
    track
    -------------------| | Resistor | -------------
    | |-----Sensor---- | |
    --------Points---------


    Where the bulky parts at each end are the solder pads, and the dashed
    lines around the word 'Sensor' are tracks for the sense points.

    This permits high accuracy measurement of the voltage across the
    resistor without (as noted above, again) worrying about the resistance
    in the sense lines. Very commonly used in relatively high current
    supplies.

    Cheers

    PeteS
     
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