Kelvin Sensing?

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Wire resistance +--------+ =A6 OUTPUT=A6=3D=3D=3D=3D=3D=3D=3D/\\/\\=3D=3D=3D=3D=3D=3D=3D+ =A6 =A6 /=A6 =A6 High =A6----------------/ =A6 =A6 Sense=A6 Kelvin lead / =A6 =A6 \\ LOAD =A6 Low =A6 Kelvin lead / =A6 Sense=A6---------------- \\ =A6 =A6 \\ =A6 =A6 =A6 \\=A6 =A6 RETURN=A6=3D=3D=3D=3D=3D=3D=3D/\\/\\=3D=3D=3D=3D=3D=3D=3D+ +--------+

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.

D'oh. Forgot I was working in extended ASCII.

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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.

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:

John

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.

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

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