Kelvin Sensing?

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

1. Bill GrayGuest

What is Kelvin Sensing?

Bill

2. JeffMGuest

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

Wire resistance
+--------+
¦ OUTPUT¦=======/\/\=======+
¦ ¦ /¦
¦ High ¦----------------/ ¦
¦ 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. JeffMGuest

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

View with monospaced font (Courier).

Wire resistance
,--------.
| OUTPUT|=======/\/\=======+
| | /|
| High |----------------/ |
| 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 LarkinGuest

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. JamieGuest

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
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
give you when they are shorted together.

6. PeteSGuest

As noted, Kelvin sensing is used in high current sense (low resistance
sensor) applications. You can find some specifically made sense
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