Kelvin Connection Measurements
©Brooke Clarke, 2003 -
Digital Multimeters, like the Hp/Agilent 34401, have a 4 wire
connection for resistance in addition to the two wire
connection. There is a special set of leads for use with the
Kelvin connection. The 34401 reads out to 0.000,1 Ohms.
The 4 wire, Kelvin, connection is very simple. One of the
pairs has a current source and the other pair is a voltmeter.
Suppose that all 4 wires had 10 Ohms of resistance and you want to
measure a 1 Ohm resistor using a 1 ma current source.
If you used a 2 wire Ohm meter then 1 ma * 1 Ohm is 1 mv, but the
leads contribute 2 * 10 Ohms * 1 ma = 20 mv of error.
You could try to use the delta DMM function by shorting the leads
and pressing zero, but the lead resistance may vary as the leads are
moved making it impossible to measure a resistance that's much
smaller than the lead resistance. The contact resistance may
also be greater or equal to the resistance of the device you're
trying to measure.
For a 4 wire, Kelvin, connection: The resistance in the voltmeter
leads has an insignificant effect since the Voltmeter input
impedance is 10 Meg Ohms. Since the meter is connected on the
device side of the lead resistance in the current pair that voltage
drop is not seen by the voltmeter. The voltmeter ends up
reading 1 mv. Note that if there is contact resistance and the
voltmeter is connected closer to the DUT than the current pair the
contact resistance will also be removed from the measurement.
Also note that it really doesn't matter what the lead + contact
resistance is, it will be removed from the measurement as long as
the current source has the compliance to keep supplying the same
current even though the voltage gets large.
By measuring an accurately known resistor using the Kelvin
connection you can determine the scale factor (calibration of both
the current source and the voltmeter). This could then be used
to improve the overall accuracy of the system.
same as Pomona
||The clips have each jaw
insulated from the other jaw and the leads are color
coded. The idea is to use these with the 34401 in
4-Wire Ohms mode and thus be able to measure very small
resistances without any contribution of the test leads or
the resistance of the connecting joint.
Here a 30 Amp Power Pole
Contact (silver plated copper) is being used as the zero
reference. Immediately after making the
connection the resistance was 4.5 milli ohms and after an
hour was 1.8 milli Ohms. Although after an hour the
variability in the reading is less, it's still a little over
1 milli Ohm.
These leads when combined with the 34401 seem good for may
0.01 Ohms, but not 0.001 Ohms.
The specification is 0.010% of reading + 0.004 Ohms.
The ability of this system to measure very near zero is at
the spec limit. But when larger values are measured it
There's no need to use NULL for removing the zero since it's
in the spec noise.
I checked a pocket Amp meter for which the calculated coil
resistance was 2.65 milli Ohms and it measured 2.8 milli
Ohms, close enough.
As a sanity check the resistance of a Greenlee test lead
from my Fluke 87 DMM was measured. The problem was the
safety shrouded Banana plug. By sticking a couple of
short bare copper 14 AWG wires into the gap between the
metal center plug and the metal shell liner and they using a
pop-sickle stick to keep the wires from shorting to each
other a Kelvin connection was made.
The reading is 0.040.5 Ohms (that's 40 milli Ohms) and has
been stable for some time. It did decrease a few milli
Ohms from when first connected but after a few minutes
Using low current DC methods, like
the 34401 for resistances that are below maybe 50 milli Ohms is
not going to work because of thermal effects. That's why
milli Ohm meters like the HP 4328A
use an AC method. The other option is to use a DC current
that's large enough that the thermal voltages are very small by
comparison. Maybe 1 Amp instead of 1 milli Amp.
Analog Devices has AN-306
System Measures Micro Ohms
. It's based on their AD630
Synchronous Balanced Modulator Demodulator which can pull a signal
out of noise that's 100 dB larger.
When measuring the voltage drop across a current shunt it's
important how you connect the lugs to the studs on the shunt.
This shunt has separate terminals for the high current and the
voltage sense and so eliminates the problem of how to connect both
to a single post. The screw is an 8-32 and the bolt is a
¼-20. For both these sizes there are standard crimp ring tongue terminals.
You can see that here the voltage sensing is done closer to the
shunt and the current supply is outside the voltage sensing.
If the voltmeter leads are installed closer to the source of
current then the voltage drop in the high current contact
resistance will become part of the measurement making it less
Correct Way (just an idea, not tested)
Connect the voltage sensing leads close to the shunt and the high
current connections further from the shunt. The idea is that
there's some finite resistance between the voltage sense lug and
the current supply lug. By wiring the voltage sense leads
close to the shunt those small extra resistances are not part of
A.C. Test Method
By using an A.C. signal the problems
caused by thermal EMFs is eliminated. This is how the HP 4328A MilliOhmMeter
lowest range is 1 milli Ohm full scale. It reads 1.0 milli
ohms for the shunt shown above.
The same shunt has been on the HP 34401A in MIN-MAX mode for some
time and the data is:
MIN: -37.9 milli ohms MAX:
+3.66 milli ohms AVE: -1.19 milli
Ohms # of Readings: 516,346
Clearly all these measurements are wrong.
Connecting a Honeywell Powerpile 750 mv Generator with the sensor
at room temperature to the HP 34401 in 4-wire Ohms mode gives a
reading of +2.6 Ohms, reversing the test leads reads +2.9
Ohms. But as the thermocouple is heated the apparent
resistance climbs and reaches OVLD, i.e. off scale open ckt.
A few minutes after the cigarette lighter has been removed the
reading is still 4.2 ohms and it's decreasing slowly. But
the HP 4328A reads 2.9 Ohms with 400 mv DC across the thermocouple
(to get 750 mv you need more than a cigarette lighter flame.
There may be 500 u Ohms increase in resistance when it's heated.
Mueller - Kelvin
Clips - these are available from most distributors
Pomona Electronics - Kelvin: 5940, 6303 Probe, 6730
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page created 12 Aug 2003.