MI 3250 MicroOhm 10A
Measurement
29
5 Measurement
5.1 Four wire Kelvin method
When measuring resistance <20
Ω it is advisable to use a four wire measurement
technique (Figure 5.1), for achieving high accuracy. By using this type of measurement
configuration the test lead resistance is not included in the measurement, and the need
for lead calibrating and balancing is eliminated.
Rwire
Rwire
Rwire
Rwire
A
V
DC
Rx
MicroOhm 10A
P1
P2
C1
C2
Figure 5.1: Four wire Kelvin method
The measuring current is passed through the unknown resistance Rx using the C1 and
C2 probes. The placing of these probes is not critical but should always be outside the
P1 and P2 probes. The Voltage drop across the Rx is measured across P1 and P2 and
these should be placed exactly at the points to be measured.
Note about poor connection:
Most measurement errors are caused by poor or inconsistent connection of the
object under test. It is essential to ensure that the device under test has clean,
oxide and dirt free contacts. High resistance connection will cause errors and may
prevent the selected current to flow, because of the high resistance of C1 - C2
loop.
Note:
Ohm's law
states that the current through a conductor between two points is
directly proportional to the potential difference or voltage across the two points,
and inversely proportional to the resistance between them. The mathematical
equation that describes this relationship is:
Amper
I
Volt
U
Rx
Ohm
R
Volt
U
Amper
I
