To measure the resistance or impedance of a load, multimeters often use a technique
known as "constant current."
In this method, a source current of constant and known amperage flows across the load under
test.
The multimeter simultaneously measures the voltage across the load, and employs Ohm's
Law to calculate the resistance.
A relatively simple setup for constant current resistance measurement is the two-wire method,
also called the two-probe method.
This entails connecting two probes to the load under measurement, running a known current
through the load and measuring the resulting voltage, and then calculating the resistance.
In the diagram shown on the screen, the probes numbered 1 and 2 provide constant current
to the load under test, and also measure the voltage across the load in order to determine
the resistance.
The two-wire method works well for moderate-to-high load resistance or impedance.
However, for very low load resistances, the resistance inherent in the probe wiring and
contacts can contribute a significant percentage to the overall results, making the measurement
of the test load resistance inaccurate.
To accommodate this issue, more advanced instruments employ a setup known as the four-wire or Kelvin
method.
As the name implies, this involves using four probes in an arrangement similar to the example
shown on the screen.
In this setup, two separate pairs of probes are used.
Pair 1 and 2 (sometimes referred to as the "source" probes) provides a known quantity
of constant current, as in the two-wire setup.
In addition, the Kelvin technique employs a second pair of probes (labeled 3 and 4 above)
to sense voltage.
This second pair is often called the "sense" probes.
The placement of these probes ensures that only the voltage across the load is measured,
independent of any resistance contributed by the current (source) probes.
There is virtually no current flowing through the sense probes; nearly all the current flows
through the source probes.
This allows for a high level of measurement sensitivity and accuracy when the load under
test is of low resistance.
The ability to accurately measure low resistances can be critical in applications that involve
high current, since even a small change in resistance (for example, due to fluctuating
temperature) can have a significant effect on voltage levels.
AEMC Instruments offers a family of micro-ohmmeters capable of performing four-wire measurements
for load resistance and impedance.
This includes the recently announced Micro-Ohmmeter Model 6292.
In the example shown on the screen, the Model 6292 is measuring the resistance of a test
shunt.
Other AEMC micro-ohmmeter instruments that provide 4-wire testing include the Model 6240
and Model 6250 products.
Note that the four-wire method is not suitable for all applications.
For example, when the load resistance is high, the resistance in the probes is relatively
smaller as a percentage of the overall resistance and therefore does not introduce an unacceptable
margin of error.
Further, measuring high resistance loads requires that the constant current be kept at very
low levels.
As a result, ground leakage resistance can become a significant contributor to the overall
resistance measurement, in the process rendering the measurement inaccurate.
The Model 6292 mitigates the problem of ground leakage by offering a mode called "Both Sides
Grounded" (BSG).
In BSG mode, the Model 6292 uses an external DC current probe to measure and eliminate
ground leakage current from the total test current.
We hope you have found our brief explanation of the four-wire Kelvin method helpful.
Please check our YouTube channel for additional instructional videos about other topics in
electronics, including how to use the many products provided by AEMC Instruments.