Hioki CKT3548, Руководство пользователя, Страница: 78 / 86

Changzhou Chuangkai Electronic Co.,Ltd Edition V1.0
78
Appx. 2 Four-Terminal (Voltage-Drop) Method
The accuracy of low resistance measurement is significantly affected by the
resistance of wires between a measuring instrument and probes, and by the
contact resistance between the probes and a measurement target.
Wiring resistance varies significantly, depending on the thickness and length
of the wire. The cable used for resistance measurement is approx.
mΩ/m for
AWG24 (0.2sq) or approx. 24
24mΩ/m for AWG18 (0.75sq), for example.
Contact resistance depends on the degree of wear and contact pressure of the
probes, and the measurement current. Even for a good contact, the resistance
is several
mΩ. It is not rare for the resistance to reach several Ω.
The four-terminal method is essential for measuring very small resistance
values. With two-terminal measurements (Fig. 1), the resistance of the test
leads is included in the measured resistance, resulting in measurement errors.
The four-terminal measurements (Fig. 2) consist of the current source
terminals (SOURCE A and SOURCE B) to provide constant current, and
voltage detection terminals (SENSE A and SENSE B) to detect voltage drop.
Because of the high input impedance of the voltmeter, measurement
requires practically no current flow through the leads connecting the voltage
detection terminals to the measurement target, practically eliminating the
effects of lead and contact resistance on the measurement. Two-terminal
measurement method Four-terminal measurement method
Measurement current I flows through test object resistance R0 as well as
lead resistances r1 and r2. The voltage to be measured is obtained by E=I
(r1+R0+r2), which includes lead resistances r1 and r2.