Appendix 2 AC Four-terminal Method
A4
The instrument uses the AC four-terminal method, so that resistance measure-
ment can be carried out with the resistance of the leads and the contact resis-
tance between the leads and the object to be measured canceled out. The
following figure shows the principle of the AC four-terminal measurement
method.
An AC current (I
s
) is supplied from the SOURCE terminals of the instrument
across the tested battery. The voltage drop across the internal impedance of the
battery (V
IS
) is measured by the SENSE terminals. At this point, since the
SENSE terminals are connected to an internal voltmeter with a high impedance,
almost no current flows through the resistances R
2
and R
3
which represent the
lead resistances and contact resistances. As a result, there is almost no voltage
drop across the resistances R
2
and R
3
. Thus the voltage drop due to the lead
resistances and contact resistances is very small, and these can be canceled
out. In the instrument, a synchronized wave detection system is used, whereby
the internal impedance is separated into resistance and reactance, and the
resistive component only displayed.
If the lead resistance, the contact resistance between measured object and lead,
or the contact resistance between the lead and the instrument increases, the
instrument can no longer supply normal current to the measured object, resulting
in an abnormal measurement status indicated by "- - - - -" within the measured
resistance field. For more information on abnormal measurements, see Section
" Measurement Fault Detection" (p.39).
Appendix 2 AC Four-terminal Method
図 交流四端子法の概略図
Constant Cur-
rent Source
Voltmeter
Resistance R
Is
R
1
R
2
V
IS
R
3
R
4
Resistance
Measurement
Circuit
Values R1 to R4 are the resistances of the test leads plus contact resistances.
DC-Elimina-
tion Capacitor
Effective Resistance
Reactance
Impedance
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