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4-2. Measuring Voltage
LCR meters generally provide internal resistances to protect their own power supply circuits.
Depending on the value of this resistance, the actual voltage differential between the electrodes
of a capacitor being measured drops excessively which prevents correct measurement of
capacitance and dissipation factor of the capacitor. Depending on type of LCR meter used,
measurement of a ceramic capacitor with large capacitance such as 10
F at an assigned
voltage may be impossible due to extremely low impedance of such capacitor.
This mechanism is explained using the simple equivalent circuit model shown in Fig.8.
Measuring voltage is applied to DUT, E
dut
is a partial potential of power supply voltage E
O
divided
by impedance of DUT, Zx = R + jX, and the LCR meter
’
s internal impedance R
in
.
The measuring voltage applied to DUT, E
dut
, is determined by:
√
(R
2
+X
2
)
E
dut
=E0
√
{
(Rin+R)
2
+X
2
}
X=1/
C=1/(2
fC)
Thus, the measurement voltage of capacitor is different from the power supply voltage.
Therefore, we recommend using a LCR meter that automatically maintains the
“measur
ement
voltage
”
at a preset voltage (ALC function). When using a LCR meter not providing such function,
you have to manually measure actual voltage applied between the terminals of the capacitor
being measured, etc. and manually adjust it.
Fig.8 Measuring Voltage Applied to DUT
4-3. Capacitance Measurement Circuit Mode
Capacitance measurement circuit mode generally includes two types of circuit modes.
These are; parallel equivalent circuit mode and serial equivalent circuit mode.
(1) In case of small capacitance (see Fig.9):
A small capacitance has a large reactance, i.e. high impedance, which causes the influence
of parallel resistance Rp on the measurement to be far larger than that of the serial resistance
Rs, thus Rs can be neglected and measurement circuit provides a parallel equivalent circuit
mode.
(2) In case of large capacitance (see Fig.10):
Internal
resistance
Power
supply
