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frequency would be optimal. For example, if the capacitance of the component is to be in
the mF range, then selecting in the range of 20 Hz to 200 Hz for test frequency would give
much better results. The results will also be obvious because if the same component was
tested with 1 kHz or above, the measured readings may look erroneous on the display.
In all cases, it is best to check with the manufacturer’s data sheet in order to determine the
best test frequency to use for measurement.
Inductance
Typically, a 1 kHz test frequency is used to measure inductors that are used in audio and RF
circuits. This is because these components operate at higher frequencies and require that
they be measured at higher frequencies above 1 kHz. However, a 120 Hz test signal is used
to measure inductors that are used for applications such as filter chokes in power supplies,
which are typically operated at 60 Hz AC (in U.S.) with 120 Hz filter frequencies.
In general, inductors below 2 mH should be measured at 1 kHz frequency while inductors
above 200 H should be measured at 120 Hz.
In all cases, it is best to check with the manufacturer’s data sheet in order to determine the
best test frequency to use for measurement.
Choosing a Measurement Circuit Model
There are two measurement circuit models to choose from, series or parallel, when
characterizing components under test. The LCR meter will make measurements regardless of
which model is chosen, but components will display more accurate results if the correct
measurement model is selected. To determine the better model to choose depends on the
impedance of the component at the specified frequency.
Capacitance
The impedance in a capacitor is negatively proportional. Therefore, the larger capacitance
means the lower impedance, the smaller capacitance reverts to higher impedance. Figure
16 shows the equivalent circuit of capacitor. If the capacitance is small, the parallel
resistance (R
P
) becomes large and more relevant than the series resistance (R
S
). If the
capacitance is large, the series resistance becomes dominant and the parallel resistance
becomes insignificant. Hence, it is proper to use the parallel model for low capacitance
measurement and series model for high capacitance measurement.
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