An LCR meter is a digital measurement instrument. Like a digital multimeter, it can be used to measure inductance, capacitance and resistance of components. In addition to this, an LCR meter can also measure the AC resistance of a resistor, equivalent series or parallel resistance, D factor (Dissipation) and Q (Quality) factor of inductors and capacitors.
Capacitors and inductors are often used in frequency sensitive applications like filter and radio tuning circuits. With inductors and capacitors, their impedance changes with frequency. For a pure inductor or capacitor, this impedance just consists of the reactance. However, all components contain some parasitic resistance and hence the term impedance (which includes both resistance and reactance) would be a more accurate usage. This parasitic resistance can be modelled in equivalent series resistance or equivalent parallel resistance.
When we use a DMM to measure L or C, it completely ignores the series and parallel resistance of the components. Further, it makes use of DC quantities to perform the measurement. For example, to measure capacitance, the DMM sends out a constant current through the capacitor to charge it and records the time taken for the capacitor to charge to a particular voltage. From this value of time recorded, the capacitance of the capacitor is displayed by deriving it from the simple expression:
Note that the resistance is too small and hence neglected in the above expression. The capacitor is then discharged safely across an inbuilt resistor and returned to its discharged state so that it's safe for handling.
This method of using the DMM for measuring capacitance may work for casual measurements but when we intend to make use of the capacitor in frequency dependent applications like in an RC filter where the capacitor basically decides circuit parameters like the cut off frequency of the filter circuit, we need more accurate measurements.
A true sense of the quality and expected performance of the L or C component can be inferred by measuring its impedance specifically over the intended range of frequency of operation of the circuit in which we wish to use the L and C component. This requires measuring the components parameters at a test frequency close to that of the circuit operation frequency – this is not possible with a DMM as it only performs measurements with the help of DC quantities.
An LCR meter, on the other hand, has an inbuilt oscillator which can produce small signal AC over a range of frequencies. This oscillator supplies an AC bridge. To completely understand the working of the LCR meter, it's beneficial to review the concept of AC bridges.
As you can see in the schematic diagram below, an AC bridge consists of 4 arms and has 4 nodes (nodes a, b, c, and d). The detector (D) is connected between node a and node b while the AC supply is connected between node c and node d. One of the arms (say Arm 1) is reserved for the device under test (DUT).
When the AC bridge is under unbalanced condition, there is a potential difference between node a and node b and hence across the detector (D). In this case, there's a current flowing through the detector. Now, in order for the AC bridge to be in balanced condition, the current through the detector (D) should be zero. This can be done by varying the impedance of the Arms 2, 3, and 4 until the potential difference between node a and node b becomes 0. Under balanced condition,
from which we get,
The construction of an LCR meter is as shown in the schematic image below. The user can measure the required parameters of the component at any of the test frequency settings available - 100 Hz, 1 kHz, 10 kHz, and even 100 kHz for top end meters. The DUT is plugged into arm 1. Arm 4 of the bridge as shown has two sub circuits - D setting and Q setting. The respective sub circuit is connected into the arm depending on whether the user wants to measure the Dissipation factor or Quality factor of the component. By varying the resistances in arms 2, 3, and 4, a balanced condition is obtained and the impedance of the DUT is found out. This calculated value is then displayed on to the front panel as the measured value.
It is possible to measure the parasitic resistance of the elements in both equivalent series and equivalent parallel resistance modes. The auto mode in the LCR meter intelligently models the resistance based on the components L/C value. However, the basic idea behind choosing a mode to measure the resistance can be summarized from the graph below:
The subscripts p and s stand for parallel and series mode respectively.
The numerous superior functionalities of the LCR meter makes it an obvious choice when we want accurate measurements for application at a particular frequency like in audio circuits, radio frequency circuits, filter circuits and laboratory projects. For hobby projects, however, a DMM will be enough.