LCR meters or LCR bridges are test equipment used to measure components’ inductance, capacitance, and resistance. These are used for inspection to ensure that the components are not faulty. They are used in development laboratories that require testing and measuring the actual performance of specific parts. LCR meters derive their name from inductance, capacitance, and resistance denoted by the letters L, C, and R. Certain versions of LCR meters employ a bridge circuit format as the base of its circuit, which also contributes to the name.
Types of LCR Meters
A wide variety of LCR meters are available, starting from a simple version that provides indications of the impedance only by converting the values to inductance or capacitance. Sophisticated LCR meters can measure the actual inductance or capacitance, the equivalent series resistance of capacitors and the Q factor of inductive components. It makes the LCR meter a valuable tool for assessing the overall performance or quality of the component.
Two types of circuit techniques are used as the basis of an LCR meter. They are as under:
The bridge method is based on the Wheatstone bridge concept, where the aim is to go for a situation wherein the bridge is balanced with no current flow through the meter. The bridge positions at the balance points are used to determine the value of the component under test. This method is usually used for lower frequency measurements – often measurement frequencies of up to 100 kHz.
In the bridge method, the DUT or device under test is placed in a bridge circuit, and its value is determined from the settings for the other elements in the bridge. LCR meters using this technique are known as LCR bridges.
The current-voltage method is generally used for components used for higher frequency applications. It provides a highly accurate measurement technique for high frequencies and over a wide range of values. Popularly known as the RF I-V measurement method, this technique uses current and voltage measurements. However, it uses an impedance matched measurement circuit as the frequencies are high. A precision coaxial test port may also be employed in some instances of very high frequency and high precision measurements.
To get accurate measurements using an LCR meter or LCR bridge, take care of the following:
Effect of lead length – the lead length starts to impact at frequencies above 1 MHz or so. A rough estimate for lead inductance is around 10 nH per cm of lead. For the best results, keep the leads as short as possible.
Measure at operational frequency – while taking measurements with an LCR meter, use a test frequency as close to the actual operating frequency as possible. It will ensure that any stray effects or changes due to frequency are minimal. For instance, inductor cores may display different properties at different frequencies, which can make a noticeable difference in some cases.
Adjust test amplitude – just as it is a good practice to measure as close to the operational frequency as possible, the same is true for the test amplitude. This is because the component values may vary according to the signals applied. This is especially true in the case of inductors that use cores such as ferrite, which may introduce losses, which may be amplitude dependent.
Discharge capacitors before measurement – as some capacitors might carry a residual charge in certain situations, it is best to discharge them before taking any new measurements.
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