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Monday, 26 August 2019

Digital LCR Meter Block Diagram and Working


The principle involved in the measurement of the component values is the measurement of voltage across, or current through the device under test. The voltage can be directly measured with a D.V.M if the voltage developed is made proportional to the value of component under test. If the current through the component under test represents the value of component then the current can be converted in to voltage. That voltage can be measured with the D.V.M. In either case the voltage measured with the D.V.M. can be made to represent the value in precise and proper units of the value of the component under test.

The best example of this type of measurement is the resistance measurement that is done in a digital multimeter. Actually a constant current source supplies the current through the internal range resistor, and the unknown resistor forming a potential divider circuit. The part of the voltage across the unknown resistor is given to the D.V.M., which intern directly gives a readout of the value of resistance in ohms. Selection of proper values of range resistors, and switching them results in the range selection.

(a) Description of Block Diagram:


The block diagram of a digital L.C.R. bridge or L.C.R. meter is shown in Figure. It consists of a 1 kHz oscillator and a current to voltage converter, it is nothing but an operational amplifier. The oscillator output reaches the current to voltage converter through a selectable source resistance Rs and the component under test (DUT). The output of the current to voltage converter and the differential amplifier along with the associated feedback circuit goes to the control switch. From the control switch the output is fed to the average voltage detector and the phase sensitive detector. These two outputs of the A.V.D. and P.S.D. act as reference and input signals respectively to the digital voltmeter module. The digital voltmeter, displays the value of inductance or capacitance depending on the component connected to the test terminals.

In addition to the above blocks we find a constant current source, a range selector and two terminals for the unknown resistance. They are connected appropriately to the D.V.M. module. This arrangement is exclusively for the measurement of resistance. The measurement of resistance is done as is done in the case of Digital Multimeters.



(b) Working Principle Involved in Measuring L and C:


In the block diagram illustrated in Figure, DUT represents device under test that is the component under test. A capacitor, or an inductance can be connected across these terminals to estimate its value. An oscillator working at 1 kHz frequency is used to apply a test signal to the component under test through a selectable source resistance Rs. An operational amplifier works as a current to voltage convertor. It has a range selector resistor Rr , incorporated in its feedback path. The operational amplifier drives the junction of component under test and Rr. to a virtual ground. Therefore Rr, will not change the current through the unknown component (or the component under test or device under test DUT).

Therefore the voltage across the unknown component is E1 , as is marked in the block diagram. The signal current will flow through Rr . It produces a voltage across Rr , which is proportional to the current through the unknown component.

The voltages E1 and E2, are vector quantities. Therefore they define the characteristics of the device at a given test frequency and signal level.

Mathematically, E1 α V and E2 α I
The capacitance C α 1/V α E2/E1
The inductance L α V/I α E1/E2

These ratios are adopted in the measurement modes and are displayed using dual slope converter module.

Working of the digital L.C.R. meter: The values of Rs and Rr will be selected depending on the impedance of the unknown component.

Inductance is measured in the series equivalent mode. The impedance of the unknown inductance is usually low at the test frequency. The value of Rs , is selected to be much higher than the impedance of unknown inductance's impedance. This results in a constant current drive through the unknown inductance. The magnitude of current will be given by the value of Rs.

Capacitance will be measured in the parallel equivalent mode of operation. The impedance will be high, Hence Rs, will be a much lower value, than the impedance offered by the capacitor at the test frequency. This results in the constant voltage drive.

The values of Rs and Rr, in any selected range are equal. Therefore equal voltage drops are obtained across Rs and Rr, with same signal current flowing through them.

The signal voltage E1, is allowed through the differential amplifier. Then it is given to a control switch SWA. This signal voltage E2, is also given to the control switch SWA, supplies the greater of E1 or E2 to the average voltage detector. The lesser one is given to the phase sensitive detector P.S.D.

The outputs of A.V.D. and P.S.D. are steady voltages (D.C. voltages). They are given to the D.V.M. module as reference and input signals. Hence the D.V.M displays the value.

(c) Resistance Measurement:


The resistance measurement is effected using the constant current source and range resistors as is done in a digital multimeter. Separate terminals are provided for measuring the resistance values.

This is a simple block diagram and illustrates only the basic principle of digital LCR meter. Commercial models have different circuit configurations. However in principle the digital component testers use the principle in measuring the voltage caused by the component or measuring the voltage caused by the current flow in the component, to estimate the value of the component under measurement.


(d) Specifications of Digital L.C.R. Meter:

The following are the specifications of a typical Digital L.C.R. meter:

Designation
Specifications
1. Measuring Range:
Inductance :
Capacitance :
Resistance :

0-2 Henries in 5 decades.
0-2 Micro Farads in 5 decades.
0-2 Mega Ohms in 5 decades.
2. Resolution :
0.1 mH/mF/ohm
3. Accuracy :
+ 1% + I digit.
4. Source :
1 kHz for L & C and D.C. for Ω
5. Compliance stress :
1 Vpp AC/100 m App. AC.
6. Guard :
Terminals provided
7. External Polarization :
External Polarization is applicable for capacitance measurement.
8. Effect of `Q' :
Within the accuracy of Q > 1
9. Power supply :
230 V + 15% 50 Hz




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