Microcontroller Based Lcr Meter

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  • Topic: Voltage, Capacitor, Electrical impedance
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M ~ E S S O R S AND

MICROS~TEMS
ELSEVIER
Microprocessors and Microsystems 20 (1996) 297-301

Technical application

Microcontroller based LCR meter
M.A. Atmanand, V. Jagadeesh Kumar
Department of Electrical Engineering, Indian Institute of Technology, Madras 600 036, India Received 19 March 1995; revised 10 May 1996

Abstract

A microcontroller based scheme for LCR measurement is described. The unknown element (an inductor or a capacitor or a resistor) is measured employing a non conventional ac bridge. The element to be measured forms one arm (side) of the bridge and the second (series) arm is made up of a simple resistor. A Multiplier type Digital to Analog Converter (MDAC), controlled by a microcontroller, serves as the other two arms. The microcontroller, after obtaining quadrature condition between the bridge output and one of the designated bridge voltages, acquires the current through and voltage across the series connected resistor. With these values and the value of digital input to the MDAC, the parameters of L or C or R values are evaluated by the microcontroller and displayed in appropriate display fields. The scheme was implemented using an Inte18751 microcontroller. An overall accuracy of the order of +1.0% was achieved for the prototype with a 12-bit MDAC having an accuracy of 4-0.2%. Keywords: LCR measurement; Microcontroller-application; Quasi-balanced bridge; PSD-application

1. Introduction

Measurement of components (inductance L, Capacitance C or resistance R) is essential in many fields of electrical and electronics engineering. Several schemes for LCR measurement have been developed for general as well as specific applications. These methods can be normally grouped into (a) bridge methods and (b) direct methods. In a class of direct methods o f measurements, a known current is passed through the unknown impedance Z (where Z could be L, C or R) and the resulting voltage across it is measured and used for computation [1,2]. Direct methods are also reported, wherein the unknown impedance is connected in series with a standard resistance and the series combination is excited by a sinusoidal source [3,4]. The various voltages across the standard resistance, impedance and the source are used for the computation o f impedance. The earliest and the most accurate methods of measurement o f an unknown impedance are the bridge methods [5,6]. Since in these ac bridges, balance is obtained by varying two parameters, convergence towards balance would involve several steps. Complex automatic balancing techniques have 0141-9331/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PH S0141-9331(96)01095-2

been developed for a specific application of these bridges [7,8]. To overcome the problem of balancing in conventional ac bridges, quasi balancing methods have been proposed. These quasi balanced bridge-forms require adjustment of only one variable element and hence convergence to the balance condition, in general, is obtained with few steps. However, in conventional quasi balanced bridge forms, two independent quasi balances are obtained and the parameters of the unknown Z computed therefrom. In most of these types of bridges, balancing involves achieving a "minimum" on the detected output. The sensing of such a minimum detected voltage/ current becomes difficult in most cases and under certain conditions the detection becomes impossible [9]. We now propose a new approach to a conventional quasi balanced bridge. With an added Phase Sensitive Detector (PSD), the "quasi balance" condition is indicated by obtaining a zero at the output of the interposed PSD. Hence sensing of quasi balance can be easily implemented. Instead of obtaining a second quasi balance, here certain circuit responses are measured and the value of Z is computed. Hence, unlike the conventional ac bridges, the accuracy of the proposed method would be mainly dictated by the accuracy with which the circuit responses are...
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