A precise regulator circuit of excitation current for single and double coil transducers used in contactless measurements of electrical conductivity

W pracy przedstawiono układ precyzyjnego regulatora prądu cewki wzbudzenia dla przetworników indukcyjnościowych jedno i dwucewkowych wykorzystywanych w bezstykowych pomiarach konduktywności elektrycznej, który został opracowany w oparciu o wzmacniacz z napięciowo regulowanym wzmocnieniem. Prezentowany regulator prądu umożliwia precyzyjne utrzymanie prądu o ustalonej wartości w obwodzie cewki wzbudzenia przetworników indukcyjnościowych w szerokim zakresie częstotliwości pracy. Układ został wykonany w technologii montażowej SMD, a następnie poddany badaniom sprawdzającym założenia i wymagania odnośnie dokładności regulacji prądu cewki wzbudzenia przetworników indukcyjnościowych.The paper presents a precise current regulator circuit for the excitation coil of single and double coil inductive transducers used in contactless measurements of electrical conductivity. The proposed current regulator circuit for inductive transducers is free from disadvantages of solutions based on the programmable devices that need software algorithms to determine the desired value excitation current. Moreover, the use of analog circuit allows obtaining better performance and reliability in a wide range of operating frequencies. This current regulator circuit uses a wide dynamic range AD8336 variable gain amplifier (VGA) as the gain control element [10]. The current regulator circuit provides precise current regulation in the circuit excitation coil of the inductive transducers up to 270 kHz. Because of the wide range of operating frequencies and the wide range of input voltages, the current regulator circuit can be widely applied in the systems in which the precise regulation is required for the resistive or inductive loads. The current regulator was made in the SMD technology and was tested to check the assumptions and requirements for the current regulation accuracy for inductive transducers. From the tests and analysis of the results obtained there was determined the current regulation accuracy equal to 0.7% for the average value of the current in the relevant frequency range of the regulator circuit. It is the sufficient accuracy to meet the requirement of maintaining a constant excitation current in inductive transducers in both cases of the measurement [1] in contrast to the situation when such a regulation would not be applied