Research On Constant Current Source For Magnetic System Of Joule Balance

A design scheme of constant current source for the magnetic field system of the JouleBalance at present is introduced in this paper. The fundamental circuit of this constant currentsource is based on the serial feedback constant current circuit. At the same time, an ingeniousvoltage drop feedback compensation circuit for the adjustment tube is devised due to thevariation of the load, which solves the key issue of apparent drift of the output currentbecause of the decrease of voltage between the drain and the source electrode of theadjustment tube due to the increasing of the resistance of the magnetic field coil in the courseof regular working hours.In this paper, the basic structure principle of the constant current source for the magneticfield of Joule Balance is firstly introduced and the ideal formula is deduced. Afterwards, theexternal and the internal factors that influence the stability of the current are analysedrespectively and the selection and design basis of the critical component of the constantcurrent source including reference voltage, sample resistor and error amplifier are determinedultimately. Meanwhile, the design concept of the auxiliary circuit including main powercircuit, auxiliary power circuit, current test circuit and protect&compensation circuit arepresented in detail. Considering apparent drift of the output current due to the decrease ofvoltage between the drain and the source electrode of the regulator caused by the increasedresistance of the constant current source’s load coil, a special feedback compensation circuitfor the regulator is devised. This design solves the limitation and deficiency of traditionalcontrol methods for maintaining the voltage of the regulator used in the design of highprecision constant current source.Three special application and design are mainly discussed in this paper. Firstly, DCVoltage Standard732B is applied in the fundamental stable current circuit of the constantcurrent source for the magnetic system, and the stability of the reference voltage in circuitreaches10^-8per hour when the constant current source is working. Secondly, a sample resistorof low load coefficient and high-power is devised and its stability reaches10^-8per30minutes, which has been proved by experiment. Thirdly, a special feedback compensation circuit forthe voltage between the regulator of the constant current source is devised, which solves theproblem of apparent drift along with time due to the power heating of the magnetic field coil,and the stability of the current is greatly improved. Based on the three critical techniquesabove, a constant current source satisfying the magnetic field system of the Joule Balance atpresent has been achieved.The test results show that the voltage between the drain and the source electrode of theregulator in the constant current source for the Joule Balance only decreases about0.4V whenthe resistance of the magnetic field coil increases145during the normal workingcondition. In30minutes, the relative standard deviation of the current is2.0E-7, and thestability is2.4E-7, which attains the design requirements.