Soil Testing Instrument Metering Circuits, Temperature Compensation Measures And Methods

Temperature is an important factor which affected the precision of the electrical instruments. Acute change of temperature had an uncontrollable effect on the circuit. This paper briefly summarized the ways of circuit's thermal compensation. Through the experimentation with the photoelectric signal circuit of TYZ-3 intelligent soil nutrition gauge, JFET was found as the major cause of circuit's thermal drift. And the output voltage drift of the circuit, which was dued to thermal drift, varied according as the different grid voltage of JFET. At the grid voltage of(-1.370v~-1.390v),(-1.390v~-1.410v),(-1.410v~-1.430v),( -1.430v~-1.460v ) ; the corresponding thermal drift of output voltage was 23mv,230mv,1275mv,1105mv. And the thermal drift curves were measured in different ranges of grid voltages in order to work out compensatory temperature-voltage curves which eliminated the thermal drift. The means of the using of thermal resister network for circuit's thermal compensation was adopted, which was based on compensatory temperature-voltage curves. The mathematical model of thermal resister network was offered by the least square method. And the parameters were calculated by the mathematical model, including the values of five resisters of the thermal resister network. The experimental result of the circuit, which integrated the thermal resister network, had shown that the circuit's output shows less voltage drift across the whole temperature range(26℃~40℃) than before. At the grid voltage of(-1.370v~-1.390v), thermal drift of output voltage changed from 23mv to 11mv; at the grid voltage o(f-1.390v~-1.410v), thermal drift of output voltage reduced from 230mv to 36mv; at the grid voltage o(f-1.410v~-1.430v), thermal drift of output voltage reduced from 1275mv to 162mv; at the grid voltage of(-1.430v~-1.460v), thermal drift of output voltage reduced from 1103mv to170mv. There are not universal ways of solving the problem of circuit's thermal drift, this paper maybe provide some useful idea for other similar electrical instrument designers.