Design And Application In Aoutmatic Weather Station Of Program-controlled Active Resistance System

Variable resistors are usually used to simulate resistive sensors, especially in instrument calibration and calibration. But the accuracy of existing variable resistances is limited, and the resolution is low. In the actual operation, we need to manually change the resistance value , and using two-port network measurement is to reduce the measurement accuracy, that cannot meet the requirement of high-precision instrument verification and calibration. Therefore, the research of high precision. high resolution variable resistors which can be adjusted automatically becomes very necessary.The drain-source channel resistance of the junction field effect transistor is controlled by the gate-source voltage, by using the principle and using the single-chip to control the gate-source voltage, the high-precision variable resistor is realized. The performance of the variable resistor is analyzed by the derivation of the JFET current formula and scanning simulation of the P Spice circuit, and the method improving the nonlinear and the dynamic range of the JFET variable resistance is proposed. Based on above, combining the CPU、matrix keypad、LCD. D/A converter and output module of resistance signal with JFET, the programmable JFET variable resistor is designed.Aiming at the automatic channel station temperature channel verification, the original four - wire measurement method based on resistance array is improved with the real - time measurement, and a four - wire program - controlled active resistance method is proposed, which needs parameters of temperature channel verification.The precision of improved resistance array measurement method is better, but it can only simulate a limited number of temperature values, while the design of programmable active resistance measurement method can simulate temperature varying continuously. The range of the module variable resistor is 75.703～141.004Ω ,and the resolution is below 0.025 Ω , which meet the resistor range from 80.305 to 130.894 Ω and resolution below 0.03 Ω requirements of the temperature channel of automatic weather station signal simulator. In order to improve the accuracy, the five variables are used to fit the control voltage and the variable resistance. This variable resistor provides standard variable resistor signals for the test of the temperature channel of data collectors.