| Temperature is an important factor which affects the precision of the electrical instruments. Acute change of temperature has random and uncontrollable effect on the circuit. This thesis briefly summarized the ways of circuit's thermal compensation. Through the experiment with the circuit's thermal drift, of TYZ-3 intelligent soil nutrition gauge, J-type field effect transistor(JFET) was found as the major cause of circuit's thermal drift. The thermal drift varied according as the different grid voltage. The thermal drift curves were measured with different ranges of grid voltages.Proportional Controller (P) is used to solve the problem of the thermal drift. P is a kind of basic method to successive temperature controlling. Its characteristic is the deviation value in direct proportion relation that outputs and inputs. That is to say, the deviation value of being adjusted and the position of the performance organization keep the proportion for deviation signal, proportional controller is an amplifier the coefficient can be adjusted.Temperature drift with different grid voltage is different, too. So constant temperature equipment which be controlled by P was adapted in thermal compensation to solve the problem of the thermal drift, and to overcome the compensation deviation margin, that produced by adapting the compensation in segments. Since the constant temperature circuit can control the temperature in the range of (30℃ ±0.1℃), with the precision of 0.2℃. The result of compensation attains 69%~99%. The unstable output under the grid voltage of JFET Which produced by temperature drift was finish well.Different ways was used to different electronic instruments. Thermal compensation has no foxed method and regulation. In this topic we adopted the constant temperature circuit. It maybe provided some useful idea for other similar electrical instrument designers.
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