Development Of High Precision Standard Resistance

The paper summarizes the classification,characteristics and development trend of resistors,and makes comparative analysis of different standard resistors and resistance elements.It points out that standard resistors have been developed very rapidly.The material,structure and processing level are the main factors that determine the performance of the resistors.According to the discussion of the development trend of domestic and foreign standard resistors,it is found that metal-encapsulated alloy foil resistors and air standard resistors are advanced development directions.However,China’s development in these two areas lags behind foreign countries.The significance and content of this research were pointed out.This text has carried on systematic research to the characteristic of the standard resistor.The principle and characteristics of standard resistors are studied in depth from the aspects of material characteristics,selection,temperature coefficient characteristics and stability of resistance elements.It is explained that the material of the resistor determines its basic performance,while the circuit design,structure and production process of the resistor profoundly affect its temperature coefficient and stability.During the research process,a set of automatic precision resistance measurement system with high accuracy and reliable performance was built to accurately measure the resistance value,temperature coefficient of resistance and long-term stability index of the resistors.A rigorous uncertainty evaluation was carried out to ensure that the system has a sufficient level of accuracy to meet the requirements for the development of high-precision standard resistors.We thoroughly studied the method of resistor temperature coefficient compensation,and explored the use of parallel NTC network as a compensation scheme for negative temperature coefficient resistance.At the same time,a design scheme of series-parallel hybrid compensation of resistance temperature coefficient was proposed,and compensation circuits were designed.During the compensation process,multiple groups of resistance elements with close temperature coefficient β values but opposite signs are screened out.Use the equal resistance value matching method to match temperature coefficient β of the equivalent resistance to the design target,while taking into account the redundant design of power and current;then the artificially wound compensation resistors with opposite temperature coefficient a are used to compensate temperature coefficient a of the total resistance,which make the temperature coefficient of the overall resistance approach zero,so as to achieve the purpose of temperature coefficient compensation.Experiments show that this method can effectively reduce the temperature coefficient of the standard resistance circuits.In order to improve standard resistor ’s stability,a new structural scheme of standard resistors is proposed:the circuit board and cylindrical bracket made of polytetrafluoroethylene(Teflon)are designed and manufactured,and high-insulation structural components such as connecting wires with a Teflon outer skin are used to reduce the impact of leakage current on the resistance value;design and manufacture metal double-shell shielded and sealed structural components,put the main resistance circuit component into a small sealed alloy metal inner shell,and then put it into a larger aluminum alloy metal outer shell for double-layer sealing to avoid the influence of humid air and oxidation corrosion from the outside.The two shells are made of heavy aluminum alloy material which can reduce the thermoelectric potential and shield the interference of electromagnetic waves.The above measures have effectively improved the stability of the standard resistance.In this paper,various factors affecting the long-term and short-term stability of resistors are analyzed in detail,and a forced accelerated aging treatment technology suitable for resistor devices is proposed.And we carried out high and low temperature cycle aging treatment on all resistance devices with the technology.The precision resistance measurement system was used to evaluate the performance of the developed standard resistance.The evaluation data proves that the specifications of the standard resistors fully meet the design objectives,as the accuracy is better than ±100ppm,the temperature coefficient a is less than 2ppm/℃,the temperature coefficient β is less than 0.1ppm/℃2,and the long-term stability is better than 8ppm/year.Finally,a set of high-precision air DC standard resistors that are at the leading level in China have been developed.In the transmission of the value of DC resistance,it can completely replace the traditional oil-immersed standard resistors and approach the level of imported products.