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Abstract:
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Standard resistors that are used to measure current and are designed to dissipate relatively high levels of power are known as current shunts. This paper will discuss some of the many different types of single range current shunts in use today, and describe self-heating effects and the kinds of errors associated with specific current shunt designs. Most older, classic DC single-range shunts were constructed using the copper-manganese resistance alloy Manganin, in the form of ribbons of varying thickness. These resistance elements ranged from long, folded ribbons to heavy, parallel short sections, for resistance values between 10 milliohms and 10 micro-ohms, and could dissipate as much as 100 watts at full power. With proper heat treatment, the characteristics of the Manganin alloy can be used to produce a broad maximum in resistance near the rated power load. However, because this loading can cause the shunt temperature to rise by as much as 50 Degrees Celsius, there still can be a relatively large fractional change in value from lower to higher current levels. Zeranin is one alloy being used in the construction of newer standard high current shunts. Alternative materials such as Evanohm, with better temperature characteristics, have been used recently in the construction of shunts, but the type of material is only one of many factors that affect high precision measurements. Improved repeatability can be achieved by attaching a copper-constantan (T-Type) thermocouple to the center of a high-power shunt. The reason for attaching a thermocouple is that it allows accurate determination of the temperature versus resistance characteristic (TCR). Tests have also been conducted by immersing a single range shunt in a temperature-regulated oil bath. By applying a lower current but changing the temperature of the bath, one can derive the TCR curve. I will also discuss the importance of the length of time it takes for some shunts to reach both temperature equilibrium and resistance equilibrium. Because of the effects of non-uniform temperature distribution, these are not the same length of time for some shunts. Other topics include how errors in measuring current shunts can be reduced by making symmetric, low-resistance connections and considering how the current distribution and the placement of the potential terminals affect the measurement
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