1. What Is Power Coefficient of Resistance (PCR)

    

        Power Coefficient of Resistance (PCR) is the relative change in resistance value when the resistor changes unit power. Both the power coefficient and the temperature coefficient of resistance (TCR) reflect the effect of temperature on the resistance of the resistor. The difference is that the power coefficient of resistance is the relative change in resistance due to self-heating after power is applied to the resistor, while the temperature coefficient of resistance is the rate of change of resistance drift caused by ambient temperature through thermal conduction.

2. How Does The Power Coefficient Affect The Coaxial Shunt?

    Due to its excellent AC-DC difference and phase shift performance, coaxial shunts are widely used in measurement laboratories to calibrate broadband current sources. However, in practical, the following problems often occur:

    (1)It is necessary to load the current into the coaxial shunt and reach thermal equilibrium before starting the measurement. The typical thermal equilibrium time for a single measurement is about 10 minutes. The usual broadband power source has multiple current ranges, which makes the calibration time-consuming.

    (2)When the same shunt is loaded with different currents, the resistance value changes due to inconsistent heat generation, which introduces an uncertainty component caused by the power coefficient. For applications with higher accuracy requirements, shunts of different specifications need to be replaced and the data needs to be corrected, which will further reduce the calibration efficiency.

3. Optimization of Calibration Using Ultra-Low Power Coefficient Coaxial Shunt

    TUNKIA analyzed the above-mentioned problems existing in the use of coaxial shunts. By optimizing the heat dissipation structure, selecting ultra-low temperature coefficient resistive elements, increasing the number of resistors, etc., we have developed the TH0420 Reference Coaxial Current Shunt with ultra-low power coefficient. We apply a nominal DC current of 100A to the two types of shunts, measure the changes in resistance over time, and draw the resistance time drift curve of the coaxial shunt to compare TH0420 its former version TH0400 Precision Coaxial Current Shunt (fully benchmarked with international first-tier products), and the result is shown in Figure 1:

Figure 1 Resistance Time Drift Curve of A Typical Coaxial Shunt After Applying Current

1) Preliminary Conclusion

   The relative change in resistance of TH0420 at 1 minute is better than 5ppm, and after reaching thermal equilibrium the relative change in resistance is only 8ppm, which is 1/7 of TH0400.

    The measurement setup time of TH0420 only takes 4 minutes, while that of TH0400 requires at least 11 minutes. It can be seen that the lower the power coefficient of the coaxial shunt, the shorter the measurement setup time (thermal equilibrium time) and the better the performance.

2) Theoretical Analysis

    The lower the power coefficient, the smaller the additional measurement uncertainty and the more accurate the measurement results. In actual work, the resistance of the resistor will change due to self-heating, and additional measurement uncertainty δP will be introduced, which is calculated as follows:

Note: I_m is the working current, I_n is the nominal current

    Taking TH0400 and TH0420 whose nominal current I_n is both 100 A as an example, calculate the additional measurement uncertainty δP introduced when the operating current I_m is 90 A and 60A respectively. The results are shown in Table 1:

Table 1 Measurement Uncertainty δP Introduced By Typical Coaxial Shunts At Different Operating Currents

    It can be seen from the table 1: 1) The larger the operating current I_m, the greater the self-heat generated, and the larger δP; 2) The lower the power coefficient, the smaller δP, and the smaller the interference to the measurement results. In actual work, coaxial shunts with low power coefficients should be selected as much as possible.

4. TUNKIA TH0420 Reference Coaxial Current Shunt