AC-DC current transfer step-up and step-down calibration and uncertainty calculation

New AC-DC current transfer standards are built for the current range from 0.3 mA to 20 A and the frequency range from 10 Hz to 100 kHz. A three-dimensional PTB-multijunction thermal converter (MJTC) with a heater resistance of 27 Ω is the basic standard for the AC-DC current transfer at currents from 20 to 50 mA. At lower currents, planar MJTCs (PMJTCs) with heater resistances from 90 to 960 Ω are used in a step-down procedure. Coaxial current shunts in parallel with PMJTCs extend the current range up to 20 A. The standard measurement uncertainties are in the range from 0.2 μA/A at 25 mA and 1 kHz to 50 μA/A at 20 A and 100 kHz. The high sensitivity of the PMJTC allows bigger steps in the step-up calibration and some smaller steps between these steps. Calibration of all these steps leads to an overdetermined system to check for the validity of the assumptions made in the step-up procedure. This results in reduced uncertainties at the different current levels compared to the old method.