AC-DC Transfer / AC-DC Voltage & Current Transfer

The calibration of electrical alternating quantities is achieved by a procedure called AC-DC Transfer which is the comparison of alternate to direct voltage or current. This comparison is based upon the thermal effect that AC and DC voltages/currents have on standard Planar Multijunction Thermal Converters (PMJTC). These standards consist of many thermocouples connected in series providing an output voltage proportional to the voltage applied on them which is respectively proportional to the rise of temperature. The complete alternating to direct voltage standard is a set of Planar Multijunction Thermal Converters and a scale up from 1mV to 1000V (in case of ac current transfer PMJTCs are combined with current shunts). Every thermal converter (paired with a shunt in case of current) is a step of the full scale up. To calibrate every step, alternating and direct voltage/current of the same rms value is applied consecutively and the output voltage is recorded. There is a difference between the amount of AC and DC voltage/current needed to achieve the same amount of heat on the thermal converters. This difference constitutes the AC-DC Transfer Difference which is represented by the Greek letter δ and it is measured in ppm. When δ is traceable to a national laboratory, any AC voltage/current device can be calibrated based on this standard.