AC-DC voltage transfer module with thin-film multijunction thermal converter

An ac-dc voltage transfer module has been designed around the planar multijunction thermal converter (PMJTC) to measure ac voltage by an automatic ac-de transfer. It operates in the frequency range from 10 Hz up to 1 MHz with an uncertainty below 20 V/V for frequencies up to 100 kHz and below 50 V/V up to 1 MHz. The module uses the standard PTB-PMJTC with one heater in an isothermal way. In the first step of the ac-dc transfer procedure, it applies the internal dc reference voltage to the heater to calibrate the system, and in the second step, it applies the unknown ac voltage from the input side to the same terminal of the heater and a dc feedback voltage from the other terminal. The feedback loop adjusts the output voltage of the PMJTC to the same value as with the reference dc. From the feedback dc value and the reference dc value, an accurate value for the ac voltage is calculated. With this isothermal design, the time for an accurate ac-dc transfer is reduced considerably to several seconds.