AC-DC transfer difference of the PTB multijunction thermal converter in the frequency range from 10 Hz to 100 kHz

The ac-dc transfer difference of the multijunction thermal converter (MJTC) developed at Physikalisch-Technische Bundesanstalt (PTB) has been evaluated for various heater resistances between 27 and 700 Ω. By optimizing the heater resistance for voltage transfer to 190 Ω and for current transfer to 27 Ω, transfer differences of a few parts in 10⁷ are achieved up to 100 kHz. Transfer differences caused by thermoelectric effects could be shown to be smaller than 10⁻⁸, due to the nearly ideal periodic structure of the converter and the special construction of the intermediate leads between heater and vacuum throughleads. An increase of the transfer difference at low frequencies is caused partly by the change in the real part of the heater impedance, which can be compensated by adding series or parallel resistors to the heater. Another part is found to be a current transfer difference which can only be diminished to a few parts in 10⁷ by increasing the number of thermocouples or decreasing the input power.