The basic unit of thermal voltage and current converters is the thermoelement (TE). A TE consists of a heater structure and one or more thermocouples to sense the heat generated when an electrical signal is applied to the heater.
A traditional Single Junction Thermal Converter (SJTC) uses one thermocouple at the midpoint of the heater and encloses both heater and thermocouple in a vacuum bottle to improve its sensitivity. Although SJTCs are rather poor voltage measuring devices, in 1948 Frank Hermach at NBS found that they make excellent ac-dc transfer devices, and the discipline of ac-dc transfer or ac-dc difference was born. The traditional SJTC can be used from 10 Hz to 100 MHz with a best uncertainty of about 10-6 over the mid-audio frequency range. A typical SJTC is used at 2 V or less.
The fundamental limitation on the performance of an SJTC are thermoelectric errors (Thomson and Peltier effects) in the heater due to the rather large (about 200 ºC) temperature gradient along the heater. To reduce these thermoelectric errors, the Multijunction Thermal Converter (MJTC) uses as many as two hundred thermocouples spaced along a much longer heater wire. Because the sensitivity of the MJTC is greater than that of the SJTC, it can be used at lower voltages, which produce a smaller temperature gradient (about 50 ºC) along the heater, and significantly reduce the thermoelectric errors. The best accuracy for MJTCs is about 5x10-7 from 40 Hz to 10 kHz (the longer heater wire introduces frequency-dependent errors at rather low frequencies.) Some MJTCs have maximum rated voltages of 10 V, although most are rated at 5 V and below.
Recently, MJTCs made using thin-film semiconductor fabrication techniques have been introduced as working standards at NIST and other national laboratories. These devices have the accuracy specifications of traditional MJTCs, but are useful at frequencies from 10 Hz to 1 MHz, and, if fabricated on a low-loss substrate like quartz, up to 100 MHz. These Planar MJTCs (PMJTCs, although most often they are simply called MJTCs) generally have a maximum rated voltage of 2V to 3 V.
Thermoelements may also be used to measure the ac-dc difference of ac current. In general TEs are made in 2.5 mA, 5 mA, and 10 mA versions, but have been available with current ratings of 500 mA. In fact, NIST has special TEs that are rated at 20 A. Most often, a thermoelement is used as a primary standard for ac current, and that primary ac-dc difference is transferred to a precision shunt for extension of the current range. As current converters, TEs have an accuracy of about 10-6 at 5 mA and mid-audio frequencies.