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Thermal Converters:

As we’ve noted, a typical thermoelement (TE) has a rated input voltage of 2 V or less, yet most applications for ac voltage measurement require applied voltages of up to 1000 V. To measure high voltages without destroying the TE, a range resistor is usually placed in the signal path before the TE. For a typical 400 Ω, 5 mA TE, a resistor with a value of 199 600 Ω will allow a 1000 V signal to be applied to the resistor/TE combination. Such a combination of resistor and TE is generally called a thermal voltage converter (TVC).

Although a series resistor is required to measure voltages up to 1000 V, the resistor, unless it is made exceptionally well, will contribute frequency-dependent errors to the ac-dc difference of the TVC. Parasitic capacitances between the resistor and enclosure provide a leakage path for ac current  (but not dc current) back to ground, bypassing the TE, and creating a positive ac-dc difference, because more ac voltage is required to match the magnitude of the dc signal. This frequency-dependent error can be reduced by placing a shield at the high-voltage end of the resistor. In this case the leakage currents are reduced because the shield is at the same potential as the high-voltage end of the resistor. No shield is necessary at the low-voltage end, because the potential difference between the resistor and enclosure is smaller.

Because the resistor is dissipating a fair amount of power (up to 5 W) it may become very hot. Heating the resistor and its enclosure may introduce errors from changes in geometry. In addition, leakage currents through the dielectric loss in the resistor form, and proximity effects in wire-wound resistors may introduce additional errors. In general these errors are greatest at high frequencies.

Since the typical TE has a current rating of a few milliamperes, ac current is often measured by connected the TE to the potential terminals of a precision shunt resistor to form a thermal current converter (TCC). The shunt has a smaller resistance than the TE, and the applied current effectively  bypasses the TE. Shunts are also susceptible to heating effects, generally because of changes in geometry between the current terminals and potential terminals of the shunt. In addition, current shunts are more sensitive to lead connections than are voltage converters, and significant errors may be introduced by large current loops in the input leads. As with TVCs, shunt performance is generally worse at higher frequencies and currents.


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Coaxial TVCs
Photograph of coaxial thermal voltage converters connected to a tee

For More Information:

Thomas E. Lipe
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