A differential thermal wattmeter for the AC/DC transfer of power

A differential thermal wattmeter for the ac/dc transfer of power is described. A differential multijunction thermal converter is used as a “sum and difference” multiplier, with both ac and dc voltage and current signals simultaneously applied to both heaters. Dc feedback to the current input speeds up the response and maintains thermal balance between the heaters, so that the output meter becomes a null indicator. This mode of operation reduces and can eliminate the need for quadratic response. The requirement for exact matching of the thermal converters is also eliminated by effective interchange of the heaters. The converter heaters are protected against burnout, and crest factors of 4 in the ac current or voltage signals are permissible without degradation of the accuracy. Maximum meter sensitivity is 100 ppm of nominal power for full deflection, with a resolution of better than 20 ppm, and a linear scale. The instrument is designed for use at power frequencies of 50–1000 Hz, with ac/dc transfer errors of less than 30 ppm over this range. The wattmeter can be used up to 20 kHz with reduced accuracy.