A Thermoelement Comparator for Automatic AC-DC Difference Measurements

AC-DC differences in thermal voltage converters (TVC´s) are determined relative to a similar instrument, from imbalance EMF´s measured in a divider circuit to which both instrument outputs are connected. These EMF´s are more stable than the input voltages to the converters, because input voltage fluctuations produce nearly equal proportional changes in the converter outputs. The divider balance is therefore affected only slightly. The divider and a Lindeck potentiometer for monitoring the test instrument output are driven by stepping motors and balanced automatically. AC and DC voltages are applied by programmable power supplies whose outputs are adjusted, in response to imbalance EMF´s in the potentiometer circuit, to produce equal output from the test instrument. The system automatically calibrates one voltage range at one or more frequencies, but requires an operator to change ranges and to enter the test parameter at a computer terminal.