Bridge Circuit for Measuring Differences of Current or Voltage from a Preselected Value

A bridge circuit has been developed, using a Zener diode, which enables measurement of small changes in current or voltage from a preselected value. The circuit features increased sensitivity over a meter whose range includes the total current and is more easily used than a balanced bridge or potentiometer. The design equations are developed and the design procedure outlined for both the current and voltage difference measuring bridges, while a sample calculation is made for the current change measuring circuit. A bridge similar to that in the sample calculation was built and tested. The test circuit displayed less than 1 per cent nonlinearity of meter current vs change of input current for the design region of 460±5 ma. This shift was attributed to a slight heating of the Zener diode or a resistor at the higher test currents, but since the diode used in the test circuit had a smaller power rating than the type originally intended for use, it is not considered to be a serious limitation.