The Signal from Parallel Circuits

The electrical output, , from each of the active gage circuits shown schematically in the figure below depends upon the power supply voltage, , and the resistances of the common leadwire ( ), the active gages ( ), the individual return leadwires ( ), and the dummy gages ( ). The resistances of the signal leads are relatively unimportant because no significant amount of current flows through them when a modern instrument with a high impedance input circuit is used to measure the signal voltage.

Schematic of parallel Wheatstone bridge circuits with common power supply leadwire.
(Repeated Illustration)

The resistance of the parallel circuits between points A and C can be expressed as:

    (516.1)

provided that the resistance of the leadwire between the active gages ( and ) is negligible. Because the common leadwire has some finite resistance, it acts as a voltage divider to reduce the excitation voltage supplied to the active and dummy gages. And because it carries the sum of the currents in all the parallel circuits, the voltage drop in the common leadwire is times as great as for individual return leadwires with the same resistance.

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