| 摘要 |
There is disclosed a capacitance transmitter that is ideally suited for use with a capacitance probe for level, pressure or proximity measurement in which the capacitance of the probe member responds to the sensed physical parameter. The transmitter has only two conductors which furnish the operational voltage and which transmit the output current signal. The transmitter includes a constant base current generator that produces a minimum base current through the two connectors, an oscillator to develop a train of square-wave voltage signals, a multivibrator circuit receiving the square-wave voltage signals and cyclicly applying controlled currents in response thereto to the probe capacitor and to a reference capacitor; a charge rate discriminating circuit receiving charging rate signals from each of the probe and reference capacitors and developing an output current responsive to differences thereof, a feedback circuit from the output of the discriminating circuit to the multivibrator circuit to increase the charging rate of the probe capacitor in response to the output current and a summation circuit receiving the constant base current and the discriminating circuit output current to provide a current signal proportional to the capacitance value of the probe capacitance. Adjustable components are provided in each of the controlled current circuits to permit the fixed adjustability of the zero and the span of the instrument. A transient supression network circuit can also be provided to protect the transmitter.
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