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US 4,283,719 AGranted
Liquid level sensing circuit
AssigneeLucas Industries Limited
InventorsDavid G. Williams, Paul A. Harris
Life of this patent
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AssigneeLucas Industries Limited
InventorsDavid G. Williams, Paul A. Harris
Estimating the term…
A liquid level sensing apparatus includes a probe having a high positive temperature coefficient of resistivity. The probe is connected in a circuit including an operational amplifier and a series transistor which maintains the voltage across the probe constant for a fixed period following switching on of the apparatus and which produces a voltage signal related to the current flowing in the probe. This voltage signal is differentiated by a differentiating circuit to provide a signal indicating the rate of change of the voltage signal from which it is determined whether or not the probe is immersed in a cooling liquid. A warning lamp is actuated by a latching switch circuit when the probe is not immersed. A second example is described in which the probe current is maintained at a fixed level and the rate of change of the voltage across it is monitored.
A liquid level sensing apparatus comprising a resistive probe having a high positive temperature co-efficient of resistivity, a circuit for applying a controlled electrical signal to said probe for a predetermined test period, and means sensitive to a voltage signal developed as a result of said electrical signal applied to said probe, said voltage sensitive means including a differentiating circuit having a time constant so that the voltage sensitive means detects the rate of change of said voltage signal.
Dependencies are inferred from the claim text.