Coupling circuit for a measuring instrument

Details Coupling circuit for a measuring instrument Coupling circuit for a measuring instrument

1993-05-03

1995-05-23

Karlsen, Ernest F.

Electricity: measuring and testing

Measuring, testing, or sensing electricity, per se

Meter protection or fraud combatting

324126, G01R 136

Patent

active

054184509

ABSTRACT:
A coupling circuit for a multimeter adapted to perform multiple measurements includes a thermistor and constant current sources in series between an instrument input terminal and an instrument circuit adapted to source current for ohms measurements. Varistors shunt the constant current sources and are thermally coupled to the thermistor. An initial inrush of current is controlled by the constant current sources and when the voltage across a constant current source reaches the knee voltage of the paralleled varistor, the varistor shunts the constant current source and provides heat to the thermistor for insuring the latter will transition to its high resistance state. The thermistor then drops a substantial part of the input voltage, effectively disconnecting the low impedance ohms circuit from the input terminals and protecting the constant current source circuitry and varistors from extended application of high voltage.

REFERENCES:
patent: 3611242 (1971-10-01), Obenhaus
patent: 3976854 (1976-08-01), Ishikawa et al.
patent: 4467310 (1984-08-01), Jakab
patent: 4990987 (1991-02-01), Boucher et al.
patent: 5146150 (1992-09-01), Gyenes et al.
Supertex Inc., Application Note AN-D11, Sep. 11, 1991.

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