Coupling circuit for a measuring instrument
First Claim
1. A coupling circuit for an instrument comprising:
- input terminal means,input coupling means interposed between said input terminal means and a circuit of said instrument to couple said input terminal means to said circuit of said instrument,thermistor means in series relation with said input coupling means, andcurrent limiting means in series relation with said input coupling means,said current limiting means including active circuit means for detecting an increased current level in said current limiting means and in response thereto increasing the series impedance.
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Accused Products
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.
16 Citations
12 Claims
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1. A coupling circuit for an instrument comprising:
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input terminal means, input coupling means interposed between said input terminal means and a circuit of said instrument to couple said input terminal means to said circuit of said instrument, thermistor means in series relation with said input coupling means, and current limiting means in series relation with said input coupling means, said current limiting means including active circuit means for detecting an increased current level in said current limiting means and in response thereto increasing the series impedance. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A coupling circuit for an instrument wherein said instrument is adapted to perform multiple measurement functions, said coupling circuit comprising:
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input terminal means, input coupling means between said input terminal means and a circuit of said instrument, thermistor means, and current limiting means, both in series relation with said input coupling means, said current limiting means comprising at least one current source circuit, and a varistor in shunt relation with said current source circuit, wherein said thermistor means is disposed in heat conducting relation with said varistor so that when a predetermined voltage drop occurs across said current source circuit causing said varistor to conduct, the heat generated in said varistor is coupled to said thermistor means whereby the resistance of said thermistor means increases and the voltage drop across said current source circuit and varistor is decreased. - View Dependent Claims (8, 9, 10, 11, 12)
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Specification