Fluke Corporation

John Fluke Mfg. Co., Inc.

Heber P Farnsworth

Glade B Bacon

A protective circuit for the input of a multimeter is provided with series connected thermistors partially shunted by a varistor whereby overload voltage is distributed between the thermistors enabling protection at higher voltage levels. Shunt connected varistors protect primarily against transient overloads and are thermally coupled to the aforementioned thermistors to bring the thermistors more rapidly to their high resistance condition such that damage to the varistors is avoided and continued protection is provided.

Protective input circuit for an instrument

A thermistor device includes plural thermistor elements in series, one or more of which is shunted with a varistor so that if such thermistor heats up and drops excessive voltage, the current therethrough is diverted to the varistor until other thermistors in series also heat to a high resistance state. The thermistor and varistor elements are thermally joined so that the various thermistor elements are rapidly brought up to temperature and prevent prolonged voltage drop across a protective varistor.

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Snohomish, WA, United States of America

Heber P Farnsworth