Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-01-25
2001-07-03
Young, Lee (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C338S309000, C338S308000, C337S159000
Reexamination Certificate
active
06253446
ABSTRACT:
BACKGROUND OF THE INVENTION
There is, in a number of industries and applications, a major need for a fault current fusing resistor that is very fast opening for relatively high currents and high AC and DC voltages. There is an even greater need for such a fault current fusing resistor that is inexpensive to manufacture and small in size, and that is characterized by a high degree of safety.
To state one example, there are power system applications where power-semiconductors (transistors, thyristors, SCRs etc.) are used in circuits which manage large currents at relatively high DC voltages. An illustration is the power drivers for motors such as are used on electric trains. The power-semiconductors associated with the control circuits or drive circuits occasionally short internally, which can cause the portion of the circuit that is in the short circuit path caused by the shorted power-semiconductor to be exposed suddenly to a very high fault current and fault voltage.
The preceding paragraph sets forth one example of a fault current condition, which is not a gradual or progressive current buildup to an excessive value, but instead a sudden large step or jump in current from normal to excessive. “Normal” is the current level present in the portion of the power-semiconductor circuit to be protected (potential short circuit path) against fault current during normal operation; it is a low current typically from a few milliamps to 2 amps. “Excessive” is what is present in the short circuit current path substantially immediately upon occurrence of the short, being the high fault current that is typically in excess of 15 amps, and more typically 50 amps to 500 amps or greater-with a voltage typically in excess of 125 volts up to 1,000 volts or higher.
It is badly needed to have an economical, fast-acting, fusing device that operates at those and other relatively high (excessive) fault currents and high voltages. Fast operation would effectively protect circuit board traces, and components, in the short circuit current path.
Insofar as applicant is presently informed, fault current fusing devices which operate at relatively high currents and that can interrupt at relatively high voltages are quite large, and/or expensive, and/or slow-acting, and/or have other disadvantages.
SUMMARY OF THE INVENTION
In accordance with the device and method of the invention, there is provided a fault current fusing resistor (FCFR) that operates very quickly when exposed to the described (and other) relatively high fault currents and relatively high fault AC/DC voltages. The device opens (clears) in a way that is controlled, contained, and nonexplosive, thus substantially safe, and that does not generate debris. There is substantially no uncontained arcing, or no arcing at all.
In accordance with a method of the invention, the stated article or device is placed in a power-semiconductor circuit, or the like, and operates for great periods of time with only the normal low current passing through it. However, upon sudden occurrence of a fault current, the step change in current flow results in the stated very fast cessation of flow of the fault current.
In accordance with a preferred embodiment of the device, a relatively low value elongate resistive element (not a wire) is sandwiched between a base and a lid, and sealed therebetween as by epoxy. The resistive element within the sealed device has such characteristics that upon occurrence of a fault current flow, such current flow very quickly ceases which includes the necessary interruption at the fault voltage.
In accordance with the method and article of the invention, metal, preferably mixed with glass in a conductive film, is employed in a novel manner producing startling results.
The preferred resistive element is a resistive film provided on the base (substrate), and the film has a relatively low resistance.
The resistive film has deposited there over an overglaze (or equivalent) film.
Terminals and traces are provided on the substrate for the resistive film. In one embodiment, the terminals and substrate and traces are so disposed and related as to insure against excessive heating of the terminals (and the related circuit board portion) during normal current conditions.
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Oppenheimer Wolff & Donnelly LLP
Smith Sean
Young Lee
LandOfFree
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