Circuit protection devices

Details Circuit protection devices Circuit protection devices

1999-08-24

2001-10-09

Picard, Leo P. (Department: 2835)

Electricity: electrothermally or thermally actuated switches

Electrothermally actuated switches

Fusible element actuated

C337S014000, C337S158000, C337S167000, C338S0220SD, C361S106000

Reexamination Certificate

active

06300859

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to improvements in electrical circuit overcurrent protection devices including a current-concentrating conductor physically integrated into, and thermally coupled to, a PTC overcurrent sensor device.
Introduction to the Invention
Positive temperature coefficient (PTC) circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions is in a low temperature, low resistance state. However, if the current through the PTC device increases excessively and/or the ambient temperature around the PTC device increases excessively, and if either condition is maintained for more than the normal operating time, then the PTC device will be “tripped,” i.e., converted to a high temperature, high resistance state such that the current is reduced substantially. Generally, the PTC device will remain in the tripped state, even if the current and/or temperature return to their normal levels, until the PTC device has been disconnected from the power source and allowed to cool. Particularly useful PTC devices contain a PTC element which is composed of a PTC conductive polymer, i.e. a composition which comprises (1) an organic polymer, and (2) a particulate conductive filler, preferably carbon black and/or a conductive inorganic filler, e.g. a ceramic oxide or a metal carbide, nitride, or boride such as titanium carbide, which is dispersed, or otherwise distributed, in the polymer. PTC conductive polymers and devices containing them are described, for example in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801 and 5,378,407, and in International Patent Publication Nos. WO 94/01876, WO 95/08176 and WO 95/31816 (corresponding to U.S. Patent Application Ser. Nos. 08/710,925(Zhang et al, filed Sep. 24, 1996) and 08/727,869 (Graves et al, filed Oct. 8, 1996)), the disclosures of which are incorporated herein by reference for all purposes. Ceramic PTC materials are also well known in the art. Negative temperature coefficient (NTC) circuit protection devices containing ceramic NTC materials are also well known in the art.
U.S. Patent Application Ser. No. 08/682,067, the disclosure of which has been published as International PCT Application Publication No. WO 98/02946 on Jan. 22, 1998, describes an overcurrent protection system which will give a rapid response to even relatively small overcurrents. In that system, a sensor element and circuit interruption element are placed in series between a current source and an electrical load. The sensor element is functionally linked to the circuit interruption element via a control element, so that, when the current in the circuit exceeds a predetermined amount, the sensor element senses the overcurrent and communicates with the control element. The control element causes the circuit interruption element to change from a normal operating state to a fault state. The normal state may be either conducting or non-conducting, and the fault state will be an inverse of the normal state, i.e., respectively non-conducting (including a completely open state), or conducting, depending upon the particular overcurrent protection circuit arrangement. In a preferred embodiment, the sensor element comprises a resistive device connected in series with the load, and the control element comprises a PTC device which is thermally linked to the resistive device and is electrically connected to the circuit interruption element. When an overcurrent passes through such a system, the resistive device increases in temperature causing the PTC device to heat up and trip to a high resistance state. The PTC device is linked to the circuit interruption element so that the increased resistance of the PTC device causes the circuit interruption element to switch into its fault state. The PTC device is not placed in series with the load and therefore may operate at current levels much lower than the normal circuit current which passes through the load.
The application Ser. No. 09/248,166 (Myong, filed on Feb. 9, 1999) referenced above provided an important advance in overcurrent protection devices. The protection devices disclosed in this application comprise a generally rectangular and planar sheet of material exhibiting PTC properties comprised of an organic polymer having a particulate conductive filler dispersed therewithin. The generally rectangular, planar sheet has a first major surface and has an opposite second major surface. In accordance with principles of the invention disclosed therein a first conductive layer comprises a patterned unitary metal foil which is thermally bonded and electrically connected to the first major surface of the PTC sheet. The patterned metal foil defines a current conductor including a first terminal electrode region at one end of the sheet, a second terminal electrode region at a second end of the sheet, and a generally thinned, current-concentrating region or portion extending between the first and second terminal electrode regions. As described in this prior patent application a second conductive layer of metal foil extends substantially entirely across opposite second major surface of the PTC sheet. One drawback of this prior approach is that the current-concentrating portion of the patterned metal foil concentrated heat from overcurrent at a small areal surface region of the PTC sheet, whereas current gradients were present and were collected across the entire opposite second major surface of the PTC sheet material, resulting in a less sensitive PTC overcurrent sensor than desired. Another drawback of the prior approach is that a control current necessarily flowed from the patterned metal foil through the PTC sheet to the second conductive layer, whereas in some electrical control circuits electrical circuit isolation is desired or needed. Finally, while the device usually provided protection against most over-current conditions, a critical overcurrent level caused the prior device to overheat and self-destruct if overheating continued continuously for a period of time. One example of a potentially destructive critical overcurrent level may arise when a relay whose load-carrying contacts are protected by the prior device became fused or welded together during an overcurrent condition. If contact welding or fusing occurs, an overcurrent continues to flow between source and load, even after the relay coil circuit is opened. In this situation heating at the current-concentrating region of the patterned metal foil conductor can result in a pyrolitic reaction at the PTC sheet layer and cause a potentially devastating electrical fire. Drawbacks such as these are overcome by improvements in accordance with principles of the present invention.
SUMMARY OF THE INVENTION
In a most general aspect the present invention relates to structural improvements and refinements of electrical devices which combine a current-concentrating load-carrying conductor element and a sensor element which is thermally linked to the conductor element, and exhibits anomalous resistance/temperature behavior, such as PTC behavior.
In a first aspect, this invention provides a generally rectangular, planar electrical overcurrent sensing device having a planar substrate with a top major surface and a bottom major surface and including a patterned metal foil conductor defined along the top major surface, the metal foil conductor having a first electrode region at one end region, a second electrode region at an opposite end region, and a current-concentrating region extending between the first electrode portion and the second electrode portion. In one example, the substrate is formed of a composition which exhibits PTC behavior and which comprises an organic polymer having a particulate conductive filler dispersed therewithin. In another example, the substrate is formed of a non-conductiv

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