Electricity: electrical systems and devices – Safety and protection of systems and devices – Circuit interruption by thermal sensing
Patent
1993-12-23
1996-07-16
Gaffin, Jeffrey A.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Circuit interruption by thermal sensing
338 21, 338277, 29612, H02H 504
Patent
active
055372864
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical devices, for example circuit protection devices, and to their methods of manufacture. In particular, the devices comprise material that has a positive temperature coefficient of resistance (PTC) and that undergoes a significant and sharp increase in resistance at a specified temperature or over a specified narrow temperature range above ambient temperature.
2. Introduction to the Invention
PTC materials, which may be polymeric or ceramic, are known for use in electrical devices such as heaters, and also for protecting electrical circuits against excessive current or temperature. The excessive temperature may itself arise simply from current flowing through the device, or may be due to an increase in the ambient temperature beyond a desired value. Details of developments relating to conductive polymer PTC compositions and devices comprising them, are given for example in U.S. Pat. Nos. 4,017,715, 4,177,376, 4,246,468, 4,237,441, 4,238,812, 4,329,726, 4,255,698, 4,272,471, 4,445,026, and 4,327,351, and GB 2,038,549. It has been proposed to use devices comprising PTC elements to protect circuits against fault conditions arising from excessive temperatures and/or circuit currents in for example U.S. Pat. Nos. 2,978,665, 3,243,753 and 3,351,882, U.K. Pat. No. 1,534,715, the article entitled "Investigations of Current Interruption by Metal-filled Epoxy Resin" by Littlewood and Briggs in J. Phys D: Appl. Phys, Vol. II, pages 1457-1462, and the article entitled "The PTC Resistor" by R. F. Blaha in Proceedings of the Electronic Components Conference, 1971, and the report entitled " Solid State Bistable Power Switch Study" by H. Shulman and John Bartko (August 1968) under Contract NAS-12-647, published by the National Aeronautics and Space Administration. U.S. Pat. Nos. 4,238,812 and 4,255,698 disclose practical circuit protection devices comprising conductive polymer PTC elements.
The disclosure of each of the patents and publications referred to above is incorporated herein by this reference.
The present invention is concerned particularly, though not exclusively, with electrical devices comprising PTC material, preferably polymeric, for use in circuit protection, and U.S. Pat. Nos. 4,238,812 and 4,329,726 referred to above for example, disclose such devices. PTC circuit protection devices are such that under normal operating conditions, determined by the current and temperature rating of the PTC material, they exhibit very low resistance to the flow of current therethrough. Under fault conditions, of excessive current and/or temperature, the PTC material heats up, significantly increases its resistance and thus switches off the flow of current therethrough, thereby protecting an associated electrical circuit. The higher the resistance of the material after passing through the switching transition, the lower is the residual current that can flow through the device, and thus the more effective is the device in protecting its circuit. Products embodying these principles are presently sold by Raychem Corporation under its tradename POLYSWITCH. Some of such products are formed from a substantially homogeneous sheet of polymeric PTC material. The sheet is coated over each of its major surfaces with electrically conductive material, to act as electrodes, and disc, rectangular, or other, shaped devices are stamped therefrom.
The resistivity of the PTC material of POLYSWITCH devices typically does not exceed 10 ohm-cm, and taking a typical value of 5 ohm-cm and a disc configuration with typical dimensions of diameter 2 cm and thickness 0.05 cm, the resistance at room temperature (i.e. about 20.degree. C.) of the device is typically 0.08 ohm. Although other geometries, for example rectangular, can be employed, the resistance values provided by devices that are of a size that can conveniently be handled manually do not vary significantly from the values given above. If it is desired to make such devices of different resistance
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Fang Shou-Mean
Gozlan Gilles R.
Burkard Herbert G.
Gaffin Jeffrey A.
Gerstner Marguerite E.
Raychem S.A.
Sherry Michael
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