Circuit protection devices comprising PTC elements
US4238812A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | Dec 1, 1978 |
| Grant date | Dec 9, 1980 |
| Priority date | — |
| Expiry date | Dec 1, 1998 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH01C7/027
- WIPO fieldElectrical machinery, apparatus, energy
- WIPO sectorElectrical engineering
Abstract
Circuit protection devices comprising PTC elements, and circuits containing such devices. The devices, which are particularly useful in circuits carrying a steady state current of 0.5 amp or more, can protect the circuit against excessive current, e.g. as a result of a short or a voltage surge, or against excessive temperature, or both. The PTC element is composed of a material, preferably a conductive polymer, having a resistivity less than 10 ohm. cm in the normal operating condition of the circuit, and the device comprises electrodes such that current flows through the PTC element over an area of equivalent diameter d with an average path length t such that d/t is at least 2. The circuit has a normal operating condition in which the device has a low resistance and is in stable thermal equilibrium with its surroundings; however, when a fault condition occurs, the device generates heat by I.sup.2 R heating at a rate which exceeds the rate at which heat can be lost from the device, thus causing the temperature and resistance of the device to rise until the device reaches a new, high temperature, stable thermal equilibrium state. In order to ensure that the circuit current is reduce…
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.