Inventors:
Lee M. Middleman - Portola Valley CA
Joseph H. Evans - Palo Alto CA
Donald F. Pettengill - Palo Alto CA
Assignee:
Raychem Corporation - Menlo Park CA
International Classification:
H02H 504
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 reduced to a sufficiently low level, the ratio of (a) the power in the circuit in the normal operating condition to (b) the power in the circuit when the device is in the high temperature equilibrium state, is at least 10, preferably at least 40.
