초록 ▼

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 a

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 I2R 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 8, preferably at least 40.

대표청구항 ▼

An electrical circuit which comprises (1) a source of electrical power; (2) a circuit protection device comprising at least two electrodes and a PTC element composed of a PTC composition having a switching temperature Ts; and (3) other circuit elements which are connected in series with said PTC ele

An electrical circuit which comprises (1) a source of electrical power; (2) a circuit protection device comprising at least two electrodes and a PTC element composed of a PTC composition having a switching temperature Ts; and (3) other circuit elements which are connected in series with said PTC element and which have an impedance RL ohms; said electrical circuit having a normal operating condition in which (A) current flows through said PTC element over an area of equivalent diameter d with an average path length t such that d/t is at least 2; (B) said device is at a temperature Tdn at which the device has a resistance Rdn which is (a) less than 1 ohm; and (b) less than 0.5×RL ohm, and at which said PTC composition has a resistivity of less than 10 ohm. cm.; (C) said device is in contact with a medium which is at a temperature Tn; and (D) there is a stable equilibrium between the rate at which the device generates heat by I2R heating and the rate at which heat is lost from the device; and said device having an electrical power/temperature relationship and being capable of losing heat to said medium at a rate such that (a) if said medium is heated slowly from Tn while maintaining RL and the voltage of the source of electrical power substantially constant, the temperature of the device increases slowly until the equilibrium between the rate at which the device generates heat by I2R heating and the rate at which heat is lost from the device becomes unstable, at which unstable equilibrium point the medium is at a temperature Tcrit, the device is at a temperature Td trip and has a resistance Rd trip, and the rate at which the resistance of the device changes with temperature, dRd trip/dTd trip is positive; and (b) if said medium is then heated just above Tcrit, the rate at which the device generates heat by I2R heating exceeds the rate at which heat can be lost from the device and thus causes the temperature and the resistance of the device to rise rapidly and the circuit current to fall until a high temperature stable equilibrium point is reached at which the rate at which the device generates heat by I2R heating is equal to the rate at which heat is lost from the device; at which high temperature stable equilibrium point, the device is at a temperature Td latch and has a resistance Rd latch which is such that the ratio of the power in the circuit in the normal operating condition to the power in the circuit at said high temperature stable equilibrium point, the Switching Ratio, is at least 8.