US 4,238,812 AGranted

Circuit protection devices comprising PTC elements

AssigneeRaychem Corporation

InventorsLee M. Middleman, Joseph H. Evans, Donald F. Pettengill

Life of this patent

Estimating the term…

Loading patent…

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.

Claims63

Dependent claims

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 T.sub.s ;
and
(3) other circuit elements which are connected in series with said PTC element and which have an impedance R.sub.L 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 T.sub.dn at which the device has a resistance R.sub.dn which is
(a) less than 1 ohm;
and
(b) less than 0.1.times.R.sub.L 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 T.sub.n ;
and
(D) there is a stable equilibrium between the rate at which the device generates heat by I.sup.2 R 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 T.sub.n while maintaining R.sub.L 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 I.sup.2 R 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 T.sub.crit, the device is at a temperature T.sub.d trip and has a resistance R.sub.d trip, and the rate at which the resistance of the device changes with temperature, dR.sub.d trip /dT.sub.d trip is positive;
and
(b) if said medium is then heated just above T.sub.crit, the rate at which the device generates heat by I.sup.2 R 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 I.sup.2 R 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 T.sub.d latch and has a resistance R.sub.d 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 10.

15 more elements

A circuit protection device which comprises a PTC element composed of a PTC composition having a switching temperature T.sub.s and at least two electrodes which can be connected to a source of electrical power and which, when so connected, cause current to flow through said PTC element;

said device being such that a test circuit which consists of said device, a source of power having a voltage selected from 10 volts and 100 volts and a resistor of selected resistance in series with said device, said device being in still air and said resistance being selected so that when the air is at 25.degree. C. there is an unstable equilibrium between the rate at which the device generates heat by I.sup.2 R heating and the rate at which heat is lost from the device, has a stable 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 T.sub.dn at which the device has a resistance R.sub.dn less than 1 ohm and at which said PTC composition has a resistivity of less than 10 ohm. cm.;
(C) the air is at a temperature T.sub.n which is less than 25.degree. C.;
and
(D) there is a stable equilibrium between the rate at which the device generates heat by I.sup.2 R heating and the rate at which heat is lost from the device;
and said device in said test circuit having an electrical power/temperature relationship and being capable of losing heat to the air at a rate such that
(a) if the air is heated slowly from T.sub.n while maintaining said resistor and said source of power substantially constant, the temperature of the device increases slowly until the equilibrium between the rate at which the device generates heat by I.sup.2 R heating and the rate at which heat is lost from the device becomes unstable, at which unstable equilibrium point the air is at a temperature of 25.degree. C., the device is at a temperature T.sub.d trip and has a resistance R.sub.d trip, and the rate at which the resistance of the device changes with temperature, dR.sub.d trip /dT.sub.d trip is positive;
and
(b) if the air is then heated just above 25.degree. C., the rate at which the device generates heat by I.sup.2 R 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 I.sup.2 R 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 T.sub.d latch and has a resistance R.sub.d latch which is such that the ratio of the power in the circuit in the stable operating condition to the power in the circuit at said high temperature stable equilibrium point, the Switching Ratio, is at least 10.

8 more elements

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 T.sub.s ;
and
(3) other circuit elements which are connected in series with said PTC element and which have an impedance R.sub.L ohms;
said electrical circuit having a normal operating condition in which
(A) a current i.sub.n 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 T.sub.dn at which the device has a resistance R.sub.dn which is
(a) less than 1 ohm;
and
(b) less than 0.1.times.R.sub.L 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 T.sub.n ;
and
(D) there is a stable equilibrium between the rate at which the device generates heat by I.sup.2 R 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 elements of the circuit are changed so that the current flowing through said device increases slowly from i.sub.n while maintaining T.sub.n substantially constant, the temperature of the device increases slowly until the equilibrium between the rate at which the device generates heat by I.sup.2 R heating and the rate at which heat is lost from the device becomes unstable, at which unstable equilibrium point the current is at a value i.sub.crit and the rate at which the resistance of the device changes with temperature is positive;
and
(b) if elements of the electrical circuit are further changed so that the current flowing through said device increases to 2.times.i.sub.crit, while maintaining T.sub.n substantially constant, the rate at which the device generates heat by I.sup.2 R 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 I.sup.2 R heating is equal to the rate at which heat is lost from the device;
at which high temperature stable equilibrium point, the device has a resistance 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 10.

15 more elements

Dependencies are inferred from the claim text.