Claims
- 1. 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 0.5.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 8.
- 2. A circuit according to claim 1 wherein said PTC element is composed of a conductive polymer.
- 3. A circuit according to claim 2 wherein said conductive polymer has been obtained by dispersing a conductive carbon black in a polymer.
- 4. A circuit according to claim 2 wherein said conductive polymer will withstand a voltage stress of at least 200 volts/millimeter at T.sub.d latch.
- 5. A circuit according to claim 2 wherein said conductive polymer has a resistivity at T.sub.dn of less than 7 ohm. cm.
- 6. A circuit according to claim 2 wherein said device has a resistance at T.sub.dn of less than 0.2 ohm.
- 7. A circuit according to claim 2 wherein said ratio d/t is at least 10.
- 8. A circuit according to claim 2 wherein R.sub.dn is less than 0.1.times.R.sub.L.
- 9. A circuit according to claim 2 wherein the Switching Ratio is at least 10.
- 10. A circuit according to claim 9 wherein the Switching Ratio is at least 40.
- 11. A circuit according to claim 2 wherein R.sub.d latch is less than the resistance of the device at all temperatures between T.sub.d latch and (T.sub.d latch +10).degree.C.
- 12. A circuit according to claim 11 wherein R.sub.d latch is less than the resistance of the device at all temperatures between T.sub.d latch and (T.sub.d latch +50).degree.C.
- 13. A circuit according to claim 2 wherein the device has a resistance at a temperature above T.sub.d latch which is at least 10 times R.sub.d latch.
- 14. A circuit according to claim 2 wherein said device, after said circuit has been subjected to an aging treatment which consists of operating the circuit for 10 hours with said device at said high temperature equilibrium point, switching the current off, allowing the device to cool to substantially below T.sub.dn and reducing the temperature of the medium to substantially below T.sub.n, has an electrical power/temperature relationship such that the circuit has a normal operating condition as defined; reaches an unstable equilibrium point as defined when the medium is heated slowly from T.sub.n, at which unstable equilibrium point the medium has a temperature T.sub.crit/10 which is between (T.sub.crit -20).degree.C. and (T.sub.crit +10).degree.C.; and reaches a stable equilibrium point as defined when the medium is heated above T.sub.crit/10.
- 15. A circuit according to claim 14 wherein T.sub.crit/10 is between (T.sub.crit -5).degree.C. and (T.sub.crit +5).degree.C.
- 16. A circuit according to claim 14 wherein said device, after said aging treatment, has a resistance in said normal operating condition, R.sub.dn/10, between 0.5.times.R.sub.dn and 3.times.R.sub.dn.
- 17. A circuit according to claim 16 wherein R.sub.dn/10 is between 0.7.times.R.sub.dn and 1.5.times.R.sub.dn.
- 18. A circuit according to claim 14 wherein said device, after said circuit has been subjected to an aging treatment which consists of operating the circuit for 100 hours with said device at said high temperature equilibrium point, switching the current off, allowing the device to cool to substantially below T.sub.dn and reducing the temperature of the medium to substantially below T.sub.n, has an electrical power/temperature relationship such that the circuit has a normal operating condition as defined; reaches an unstable equilibrium point when the medium is heated slowly from T.sub.n, at which unstable equilibrium point the medium has a temperature T.sub.crit/100 which is between (T.sub.crit -20).degree.C. and (T.sub.crit +10).degree.C.; and reaches a stable equilibrium point as defined when the medium is heated above T.sub.crit/100.
- 19. A circuit according to claim 19 wherein T.sub.crit/100 is between (T.sub.crit -5).degree.C. and (T.sub.crit +5).degree.C.
- 20. A circuit according to claim 18 wherein said device, after said aging treatment, has a resistance in said normal operating condition, 6R.sub.dn /100, between 0.5.times.R.sub.dn and 3.times.R.sub.dn.
- 21. A circuit according to claim 2 wherein the heat transfer coefficient of said device, measured in still air, is 2.5 to 6 milliwatts per deg C per cm.sup.2.
- 22. A circuit according to claim 2 wherein said device is such that at each temperature between T.sub.n and T.sub.d trip the value of the quantity
- 1/R.times.dR/dT
- where R is the resistance of the device in ohms and T is the temperature of the device, does not change by more than .+-.50% when the device is subjected to an aging treatment which consists of operating the circuit for 100 hours with said device at said high temperature equilibrium point, switching off the current, and allowing the device to cool to substantially below T.sub.dn.
- 23. A circuit according to claim 22 wherein said quantity does not change by more than .+-.25%.
- 24. 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 0.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 0.degree. C. 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 8.
- 25. A device according to claim 24 wherein said PTC element is composed of a conductive polymer.
- 26. A device according to claim 25 wherein said conductive polymer has been obtained by dispersing a conductive carbon black in a polymer.
- 27. A device according to claim 25 wherein said conductive polymer will withstand a voltage stress of at least 200 volts/millimeter at T.sub.d latch.
- 28. A device according to claim 25 wherein said conductive polymer has a resistivity at T.sub.dn of less than 7 ohm cm.
- 29. A device according to claim 25 wherein said device has a resistance at T.sub.dn of less than 0.2 ohm.
- 30. A device according to claim 25 wherein said ratio d/t is at least 10.
- 31. A device according to claim 25 wherein R.sub.dn is at most 0.1 times the resistance of said resistor.
- 32. A device according to claim 25 wherein said source of power has a voltage of 100 volts and R.sub.dn is at most 0.01 times the resistance of said resistor.
- 33. A device according to claim 25 wherein said source of power has a voltage of 100 volts and the Switching Ratio is at least 60.
- 34. A device according to claim 25 wherein R.sub.d latch is less than the resistance of the device at all temperatures between T.sub.d latch and (T.sub.d latch +10).degree.C.
- 35. A device according to claim 34 wherein R.sub.d latch is less than the resistance of the device at all temperatures between T.sub.d latch and (T.sub.d latch +50).degree.C.
- 36. A device according to claim 25 wherein the device has a resistance at a temperature above T.sub.d latch which is at least 10.times.R.sub.d latch.
- 37. A device according to claim 25 which, after said circuit has been subjected to an aging treatment which consists of operating the circuit for 10 hours with said device at said high temperature equilibrium point, switching the current off, allowing the device to cool to substantially below T.sub.dn and reducing the temperature of the medium to substantially below T.sub.n, has an electrical power/temperature relationship such that the circuit has a operating condition as defined; reaches an unstable equilibrium point as defined when the medium is heated slowly from T.sub.n, at which unstable equilibrium point the medium has a temperature T.sub.crit/10 which is between 5.degree. and 35.degree. C.; and reaches a high temperature stable equilibrium point as defined when the medium is heated above T.sub.crit/10.
- 38. A device according to claim 37 such that T.sub.crit/10 is between 20.degree. and 30.degree. C.
- 39. A device according to claim 37 which, after said aging treatment, has a resistance in said stable operating condition, R.sub.dn/10, between 0.5.times.R.sub.dn and 3.times.R.sub.dn.
- 40. A device according to claim 39 whose R.sub.dn/10 is between 0.7.times.R.sub.dn and 1.5.times.R.sub.dn.
- 41. A device according to claim 37 which, after said circuit has been subjected to an aging treatment which consists of operating the circuit for 100 hours with said device at said high temperature equilibrium point, switching the current off, allowing the device to cool to substantially below T.sub.dn and reducing the temperature of the medium to substantially below T.sub.n, has an electrical power/temperature relationship such that the circuit has a stable operating condition as defined; reaches an unstable equilibrium point when the medium is heated slowly from T.sub.n, at which unstable equilibrium point the medium has a temperature T.sub.crit/100 which is between 5.degree. and 35.degree. C.; and reaches a high temperature stable equilibrium point as defined when the medium is heated above T.sub.crit/100.
- 42. A device according to claim 41 such that T.sub.crit/100 is between 20.degree. and 30.degree. C.
- 43. A device according to claim 41 which, after said aging treatment, has a resistance in said normal operating condition, R.sub.dn/100, between 0.5.times.R.sub.dn and 3.times.R.sub.dn.
- 44. A device according to claim 25 whose heat transfer coefficient, measured in still air, is 2.5 to 6 milliwatts per deg C per cm.sup.2.
- 45. A device according to claim 25 whose resistance/temperature curve in said circuit is such that at each temperature between T.sub.n and T.sub.d trip, the value of the quantity
- 1/R.multidot.dR/dT
- where R is the resistance of the device in ohms and T is the temperature of the device does not change by more than .+-.50% when the device is subjected to an aging treatment which consists of operating said circuit for 100 hours with said device at said high temperature equilibrium point, switching off the current, and allowing the device to cool to substantially below T.sub.dn.
- 46. A device according to claim 45 wherein said quantity does not change by more than .+-.25%.
- 47. 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 flow 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 0.5.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 8.
- 48. A circuit according to claim 47 wherein said stable equilibrium point is such that if the elements of the electrical circuit are restored to their original condition, while continuing to pass current through the circuit, the current which flows in the restored circuit is substantially lower than i.sub.n.
- 49. A circuit according to claim 47 wherein said PTC element is composed of a conductive polymer.
- 50. A circuit according to claim 49 wherein said conductive polymer has been obtained by dispersing a conductive carbon black in a polymer.
- 51. A circuit according to claim 49 wherein said conductive polymer will withstand a voltage stress of at least 200 volts/millimeter at said high temperature stable equilibrium point.
- 52. A circuit according to claim 49 wherein said conductive polymer has a resistivity of less than 7 ohm. cm. in said normal operating condition.
- 53. A circuit according to claim 49 wherein said device has a resistance of less than 0.2 ohm. in said normal operating condition.
- 54. A circuit according to claim 49 wherein said ratio d/t is at least 10.
- 55. A circuit according to claim 49 wherein R.sub.dn is at less than 0.1.times.R.sub.L.
- 56. A circuit according to claim 49 wherein the Switching Ratio is at least 10.
- 57. A circuit according to claim 56 wherein the Switching Ratio is at least 40.
- 58. A circuit according to claim 49 wherein the device has a resistance at a temperature above its temperature at said high temperature stable equilibrium point which is at least 10 times its resistance at said equilibrium point.
- 59. A circuit according to claim 49 wherein said device, after said circuit has been subjected to an aging treatment which consists of operating the circuit for 10 hours with said device at said high temperature equilibrium point, switching the current off, allowing the device to cool to substantially below T.sub.dn and reducing the temperature of the medium to substantially below T.sub.s, has an electrical power/temperature relationship such that the circuit has a normal operating condition as defined and the device has a resistance in said normal operating condition R.sub.dn/10, between 0.5.times.R.sub.dn and 3.times.R.sub.dn.
- 60. A circuit according to claim 59 wherein R.sub.dn/10 is between 0.7.times.R.sub.dn and 1.5.times.R.sub.dn.
- 61. A circuit according to claim 59 wherein said device, after said circuit has been subjected to an aging treatment which consists of operating the circuit for 100 hours with said device at said high temperature equilibrium point, switching the current off, allowing the device to cool to substantially below T.sub.dn and reducing the temperature of the medium to substantially below T.sub.n, has an electrical power/temperature relationship such that the circuit has a normal operating condition as defined and the device has a resistance in said normal operating condition, R.sub.dn/100, between 0.5.times.R.sub.dn and 3.times.R.sub.dn.
- 62. A circuit according to claim 61 wherein R.sub.dn/100 is between 0.7.times.R.sub.dn and 1.5.times.R.sub.dn.
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of our Application Ser. No. 965,344 filed Dec. 1, 1978, now U.S. Pat. No. 4,238,812 the disclosure of which is incorporated by reference herein.
US Referenced Citations (6)
Foreign Referenced Citations (7)
Number |
Date |
Country |
1005459 |
Sep 1965 |
GBX |
1167551 |
Oct 1969 |
GBX |
1168162 |
Oct 1969 |
GBX |
1172718 |
Dec 1969 |
GBX |
1234654 |
Jun 1971 |
GBX |
1462446 |
Jan 1977 |
GBX |
1476089 |
Jun 1977 |
GBX |
Non-Patent Literature Citations (1)
Entry |
"The PTC Resistor"--Blaha; Proceedings of The Electronic Components Conference, 1971. |
Continuation in Parts (1)
|
Number |
Date |
Country |
Parent |
965344 |
Dec 1978 |
|