Composite circuit protection devices

• US 4,780,598 A
• Filed: 02/04/1988
• Issued: 10/25/1988
• Est. Priority Date: 07/10/1984
• Status: Expired due to Term

• Current Assignee: Littelfuse Incorporated
• Original Assignee: Raychem Limited (TE Connectivity Limited)
• Inventors: Fahey, Timothy E., Carlomagno, William D., Au, Andrew N.
• Primary Examiner(s): PASCHALL, MARK H
• Application Number:
  US07/150,005
• Time in Patent Office: 264 Days
• Field of Search: 219/501, 219/511, 219/504, 219/505, 219/507-509, 219/490, 219/491, 219/494, 338/22 R, 338/22 SD, 338/25, 361/103, 361/106, 361/54, 361/58, 361/27, 361/57
• US Class Current: 219/511
• CPC Class Codes: H01C 7/027 consisting of conducting or semi-conducting material dispersed in a non-conductive organic material
• CPC Class Codes: H01C 7/13 current responsive
• CPC Class Codes: H05B 2203/006 using interdigitated electrodes
• CPC Class Codes: H05B 2203/013 Heaters using resistive films or coatings
• CPC Class Codes: H05B 2203/017 Manufacturing methods or apparatus for heaters
• CPC Class Codes: H05B 2203/02 Heaters using heating elements having a positive temperature coefficient
• CPC Class Codes: H05B 3/146 Conductive polymers, e.g. polyethylene, thermoplastics
• CPC Class Codes: H05B 3/34 flexible, e.g. heating nets or webs

Abstract

Circuit protection devices which comprise a PTC conductive polymer element and a second electrical component which is thermally coupled to the PTC element and which, when a fault causes the current in the circuit to become excessive, generates heat which is transferred to the PTC element, thus reducing the time taken to "trip" the PTC element. The second component is for example a voltage-dependent resistor which is connected in series with the PTC element under the fault conditions and is thus protected from damage.

Claims

1. Electrical apparatus which comprises

(1) a first electrical component comprising

(a) a PTC element composed of a conductive polymer which exhibits PTC behavior with a switching temperature T_s and which has a resistivity which does not decrease in the temperature range T_s to (T_s +20)°

C.; and

(b) at least two electrodes which can be connected to a source of electrical power so that current passes between the electrodes through the PTC element;

(2) a second electrical component which

(a) is physically adjacent to and physically connected to the first component so that it is in good thermal contact with the PTC element, but which is not in direct physical contact with the first component; and

(b) is electrically connected to the first component;

(3) an electrical lead which electrically connects the first and second electrical components; and

(4) an electrically insulating component which is composed of a solid material, which lies between the first and second electrical components and which is in direct physical contact with the first electrical component and with the second electrical component;

the apparatus being suitable for use in an electrical circuit in which, under normal operating conditions, the PTC element is in a lower temperature, low resistance state and which, if it is subject to a fault condition which results in excessive current in the circuit, is protected from damage by conversion of the PTC element into a high resistance, high temperature state which reduces the current to a safe level, the second component, when subjected to the fault condition, generating heat which is tranferred to the PTC element and reduces the time taken to convert the PTC element to the high resistance, high temperature state.

2. Apparatus according to claim 1 wherein the thermal gradient induced in the PTC element by heat transferred from the second component is at right angles to the direction of curent flow in the PTC element.

3. Apparatus according to claim 1 wherein the second component is a resistor selected from thick film resistors, thin film resistors, metallic film resistors, carbon resistors, wire resistors, and conductive polymer resistors.

4. Apparatus according to claim 3 wherein the second component has a resistance at 23°

C. which is at least 2 times the resistance at 23°

C. of the first component and which does not increase substantially with temperature.

5. Apparatus according to claim 1 wherein the second component has a voltage-dependent resistance.

6. Apparatus according to claim 1 wherein the second component is a varistor, a transistor, or another electronic component.

7. Apparatus according to claim 1 which comprises two second electrical components, one being a voltage-dependent resistor and the other being a resistor whose resistance is substantially independent of voltage.

8. Apparatus according to claim 1 wherein the electrically insulating component is composed of a solid material, and the second component and the electrically insulating component are substantially surrounded by the PTC element.

9. Apparatus according to claim 1 wherein the electrical lead and one of the electrodes are provided by a single piece of metal.

10. Apparatus according to claim 1 wherein the first electrical component comprises interdigitated electrodes positioned on a surface of a laminar PTC element.

11. Apparatus according to claim 1 wherein the first component comprises two electrodes which are in direct physical and electrical contact with the PTC element, and there is at least one second electrical component which lies between the electrodes in a cavity in the PTC element.

12. Apparatus according to claim 1 wherein the insulating component comprises a metal member substantially surrounded by an insulating member and extends into the PTC element between the electrodes.

13. Apparatus according to claim 12 wherein the insulating component comprises anodized aluminum.

14. An electrical circuit which comprises

(A) a source of electrical power;

(B) an electrical load; and

(C) electrical apparatus to protect the circuit from damage under a fault condition, said apparatus comprising

(1) a first electrical component comprising

(a) a PTC element composed of a conductive polymer which exhibits PTC behavior with a switching temperature T_s and which has a resistivity which does not decrease in the temperature range T_s to (T_s +20)°

C.; and

(b) at least two electrodes which can be connected to a source of electrical power so that current passes between the electrodes through the PTC element;

(2) a second electrical component which

(a) is physically adjacent to and physically connected to the first component so that it is in good thermal contact with the PTC element, but which is not in direct physical contact with the first component; and

(b) is electrically connected to the first component;

(3) an electrical lead which electrically connects the first and second electrical components; and

(4) an electrically insulating component which is composed of a solid material, which lies between the first and second electrical components, and which is in direct physical contact with the first electrical component and with the second electrical component;

said circuit having normal operating condition in which the PTC element is in a low temperature, low resistance state, and said circuit being liable to exposure to at least one fault condition in which damage to one or more components of the circuit is prevented by conversion of the PTC element into a high temperature, high resistance state which reduces the current to a safe level, the second component, when subject to the fault condition, generating heat which is transferred to the PTC element and reduces the time taken to convert the PTC element into the high resistance, high temperature state.

21. A circuit according to claim 14 wherein no current passes through the second component under normal operating conditions and the second component is in series with the first component under the fault condition.

15. A process for the preparation of an electrical apparatus which comprises

(1) a first electrical component comprising

(a) a PTC element composed of a conductive polymer which exhibits PTC behavior with a switching temperature T_s and which has a resistivity which does not decrease in the temperature range T_s to (T_s +20)°

C.; and

(b) at least two electrodes which can be connected to a source of electrical power so that current passes between the electrodes through the PTC element;

(2) a second electrical component which

(a) is physically adjacent to and physically connected to the first component so that it is in good thermal contact with the PTC element, but which is not in direct physical contact with the first component; and

(b) is electrically connected to the first component;

(3) an electrical lead which electrically connects the first and second electrical components; and

(4) an electrically insulating component which is composed of a solid material, which lies between the first and second electrical components, and which is in direct physical contact with the first component and with the second component;

which process comprises

(1) placing within a mold a device comprising said second electrical component, an electrically insulating component surrounding said second electrical component, and two electrical leads extending from said second electrical component through the insulating component; and

(2) filling the mold with a conductive polymer which exhibits PTC behavior with a switching temperature (T_s) and which has a resistivity which does not decrease in the temperature range (T_s) to (T_s +20)°

C., thereby contacting the conductive polymer with at least one of said electrical leads which thus provides at least one of said electrodes.

16. A process according to claim 15 wherein one of the leads passes through the mold at two spaced apart locations and the process includes severing said lead between said locations, whereby said lead provides both said electrodes.

17. A process according to claim 15 wherein two said devices are placed within the mold and said electrodes are provided by one lead from each of said devices.

18. A circuit protection device which comprises

(a) a PTC element composed of a first conductive polymer exhibiting PTC behavior;

(b) a ZTC element composed of a second conductive polymer which exhibits ZTC behavior and which has a resistivity at 23°

C. which is greater than the resistivity at 23°

C. of the first conductive polymer, the ZTC element being in direct physical and electrical contact with the PTC element; and

(c) at least two electrodes which can be connected to a source of electrical power;

the components (a), (b) and (c) being so arranged that when the electrodes are connected to a power source such that the PTC element is converted into a high temperature high resistance state, (a) all current paths between the electrodes pass through the PTC element and the ZTC element, and (2) a hot zone is formed at an interface between the PTC and ZTC elements and at a location on the interface which is completely surrounded by the PTC and ZTC elements.

19. A device according to claim 18 wherein each of said electrodes is in the form of a columnar member having an enlarged head, and the enlarged head of at least one of said electrodes is embedded in a ZTC element which is substantially surrounded by the PTC element.

20. A device according to claim 19 wherein the head of each of said electrodes is embedded in a ZTC element, and the PTC element substantially surrounds both ZTC elements.

22. An electrical circuit comprising

(A) a power source;

(B) an electrical load; and

(C) a circuit protection device which is in series with the load and which comprises

(1) a laminar element which is at least 0.002 inch thick and is composed of a conductive polymer composition which (a) exhibits PTC behavior and (b) comprises an organic polymer and, dispersed in the polymer, a particulate conductive filler; and

(2) a plurality of spaced-apart electrodes, at least two of which are connected to the power source to cause current to pass between the electrodes through the laminar element, each electrode comprising a plurality of distinct parts which interdigitate with distinct parts of an adjacent electrode and which are dimensioned and positioned so that

(a) when current passes between the electrodes, a substantial proportion of the current through the laminar element is parallel to the faces of the laminar element, and

(b) the ratio of the average width of the electrodes, measured parallel to the faces of the laminar element and in the direction of current flow in the laminar element, to the average distance between adjacent electrodes between which current passes, measured parallel to the faces of the laminar element and in the direction of current flow in the laminar element, is at least 0.1;

1;

said circuit having a normal operating condition in which the PTC conductive polymer composition of the circuit protection device is in its low temperature, low resistivity state.

23. A circuit according to claim 22 wherein the electrodes of the circuit protection device are interdigitated and are so positioned and dimensioned that, at all points, the distance between adjacent electrodes between which current passes, measured parallel to the faces of the laminar element, is not more than three times the average distance between adjacent electrodes between which current passes, measured parallel to the faces of the laminar element.

24. A circuit according to claim 22 wherein the conductive polymer composition of the circuit protection device has been melt-extruded and the electrodes are so positioned that current passing between the electrodes follows a path which is substantially parallel to the direction of extrusion.

25. A circuit according to claim 22 wherein the electrodes of the circuit protection device have been printed on the same surface of the laminar element.

26. A circuit according to claim 22 wherein in the circuit protection device the electrodes are interdigitated and the ratio of the average width of the electrodes to the average distance between adjacent electrodes between which current passes is from 0.4:

1 to less than 5;

1.

27. A circuit according to claim 22 wherein the conductive polymer composition of the circuit protection device has a resistivity at 23°

C. of 1 to 100 ohm.cm and the circuit protection device has a resistance at 23°

C. of 2 to 100 ohms.