CA1333411C - Composite circuit protection devices - Google Patents

Abstract

Circuit protection devices which comprise a PTC conduc-tive 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.
Alternatively, the second component is a thick film resistor which is connected in series with the PTC element.

Description

-1- 133~ tll MP906A

BACKGROUND OF THE INVENTION

Field of the Invention This invention relates to circuit protection devices comprising PTC conductive polymers.

Introduction to the Invention Conductive polymer and ceramic compositions exhibiting PTC behavior, and electrical devices comprising them, are well known. Reference may be made, for example, to U.S.
Patent Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715, 4,068,281, 4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,450,496, 4,459,473, 4,459,632, 4,475,012, 4,481,498, 4,476,450, 4,502,929, 4,514,620, 4,515,449; 4,534,889, 4,542,365, 4,545,926, 4,549,161, 4,560,498, 4,562,313, 4,647,894, 4,647,896, 4,685,025 and 4,689,475, and Canadian Application No. 578,757 (Fang, et al.), filed September 29, 1988.

Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under the normal operating conditions of the circuit, but are "tripped", i.e., converted into a high resistance state, when a fault condition, e.g., excessive current or temperature, occurs.
When the device is tripped by excessive current, the current passing through the PTC element causes it to self ~ `

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heat to an elevated temperature at which it is in a high resistance state. Such devices, and PTC conductive polymer compositions for use in them, are described for example in U.S. Patents Nos. 4,237,411, 4,238,812; 4,255,698;
4,315,237; 4,317,027; 4,329,726; 4,352,083; 4,413,301;
4,450,496; 4,475,138; 4,481,498; 4,534,889; 4,562,313;
4,647,894; 4,647,896; 4,685,025; 4,724,417; and 4,774,024;
European Patent Publication No. 38,713 (published October 28, 1981); and in Canadian Application No. 578,757 (Fang, et al.), filed September 29, 1988. When the circuit protection device is "tripped", a thermal gradient is created. Where the thermal gradient flows in the same direction as the current flow, measures can be taken to assure that the peak temperature of the thermal gradient, i.e.. the "hotline" or "hotzone" does not form near an electrode. Such preventative measures are described in U.S.
Patent Nos. 4,317,027 and 4,352,083.

A particularly important use for circuit protection devices is in telecommunications apparatus, which can be exposed to a variety of different fault conditions.
Reference may be made for example to U.S. Patents Nos.
4,068,277, 4,068,281, 4,475,012, 4,459,632, 4,562,313, 4,647,894, 4,647,896, 4,685,025, 4,724,417, and 4,774,024.

133~ ~1~ MP906A

SUMMARY OF THE INVENTION

We have now discovered that improved protection of circuits against excessive currents (and the voltages which produce such currents) can be obtained through the use of composite protection devices which comprise a PTC conductive polymer element and a second electrical component which, under at least some of the fault conditions against which protection is needed, modifies the response of the PTC ele-ment to the fault conditions in a desired way. For example, the second component may be a resistor which, under the fault conditions, generates heat which is transferred to the PTC element and thus reduces the "trip time" of the device (i.e. the time taken to convert the PTC element into a high resistance, high temperature state such that the circuit current is reduced to a safe level). The second component may function substantially only to reduce the trip time, but it is preferably part of the circuit protection system. The reduction of the current by the PTC element may serve to protect the second component and/or to protect other com-ponents of the circuit.

The use of a PTC conductive polymer in such devices offers very important advantages over the use of a PTC
ceramic. For example many PTC conductive polymers are known whose resistivity does not decrease over a temperature range between the switching temperature (Ts) and a much higher temperature, e.g. (TS+40)C, so that by using such conductive polymers, one can eliminate any danger that the additional heat supplied by the second electrical component will cause the PTC element to reach a temperature which is so far above Ts that the composition shows NTC behavior 1333~1~ MPg06A

(i.e. its resistivity decreases with an increase in temperature). PTC ceramics, on the other hand, become NTC
at a temperature which is not far above, e.g. 20 to 50C
above, their Ts. Another major disadvantage of PTC ceramics is that they are difficult or impossible to form into complex shapes (typically they are formed only into simple plates); this limits their ability to be shaped into confor-mity with the second component and to provide efficient heat-sinking of the second component. In addition, ceramics are brittle, and this tends to make them crack when they are subjected to the thermal-electrical-mechanical stresses created by "tripping" of a device in which a second com-ponent increases the rate at which the temperature of the PTC element increases. PTC conductive polymers, by contrast, can readily be shaped in almost any desired shape by a variety of techniques, e.g. molding, extrusion and sin-tering and are much better able to withstand thermal-electrical-mechanical stresses than PTC ceramics. Another disadvantage of PTC ceramics, in many cases, is that their resistivity is higher than is desirable.

In one preferred embodiment, the invention provides an electrical apparatus which comprises (1) a laminar substrate;

(2) a first electrical component which (i) is physically adjacent to said substrate and (ii) has a resistance Rl, said first component comprising (a) a laminar PTC element composed of a conductive polymer which exhibits PTC behavior with a switching temperature Ts, and ~5~ 1333~1~

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

(3) a second electrical component which (a) is physically adjacent to said substrate, (b) is in good thermal contact with the PTC
element, (c) is electrically connected in series to said first component, and (d) has a resistance R2; and (4) an electrical lead which electrically connects the first and second components.

In another embodiment, the invention provides an electrical apparatus which comprises (1) a plurality of laminar substrates;

(2) at least one first electrical component which (i) is physically adjacent to at least one of said substrates, (ii) has a resistance Rl, and (iii) comprises (a) a laminar PTC element composed of a conductive polymer which exhibits PTC behavior with a switching temperature Ts, and (b) at least two laminar electrodes which can be connected to a source of electrical power so 13~3~1 that current passes between the electrodes through the PTC element;

(3) a plurality of second electrical components, each of which (a) is physically adjacent to at least one of said substrates, (b) is in good thermal contact with at least one first component, (c) is electrically connected in series with at least one first component, and (d) has a resistance R2; and (4) an electrical lead which electrically connects at least one first component and at least one second component.

The invention further includes electrical circuits which comprise a source of electrical power, a load and a circuit protection apparatus or device as defined above. In such circuits, the first and second electrical components can be connected in series both under the normal operating con-ditions of the circuit and under the fault conditions (as for example when the second component is a surge resistor in a telephone circuit), or the second component can be one through which no current passes under normal operating con-ditions but is placed in series with the first component under the fault conditions (as for example when the second component is a VDR which is connected to ground to provide a clampdown in a telephone circuit).

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BRIEF SUMMARY OF THE DRAWINGS

The invention is illustrated in the accompanying drawing, in which Figure 1 is a cross-section through an apparatus of the invention;

Figure 2 is a cross-section on line A,A of Figure l;

Figure 3 is the equivalent circuit of the apparatus shown in Figures 1 and 2;

Figure 4 is a cross-section through a second apparatus of the invention;

Figure 5 is a cross-section on line B,B of Figure 4;

Figure 6 is a plan view of a third apparatus of the invention;

Figure 7 is a cross-section on line C,C of Figure 6;

Figure 8 is an isometric drawing of a fourth apparatus of the invention;

Figure 9 is the equivalent circuit of the apparatus shown in Figures 4 to 8;

Figure 10 is a cross-section through a fourth apparatus of the invention;
Figure 11 is the equivalent circuit of the apparatus shown in Figure 10;

Figure 12 is a cross-section through a device of the 133~ L~ ~ ~ MP906A

invention;

Figure 13 is a cross-section on line D,D of Figure 12;

Figure 14a is a plan view and Figure 14b is a cross-sectional view on line E,E of Figure 14a of a fifth apparatus of the invention;

Figure 15a is a plan view and Figure 15b is a cross-sectional view on line F,F of Figure 15a of a sixth apparatus of the invention;

Figure 16 is a cross-section of a seventh apparatus of the invention; and Figures 17a and 17b are plan views of two different sides of an eighth apparatus of the invention.

Figures 18 and 19 are cross-sections of a ninth apparatus of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the first embodiment of the invention, the second electrical component can be one which is specially designed for the particular performance characteristic required; for example, it can be composed of a ZTC conductive polymer.
However, a particular advantage of this embodiment is that it can make use of standard commercially available electri-cal components as the second electrical component, or at least can make use of standard production techniques to pro-duce suitable second electrical components. In this way, for example, it is possible to make use of a component which has a recognized utility as part of a circuit, eg. a -9- 1~33~1~

voltage-dependent resistor (VDR) such as a varistor, a transistor or another electronic component, or a resistor whose resistance is comparatively independent of voltage.
The second component can, for example, be a resistor which is a thick film resistor, a thin film resistor, a metallic film resistor, a carbon resistor, a metal wire, or a conductive polymer resistor formed by, for example, melt-shaping (including melt-extrusion, transfer molding and injection molding), solution-shaping (including printing and casting), sintering or any other suitable technique. The resistance of resistors produced by some of these techniques can be changed by laser-trimming techniques. The resistance of the resistor at 23C is preferably at least 2 times, particularly at least 5 times, especially at least 10 times or even higher, eg. at least 20 times, the resistance at 23C of the PTC element. The resistance of the resistor preferably does not increase substantially with temperature.
For high voltage applications, e.g. where the voltage is greater than about 200 V, the resistance of the resistor is generally at least 20 times, preferably at least 40 times, particularly at least 60 times, or even higher, e.g. at least 100 times, the resistance at 23C of the PTC element.
The preferred total resistance at 23C of the first and second components together will depend on the end use, and may be for example 3 to 2000 ohms, eg. 5 to 1500 ohms, but is usually 5 to 200 ohms, with the resistance of the PTC
element being for example 1 to 100 ohms, usually 1 to 5 ohms.

There can be two or more second electrical components, which can be the same or different. Preferred is an apparatus which acts as a dual hybrid integrated protector -lo- ~ 3 3 3~1~ MP906A

in which one second electrical component comprises a thick film resistor and another second electrical component comprises a voltage limiting device. If there are two or more second electrical components, the combined resistance of the second components which are connected in series with a single PTC element is the resistance used when determining the desired ratio of the resistor (or other second com-ponent) resistance to that of the PTC element. If the electrical apparatus comprises multiple PTC elements and multiple second components, the resistance of the apparatus is defined as that of each individual PTC element and its associated second components (i.e. those second components which are connected in series with the PTC element). For such apparatus, the resistance of each "unit" comprising a PTC element and second components are preferably the same.
Electrical apparatus comprising multiple first and/or second components and substrates is advantageous in providing compact apparatus. Such apparatus requires less space on a circuit board, requires a smaller encapsulation or insulation enclosure, and may respond more rapidly to electrical fault conditions due to better thermal contact between the components. Additionally, the use of multiple components provides the potential for multiple functions.
The leads which are secured to the second electrical component can function not only to connect the component to the circuit and to the first component, but can also be used to provide the electrodes of the first component. For example, one of the leads can be wrapped around an insulating member which surrounds the first component, and the PTC
polymer can be molded around the wrapped product. Alter-natively or additionally one or both of the leads can -11- 133~

be bent into a suitable configuration around, but not touching, an insulating member which surrounds the first component, and the PTC polymer can be molded around the product. These expedients result in apparatus in which the lead between the first and second components and one of the electrodes are formed by a single piece of metal. The cross-section of the leads can, if desired, be modified to pride a desired electrode configuration, e.g. a planar or curved laminar cross-section instead of a round cross-section. It is also possible to change the cross-section of a part of the lead which is not to be molded into the PTC polymer in order to provide a fuse link which will provide protection against a fault condition which cannot otherwise be taken card of by the apparatus. By making use of the leads to provide electrodes in this way, considerable advantages can be obtained in the injection molding process which is preferably used to shape the PTC
conductive polymer around the second component and the electrodes. Thus the leads help to stabilize the configuration inside the mold. If desired, one or more of the leads can be arranged so as to pass through the mold at spaced apart locations and can be severed, after molding is complete, to provide a desired electrical arrangement. For apparatus comprising a laminar substrate, leads may comprise screen-printed ink or sputtered traces.

Suitable PTC conductive polymers for use in this invention are disclosed in the prior art. The conductive polymer should have a resistivity which does not decrease in the temperature range Ts to (Ts + 20)C, preferably Ts to (Ts + 40)C, particularly Ts to (Ts + 75)C.

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The insulating element which lies between the first and second components is subject to substantial thermo-mechanical stress and should be selected accordingly.

In one preferred embodiment, the insulating element comprises a metal surrounded by an insulating material, eg.
anodized aluminum, in order to improve heat transfer from the second component to the PTC element; such an insulating element can be shaped so that it extends into the PTC ele-ment and thus delivers heat to a desired location for the hot zone between the electrodes. The use of an insulating element of this kind is particularly valuable when the second component is in the form of a disc or other shape which can-not easily be fitted within the PTC element. A second preferred embodiment comprises a laminar substrate.
Particularly preferred are substrates which are electrically insulating but have some thermal conductivity, e.g. alumina or berylia. Such substrates may be readily mounted onto a printed circuit board by means of leads. In order to minimize the size of the apparatus on the circuit board, it is preferred that the alumina (or other) substrate have maximum dimensions of 0.100 inch in thickness, 1.5 inch in width, and 0.400 inch in height. This generally allows the apparatus to be lower than the 12 mm (0.47 inch) maximum height constraint of many circuit boards.

In some embodiments, the first and second electrical components are preferably arranged so that the thermal gradient induced in the PTC element is at right angles to the direction of current flow in the PTC element. This is important because the heat flow can otherwise encourage for-mation of the hot zone adjacent one of the electrodes, which -13- 1333~ ~ ~

is undesirable. When the second electrical component lies in a cavity in the PTC element between the electrodes, the desired result is usually easy to obtain. However, if the second component is flat, conventional arrangements of the electrodes and the PTC element encourage formation of the hot zone adjacent one of the electrodes. Particularly in this situation, therefore, the first electrical component preferably comprises the novel combination of interdigitated electrodes positioned on a surface of a laminar PTC element, as described in detail in European Publication No. 158,410, published October 15, 1985. Such a first electrical component can also be wrapped around a cylindrical second component, e.g. a carbon resistor.
Alternatively, a planar device, as described in Canadian Application No. 578,757 (Fang, et al.), filed September 29, 1988, which incorporates a higher resistivity layer in the center plane of the PTC element, may be used. In many applications such laminar PTC elements are preferred because they provide better thermal contact to a laminar substrate and can be smaller than PTC elements of other configurations of comparable resistance. Such laminar PTC
elements also allow design flexibility. The PTC element may be attached directly to the surface of the laminar element or the second component, or it may be attached to the opposite side of the substrate. For circuit protection devices, the hold current (i.e. the maximum current that can flow through the device without causing the device to pass into its high resistance "tripped" state) may be influenced by the rate of heat dissipated into and out of the PTC element. Thermal transfer can be affected by the distance between the PTC element and the second component.

In the second embodiment of the invention, the PTC and ZTC conductive polymer elements are in direct contact with t~

1333~i~ MP906A

each other. As in earlier devices of this kind, the hot zone forms at the interface between the PTC and ZTC elements, but in the devices of the present invention the elements are arranged so that the hot zone is confined to that part of the interface which is completely surrounded by the PTC and ZTC elements. It had not previously been realized that this was important because the presence of air at the hot zone increases the probability of breakdown. Preferably each of the electrodes is in the form of a columnar member (eg. a wire) having an enlarged head (eg. a disc or a sphere or a loop in the member) to reduce the current density on the electrode. Preferably, the enlarged head of at least one of the electrodes is embedded in a ZTC element which is substantially surrounded by the PTC element.

In some cases the apparatus of the invention may be used to protect the thick film resistor or other second electrical component from damage caused by exposure to high temperatures. Under these conditions, the PTC element is selected such that it is converted to a high resistance state at a temperature below that which causes damage to the resistor.

Referring now to the drawing, each of Figures 1, 2, 4, 5, 6, 7, 8 and 10 shows a PTC element 1 which is contacted by electrodes 2 and 3; a lead 4 (leads 4A and 4B in Figure 1) which connects one of the electrodes to a second electrical component which is a resistor 6 (6A, 6B in Figure l); an insulating member S (5A, 5B in Figure 1) and leads 21 and 22 for connecting the device into a circuit.

In Figures 1 and 2, one lead of each of two carbon resistors is wrapped around the insulating container of the ~! 3 ~
resistor to provide one of the electrodes which contact the PTC element. In Figures 4 and 5, each of leads from a carbon resistor has been modified into a desired electrode shape and then embedded in the PTC element; the dotted lines in Figure 4 show where one of the leads was severed, after molding was complete, to provide the desired configuration.
Figure 6 and 7 show a first component which comprises inter-digitated electrodes secured to a laminar PTC element and which is secured to a flat resistor. Figure 8 shows a simi-lar first component wrapped around a cylindrical resistor.
Figure 10 shows an apparatus which comprises two second components, one a resistor, the other a VDR.

Figures 12 and 13 illustrate the second embodiment of the invention and show electrodes 2 and 3 with en~arged heads which are embedded in ZTC conductive polymer elements 8 which are in turn embedded in a PTC conductive polymer element 1.

As shown in Figures 2, 5 and 12, the PTC conductive polymer element is preferably shaped with a construction 11 to promote formation of the hot zone at a location midway between the electrodes.

Figures 14 to 17 illustrate versions of the invention wherein the insulating member 5 comprises a rigid laminar substrate, often alumina. In each version silver or other conductive paste is screen-printed in a pattern suitable for making connection to the PTC element 1 and a second electrical component.

Figures 14a and 14b show an apparatus wherein the PTC
element 1 and the second electrical component, a thick film -16- ~333~1~

resistor 6, are arranged on the same side of the substrate 5. The PTC element 1 is laminar and comprises a first conductive polymer layer 14,14' on the top and bottom of a second conductive polymer layer 13. Adjacent to each first layer is an electrodeposited nickel foil electrode 2,3. A
lead wire 4 connects the bottom electrode 3 of the PTC
element to the thick film resistor 6. Leads 21,22 for connecting the apparatus into a circuit are attached to one edge of the silver conductor pad 9 under the thick film resistor and to the top electrode 2 of the PTC element.

Figures 15a and 15b show an alternative version of the invention in which the thick film resistor 6 and the PTC
element 1 are on opposite sides of the alumina substrate 5.
Also shown is the direction of leads 21, 22 into a printed circuit board 30.

Figure 16 shows in cross-section an apparatus comprising two devices shown in Figure 14 which are packaged to minimize the space required on the circuit board.

Figures 17a and 17b show the opposite sides of the alumina substrate 5 used in a version of the invention comprising three electrical components. Two thick film resistors 6,6' are screen-printed adjacent to one another on one side of the substrate. On the other side of the substrate, two PTC elements 1,1' are positioned adjacent to a voltage limiting device 10. Electrical connections are made independently between PTC element 1 and thick film resistor 6 and between PTC element 1' and thick film resistor 6' by means of silver paste or solder leads 4,4'.
Connection is made between PTC element 1 and voltage limiting device 10 by means of lead 41. Similar connection -17- 1333~11 to PTC element 1' is made by means of lead 41'. Leads 21,22 and 23,24 are used to connect the device to the circuit. Ground lead 25 is attached to the voltage limiting device.

Figure 18 shows an apparatus in which the PTC element 1 is sandwiched between two ruthenium oxide resistors 6,6', each of which is printed onto a separate alumina substrate 5,5'. The PTC element is attached to the substrate by means of a solder layer 30 between the electrodeposited foil electrodes 2,3 and the resistors 6,6'. Wire leads 21,22 are attached to conductor pads 91,91' and allow the current to flow from the lead through a first resistor 5, through the PTC element 1, and then through a second resistor 6'.

Figure 19 shows an apparatus containing multiple components. Two PTC elements 1,1' are soldered (layer 30) onto opposite sides of a laminar substrate 5'', each side of which has been printed with resistors 61,61'. Two additional substrates 5,5' are attached to the remaining side of each PTC element. Wire leads 21,22,21',22' are attached to conductor pads 91,91' to provide two separate units which may be individually powered.

The invention is illustrated by the following examples.

Example 1 Conductive compounds A to D as listed in Table 1 were prepared using a Banbury~ mixer; each was pelletized.
Equal quantities of Compounds A and B were blended together; the blend (Compound I) was extruded into a sheet with a thickness of 0.010 inch (0.025 cm). Equal quantities of Compounds C and D were blended together and the blend .

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(Compound II) was extruded into a sheet with a thickness of 0.020 inch (0.050 cm). A laminated plaque was made by stacking 5 layers of Compound I sheets on either side of a single sheet of Compound II and attaching 0.0014 inch (0.0036 cm) electrodeposited nickel foil electrodes (available from Fukuda) by pressing at 175C and cooling under pressure. PTC elements were prepared by cutting 0.3 x 0.3 inch (0.76 x 0.76 cm) chips from the plaque. These were processed by heating at 150C for one hour, irradiating to a dose of 25 Mrad, heating a second time, irradiating to 150 Mrad, vacuum drying a second time, and heating a third time.

Electrical apparatus made in accordance with this Example is shown in Figures 14a and 14b. Conductor pads (9) made from thick film silver ink (available from ESL) were screen-printed at the edges of a 1.0 x 0.375 x 0.050 inch (2.54 x 0.95 x 0.13 cm) alumina substrate (5). A layer (6) of ruthenium oxide thick film resistor ink (ESL 3900 Series 10 ohm and 100 ohm/sq inks blended to give a resistance of 20 ohm/sq) was printed in a pattern 0.6 x 0.375 inch (1.52 x 0.953 cm) at one edge of the alumina substrate, bridging the conductor pads. A PTC element (1) with a resistance of 2.5 ohms was attached on top of the conductor pad at the other edge via solder. Connection was made between the thick film resitor and the PTC element by means of a wire (4). Lead wires (21, 22) were attached to the top surface electrode (2) of the PTC element and the edge of the thick film resistor. The resulting composite device had a resistance of about 37.5 ohms.

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TABLE I

Formulations of Compounds by Volume Percent Cpd Cpd Cpd Ç~ Cpd Cpd Material A B I C D II

Marlex~ HXM 50100 54.1 52.1 53.1 57.1 55.1 56.1 Statex~ G 28.7 30.7 29.7 25.7 27.7 26.7 Kisuma~ 5A 15.5 15.5 15.5 15.5 15.5 15.5 Antioxidant 1.7 1.7 1.7 1.7 1.7 1.7 Marlex~ HXM 50100 is a high density polyethylene available from Phillips Petroleum.

Statex~ G is a carbon black available from Columbian Chemicals.

Kisuma~ 5A is magnesium hydroxide available from Mitsui.

Antioxidant is an oligomer of 4,4'-thiobis (3-methyl-6-t-butyl phenol) with an average degree of polymerization of 3-4, as described in U.S. Patent No. 3,986,981.

Example 2 Five sheets of Compound I were laminated between two electrodeposited nickel foil electrodes. PTC elements were cut from the plaque and were processed following the procedure of Example 1. Electrical apparatus prepared in accordance with this Example is shown in Figures 15a and 15b.

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Silver ink conductor pads (9) were screen-printed on both sides of an 0.8 x 0.4 x 0.050 inch (2.0 x 1.0 x 0.13 cm) alumina substrate (5). A ruthenium oxide thick film resistor (6) was screen-printed in a 0.8 x 0.3 inch (2.0 x 0.76 cm) rectangle on one side of the substrate. The PTC
element was attached by solder to the other side.
Electrical connection between the components was made by means of a screen-printed lead (4) from the bottom electrode of the PTC element (3) to one edge of the thick film resistor (6).

Example 3 Following the procedure of Example 1, electrical apparatus was made. Two individual units were placed adjacent to one another, as shown in Figure 16, with the PTC
elements in the same plane. This packaging design allowed two units to fit into the same space on a circuit board as one unit.

Example 4 Electrical apparatus in accordance with this Example is shown in Figures 17a and 17b. Two PTC elements (1,1') were placed on one side of an alumina substrate (5) adjacent a voltage limiting device (10). Two ruthenium oxide thick film resistors (6,6') were screen-printed adjacent to one another on the opposite side of the substrate. Electrical connection was made between a resistor (6) and a PTC element (1) by means of a screen-printed lead (4). Electrical connection was also made between the PTC element (1) and the voltage limiting device (10) by means of another screen-printed lead (41). The second resistor (6') was ~333~ MP906A

connected to the second PTC element (1') by lead (4'). The second PTC element (1') was connected to the voltage limiting device (10) by similar means (41') to the first PTC
element.

Example 5 Electrical apparatus made in accordance with this Example is shown in Figure 18. A PTC element was made following the procedure of Example 1. Conductor pads (9,9',91,91') and a thick film resistor (6,6') were screen-printed onto one side of two alumina substrates as in Example 1. A PTC element (1) with a resistance of 2 ohms was positioned between the resistor on each substrate and attached with solder (30). Lead wires (21,22) were attached to a conductor pad (91,91') on each substrate so that, when connected to a source of electrical power, the current would flow from lead 21 through resistor 6, PTC element 1, and resistor 6'. The total resistance of the apparatus was 100 ohms.

Example 6 Electrical apparatus of this Example is shown in Figure 19. Two PTC elements were made following the procedure of Example 2. Two laminar substrates (5,5') were prepared as described in Example 5. A third laminar substrate (5'') was prepared by printing conductor pads and ruthenium oxide resistors (61,61') on both laminar surfaces. Using solder, the PTC elements were each positioned between a single-coated substrate (5,5') and a double-coated substrate (5'').
Four lead wires (21,22,21',22') were attached to four conductor pads (91,91').

Claims (18)

1. Electrical apparatus which comprises (1) a laminar substrate;

(2) a first electrical component which (i) is physically adjacent to said substrate and (ii) has a resistance R1, said first component comprising (a) a laminar PTC element composed of a conductive polymer which exhibits PTC behavior with a switching temperature Ts, and (b) at least two laminar electrodes which can be connected to a source of electrical power so that current passes between the electrodes through the PTC element;

(3) a second electrical component which (a) is physically adjacent to said substrate, (b) is in good thermal contact with the PTC
element, (c) is electrically connected in series to said first component, and (d) has a resistance R2; and (4) an electrical lead which electrically connects the first and second components.

2. Electrical apparatus which comprises (1) a plurality of laminar substrates;

(2) at least one first electrical component which (i) is physically adjacent to at least one of said substrates, (ii) has a resistance R1, and (iii) comprises (a) a laminar PTC element composed of a conductive polymer which exhibits PTC behavior with a switching temperature Ts, and (b) at least two laminar electrodes which can be connected to a source of electrical power so that current passes between the electrodes through the PTC element;

(3) a plurality of second electrical components, each of which (a) is physically adjacent to at least one of said substrate, (b) is in good thermal contact with at least one first component, (c) is electrically connected in series with at least one first component, and (d) has a resistance R2; and (4) an electrical lead which electrically connects at least one first component and at least one second component.

3. Apparatus according to claim 1 or 2 wherein at least one second component is a thick film resistor.

4. Apparatus according to claim 1 or 2 wherein, when electrical power flows through the first component, a thermal gradient induced in the PTC element is in the same direction as the direction of current flow through the PTC
element.

5. Apparatus according to claim 2 which comprises (i) one first component, (ii) two second components, and (iii) two laminar substrates, wherein the first component is positioned between the second components and each second component is physically adjacent to a different laminar substrate.

6. Apparatus according to claim 1 or 2 wherein the second components are thick film resistors.

7. Apparatus according to claim 1 or 2 wherein the laminar substrates are alumina.

8. Apparatus according to claim 2 which comprises (i) two first components, (ii) four second components, and (iii) three laminar substrates, wherein each first component is positioned between two second components.

9. Apparatus according to claim 8 wherein a first laminar substrate has two opposite laminar surfaces each of which is physically adjacent to a second component.

10. Apparatus according to claim 5 wherein the ratio of the total resistance at room temperature of the second components connected in series to the PTC element to the resistance at room temperature of the PTC element R1 is at least 20:1.

11. Apparatus according to claim 5 wherein the total resistance at room temperature of the first component and the two second components is at most 500 ohms.

12. Apparatus according to claim 1 or 2 wherein at least one second component is a thick film reistor which if subject to a temperature exceeding a predetermined level is subject to damage and the PTC element is converted to a high resistance state below said predetermined level.

13. Apparatus according to claim 1 or 2 which is mounted on a printed circuit board.

14. Apparatus according to claim 1 or 2 wherein the substrate is electrically insulating.

15. Apparatus according to claim 1 or 2 wherein the first and second components are mounted on the same surface of the substrate.

16. Apparatus according to claim 1 wherein the ratio of R2 to R1 is at least 20.

17. Apparatus according to claim 1 wherein the apparatus has a resistance of at most 500 ohms.

18. Apparatus according to claim 1 or 2 wherein the substrate has a thickness of at most 0.100 inch, a width of at most 1.0 inch, and a height of at most 0.400 inch.