WO2001018934A1

WO2001018934A1 - Method and apparatus for protecting electronic equipment coupled to a telephone line

Info

Publication number: WO2001018934A1
Application number: PCT/US2000/024528
Authority: WO
Inventors: David L. Rawlings; Alan J. Squillante
Original assignee: Porta Systems Corporation
Priority date: 1999-09-08
Filing date: 2000-09-07
Publication date: 2001-03-15
Also published as: AU7120400A

Abstract

A method and apparatus for protecting electronic equipment coupled to a telephone line. The apparatus includes a heat coil (36) electrically coupled in series between the electronic equipment and the telephone line, and a positive temperature coefficient device (PTC) (38) coupled in parallel across the heat coil. The PTC presents a high resistance only to excessive currents, which diverts the majority of the current through the heat coil causing the heat coil to trip and short the excess current to ground.

Description

METHOD AND APPARATUS FOR PROTECTING ELECTRONIC EQUIPMENT COUPLED TO A TELEPHONE LINE

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on U.S. Provisional Patent Application Serial No. 60/152,846, which was filed on September 8, 1999, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to protecting telecommunication equipment from excessive voltages and/or currents that may occur on telephone lines during normal operation, and more particularly to protecting such equipment without substantially increasing the resistance of the telephone line.

Description of the Prior Art

Telephone systems are designed with central switching stations to which telephone lines are connected. The central switching stations route the telephone calls placed by telephone service subscribers. To route the telephone calls, it is necessary to attach very expensive electronic equipment (e.g., switching networks) to the telephone lines. Energy surges, due to lightning, for example, on the telephone lines can damage the electronic equipment. In order to protect the electronic equipment, energy surge protection circuits are typically placed between the electronic equipment and the telephone lines.

Figure 1 illustrates a functional block diagram of a telephone system with an energy surge protection circuit 10. Prior art energy surge protection circuits have included fuses, semiconductor devices, heat coils, gas tubes, or combinations thereof. In Figure 1 , the protection circuit 10 is connected to transmission lines of a telephone or communication system including a tip input line 12, a tip output line 14, a ring input line 16, and a ring output line 18.

Telephone line surge protection modules are well-known devices. One such device is described in U.S. Patent No. 3,947,730 to DeLuca, et al., issued March 30, 1976, the disclosure of which is incorporated herein by reference. Protection modules normally accommodate both the telephone line tip (conversation) and ring circuits.

A typical protection module includes a transient over-voltage device designed to short the affected circuit to ground in case of an over-voltage condition. Early modules employed a simple air gap with a pair of arcing electrodes. The dielectric breakdown of the air in the gap enabled arcing to ground the electrodes during the over-voltage condition. Later devices employed sealed tubes incorporating a pair of spaced-apart arcing electrodes and containing a gas of known dielectric strength.

Recent developments in telecommunication switching hardware involve the use of electronic digital switches. Although such switches offer many advantages over prior art devices, they suffer from greater sensitivity to high voltage spikes and current surges. Accordingly, there is much interest in developing a telephone line surge protection module that incorporates a transient over- voltage device that can actuate more rapidly than prior art gas-tube devices. Such modules have been developed, using fast-acting, solid state, over-voltage protection devices. These solid state devices, although faster than gas tubes, are unable to tolerate as much energy dissipation as gas tubes.

In addition to over- voltage protection, telephone line surge protection modules typically include protection against excessive current, as well, most often by a thermally activated device responsive to Ohmic heating produced by the over-current. One well-known thermal device is the heat coil, wherein a coil of resistive heating wire wound about a bobbin carries the circuit current. An elastically biased ground contact is fastened to the bobbin with a low melting-point solder. When the circuit current exceeds a predetermined value, corresponding to an over-current condition, the Ohmic heat generated in the coil melts the low melting-point solder and the elastically biased ground contact is free to short the affected circuit to ground. In view of the increased delicacy of electronic digital switches used in telephone switching hardware, as well as the low energy dissipation tolerance of solid state, over-voltage protection devices frequently employed with such switches, there is a need for a fast-acting, low resistance device that provides over-current protection in telephone line surge protection modules. Prior art heat coils suffer from the disadvantage of excessive electrical resistance due to thermal and structural constraints, thereby requiring excessive Ohmic heating to trigger the heat coil.

It has become evident that certain telephone circuits requiring the fast-acting performance of devices, such as a high-resistance heat coil disclosed in U.S. Patent No. 5,646,812, which is incorporated herein by reference, are unable to accommodate the high resistance that such a heat coil introduces into the circuit. Such circuits might, for instance, include high bit-rate digital subscriber lines (hdsl), which are particularly sensitive to attenuation. These elements also require that the heat coil perform a fast response at low current levels in order to avoid damage to costly equipment commonly utilized in such systems.

The difficulty associated with satisfying the need for a low-resistance, sensitive heat coil is that the thermal mechanism of the heat coil construction requires a specific amount of energy to raise the heat coil temperature to the melting point of a special low melting-temperature, alloy solder, which retains the elastically biased ground contact from shorting the affected circuit to ground. This energy must be provided by resistive (I²R) losses so that a reduction in heat coil resistance necessarily requires an increase in current to compensate for the reduced resistance. Such a current increase is generally considered unacceptable. Lower thermal mass and selection of different solder alloys could conceivably be investigated, but the time and cost required to develop these concepts would be substantial.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and apparatus, which significantly reduce the cost and failure rate of protection circuits for telecommunication equipment.

SUBSTITUTE SHEET .(RULE 26) It is a further object of the present invention to provide a method and apparatus, which protect telecommunication equipment from excessive voltages and/or currents normally occurring on telephone lines that are caused by lightning strikes, power line crosses, and currents induced by adjacent power lines. It is yet a further object of the present invention to provide a method and apparatus for protecting telecommunication equipment, which respond to a slight excess current on a telephone line, which is preferably about 250 mA, in a relatively short time, which is typically less than about 210 seconds.

It is another object of the present invention to provide a method and apparatus for protecting telecommunication equipment, which interrupt an excessive current flow on a telephone line by shorting the offending supply conductor to ground as opposed to opening the circuit.

It is still another object of the present invention to provide a method and apparatus, which protect telecommunication equipment from excessive current and introduces a minimum amount of resistance between a telephone line and the equipment to be protected.

It is yet another object of the present invention to provide a fast-acting, low- resistance heat coil assembly, which can be used in a telephone line, surge protection module for protecting sensitive digital switching hardware. It is a further object of the present invention to provide a fast-acting, low- resistance heat coil assembly that can readily be mass-produced using automated equipment.

It is still a further object of the present invention to provide method and apparatus for protecting telephone equipment, which overcome the disadvantages of known telephone protection modules.

In accordance with the present invention, an apparatus for protecting electronic equipment coupled to a telephone line is provided, which includes a heat coil electrically coupled in series between the electronic equipment and the telephone line, and a positive temperature coefficient device (PTC) electrically coupled in parallel across the heat coil. The (PTC) has a first resistance in response to a first current level and a second resistance in response to a second current level. The first resistance is less than the second resistance, and the first current level is less than the second current level, which diverts the majority of the current through the heat coil causing the heat coil to trip and short the excess current to ground. A fuse or fusing wire may be substituted for the PTC.

In further accordance with the present invention, a method for protecting electronic equipment coupled to a telephone line is provided, which includes the steps of coupling a heat coil electrically in series between the electronic equipment and the telephone line, and coupling a PTC electrically in parallel across the heat coil. A fuse or fusing wire may be substituted for the PTC.

These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a conventional protection circuit used to protect electronic equipment coupled to telephone lines.

Figure 2 is a schematic diagram showing a protection circuit formed in accordance with the present invention, which includes positive temperature coefficient devices (PTC). Figure 3 is a schematic diagram showing the protection circuit formed in accordance with the present invention, which includes fuses.

Figure 4 is a side, cross-sectional view of the protection circuit formed in accordance with the present invention, which shows a heat coil in an untripped state.

Figure 5 is a side, cross-sectional view of the protection circuit formed in accordance with the present invention, which shows the heat coil in a tripped state. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 2 shows a protection circuit 20, which essentially includes two types of circuits that provide protection for telecommunication equipment under slightly different circumstances. The first circuit is commonly referred to as a surge arrestor circuit 21 and functions to short excess current to ground under various over- voltage conditions typically existing for relatively short durations of time. The second type of circuit includes a heat coil 36 and operates to short excess current to ground when an over-current condition persists for a longer duration of time, such as greater than about 60 seconds. The following is provided to briefly describe the electrical connectivity and operation of the surge arrestor circuit 21 shown in Figure 1. The circuit 21 preferably includes a diode bridge having preferably four diodes 24, 26, 28, 30, and a zener diode 22. The cathode of diode 24 is preferably coupled to the cathode of diode 26 at node A, and the anode of diode 26 is preferably coupled to the cathode of diode 28 at node B. The anode of diode 28 is preferably coupled to the anode of diode 30 at node C, and the cathode of diode 30 is preferably coupled to the anode of diode 24 at node D.

The cathode of the zener diode 22 is preferably coupled to node A, and the anode of the zener diode 22 is preferably coupled to node C. In addition, the surge arrestor circuit 21 preferably includes diodes 32 and 34. The cathode of diode 32 is preferably coupled to the cathode of the zener diode 22 at node A, and the anode of diode 34 is preferably coupled to the anode of the zener diode 22 at node C. Both the anode of diode 32 and the cathode of diode 34 are preferably coupled to ground. Node D is preferably coupled to the tip input signal 12, and node B is preferably coupled to the ring input signal.

The operation of the surge arrestor circuit will now be described under various overvoltage conditions. If an excessive positive voltage appears from a tip signal 12 (positive) to a ring signal 16 (negative), then diodes 24, 22, and 28 are biased on, and the current through these diodes reduces the voltage between the tip and ring signals. If an excessive positive voltage appears from the tip signal 12 (positive) to ground, then diodes 24, 22, and 34 are biased on, and the current through these diodes reduces the voltage between the tip signal 12 and ground. If an excessive positive voltage appears from the ring signal 16 (positive) to ground, then diodes 26, 22, and 34 are biased on, and the current through these diodes reduces the voltage between the ring signal 16 and ground. Similarly, if an excessive negative voltage appears from the tip signal 12

(negative) to the ring signal 16 (positive), then diodes 26, 22, and 30 are biased on, and the current through these diodes reduces the voltage between the tip and ring signals. If an excessive negative voltage appears from the tip signal 12 (negative) to ground, then diodes 32, 22, and 30 are biased on, and the current through these diodes reduces the voltage between the tip signal 12 and ground. If an excessive negative voltage appears from the ring signal 16 (negative) to ground, then diodes 32, 22, and 28 are biased on, and the current through these diodes reduces the voltage between the ring signal 16 and ground.

It is anticipated that the zener diode 22 can readily be replaced by a semiconductor device in the so-called generic "thyristor family", such as a reverse blocking diode thyristor or a bidirectional diode thyristor (diac). The zener diode 22 is preferably described as "clamping" the voltage to a specified value, whereas the thyristor is preferably described as a "crowbar" for excessive current. Further details concerning the operation of the diode bridge and zener diode 22 can be found in V. Veley, "Benchtop Electronics Handbook", McGraw-Hill Book Company, pp. 288- 294, (1998), and G. Deboo and C. Burrous, "Integrated Circuits and Semiconductor Devices: Theory and Operation", McGraw-Hill Book Company, pp. 365-458, (1977), which are incorporated herein by reference.

The protection circuit 20 also includes a parallel combination of the heat coil 36 and a positive temperature coefficient device (PTC) 38 electrically coupled in series with both the tip output line 14 and the ring output line 18. Specifically, the parallel combination of the heat coil 36 and the PTC 38 is electrically coupled in series between the tip input line 12 and the tip output line 14. Another parallel combination of a heat coil 38 and a PTC 36 is electrically coupled in series between the ring input line 16 and the ring output line 18. The heat coils 36 are preferably fast-acting, high-resistance heat coils, such as the device disclosed in U.S. Patent No. 5,646,812, and preferably have a nominal resistance of about 14 ohms. The PTC 38 is preferably a device, such as Part No. TR250-145-B-0.5 manufactured by Raychem Corporation, Menlo Park, California, 94025-1164 or Part No. B59830-C80-A70 manufactured by Siemens Electro-Mechanical Inc., Princeton, Indiana, 47671-0001. The PTC 38 preferably has a nominal undisturbed resistance of about 5 ohms.

The PTC 38 preferably exhibit a minimal resistance in response to currents below a predetermined threshold level. However, as the current applied to the PTC 38 is increased beyond this threshold, the resistance of the PTC 38 increases drastically. Therefore, under normal circumstances, the protection circuit 20 presents a parallel resistance from the tip input line 12 to the tip output line 14, which preferably includes about 14 ohms from the heat coil 36 and about 5 ohms from the PTC 38. Thus, the equivalent series resistance on the tip line is only about 3.68 ohms. Such a low resistance is highly desirable and minimizes the attenuation of telecommunication signals passing through the protection circuit 20 under normal operating conditions.

In contrast, when an excessive current begins to flow through the protection circuit 20, the PTC 38 will very quickly react to the high current by switching to its high resistance state. At this point, the excessive current will be diverted primarily through the heat coil 36, which preferably only presents 14 ohms. The excess current diverted through the heat coil 36 causes the heat coil 36 to trigger or fire, which grounds the excess current and prevents damage to electronic equipment coupled to the telephone or telecommunication lines.

It is anticipated that through selection of PTC and heat coil characteristics, a variety of current-versus-time and resistance parameters can be obtained. Thus, the protection circuit 20 formed in accordance with the present invention advantageously provides for a fast reaction time at low current levels in addition to the desirability of grounding the conductor carrying excess current as opposed to opening the circuit between the electronic equipment and the telephone line. Figure 3 shows a second embodiment of the protection circuit formed in accordance with the present invention, in which fusing wire or a fuse 40 has been substituted for the PTC 38 shown in Figure 1. The fuse 40 is preferably a device, such as Part No. 5HFP manufactured by Bel Fuse, Inc., Jersey City, New Jersey, 07302.

Under normal operating conditions, the majority of the currents flow through the fuses 40 in the tip and ring lines. As current flowing in these lines exceeds the rating of the fuses 40, the fuse 40 will open, which will divert all of the current through the heat coil 36. The heat coil 36 will then fire and divert the current to ground. The fuses 40 preferably have a resistance of about 1.7 ohms and about a 0.125 amp rating. Thus, the net parallel resistance in each of the tip and ring lines contributed by the fuse 40 and the heat coil 36 is about 0.116 ohms.

Figure 4 shows a side, cross-sectional view of the heat coil 36 electrically coupled in parallel with the PTC 38 or fuse 40. A brief discussion of the construction and operation of the heat coil 36 will now be described. The heat coil 36 includes an elastically biased contact member. Such a member may be formed of, for example, the assembly of a spring 42, insulated spring follower 44 with conductive shell 46, and a contact pin 48. The contact in 48 is preferably received in the follower 44 and can be retained by, for example, a press fit. The conductive shell 46 is preferably disposed about the follower 44 and can be retained by, for example, crimping. The spring 42 is preferably disposed about the follower 44 for stable transmission of force.

The contact pin 48 can be made of, for example, free-machining brass, while the spring 42 is preferably made of phosphor bronze plated with 60/40 tin-lead solder. The follower 44 is preferably made of a polycarbonate, such as that sold by the General Electric Company, Schenectady, Massachusetts 12306 under the trademark LEXAN, and the shell 46 is preferably made of free-machining brass plated with 60/40 tin-lead solder.

The heat coil 36 also includes a bobbin 50. The bobbin 50 is secured to the elastically biased contact member at a first temperature range to prevent relative motion between the bobbin 50 and the contact member. However, at a second temperature range, the bobbin 50 and the contact member are movable with respect to one another. One method of accomplishing this is to provide a bore 52 through the bobbin 50 for receipt of the contact pin 48, as shown in Figures 4 and 5. The contact pin 48 may then be fastened to the bobbin 50 by a fillet of low melting point solder 54. The bobbin 50 is also preferably formed from free-machining brass. At the first temperature range, below the melting point of the solder fillet 54, the contact pin 48 and the bobbin 50 are rigidly fastened together. This corresponds to an "untripped" or "unfired" condition of the coil 36, wherein the protected circuit is not grounded. At the second temperature range, above the melting point of the solder fillet 54, the contact pin 48 and the bobbin 50 are free to move relative to each other. This corresponds to a "tripped" or "fired" condition of the heat coil 36, wherein the protected circuit is grounded. A bobbin receiving chamber 55 can be provided for at least partially receive the bobbin 50 in the "tripped" condition as shown in Figure 5.

The heat coil 36 also includes a length of resistive heating wire 56, a plurality of turns of which is wound around the bobbin 50. A first end 58 of the wire 56 is preferably connected to the conductive shell 46, while a second end 60 of the wire 56 is preferably connected to the bobbin 50. The electrical interconnections of the wire 56 may be implemented by, for example, capacitive discharge welding. It is to be understood that, while the wire 56 is wound about the bobbin 50 in the illustrative example, alternative configurations could be provided with the wire 56 wound about, for example, the contact pin 48 or both the contact pin 48 and the bobbin 50, and wherein the bobbin 50 is included primarily for structural purposes.

A path for current flow is provided through the spring 42, shell 46, wire 56, and bobbin 50. It will be appreciated that the current flowing in the circuit to be protected passes through the wire 56. Accordingly, the wire 56 will be subject to Ohmic heating with a thermal power dissipation given by the following equation:

P=I²R, (1) where P is the power dissipation in watts, I is the current in amps, and R is the resistance of the wire in ohms. The Ohmic heating is used to melt the solder fillet 54 in a predetermined time and at a predetermined current value. As shown in Figure 4, in the "untripped" or "unfired" condition, the spring 42 biases the shell 46, follower 44, and contact pin 48 (which is preferably a press fit in the follower 44) towards a ground plane 43. However, the bobbin 50 bears against the contact surface 45, and the contact pin 48 is prevented from touching the ground plane 43 by virtue of a shear load developed in the solder fillet 54. It will be apparent that the opening 41 in the contact surface 45 is provided to permit protrusion of the contact pin 48.

Referring now to Figure 5, it can be seen that in the "tripped" or "fired" condition, the solder fillet 54 in its melting state, can no longer support the shear load imposed by the spring 42. Thus, the contact pin 48 and the bobbin 50 are free to move with respect to one another and the contact pin 48 can contact the ground plane 43, thereby grounding the affected circuit. It will be appreciated that only the solder fillet itself needs to enter the second temperature range. However, any structure between the heat coil and the fillet 54 will inherently have a higher temperature than the fillet 54.

As shown in both Figures 4 and 5 the PTC 38 or fuse 40 is preferably coupled electrically in parallel with the equivalent resistance provided by the resistive wire 56 by coupling one end of the PTC or fuse 38, 40 to the conductive shelf 46 and the other end of the PTC or fuse 38, 40 to the bobbin 50. It is anticipated that the PTC or fuse 38, 40 can be coupled to the heat coil 36 at different absolute locations while remaining substantially equivalent to the circuits shown in Figures 2 and 3, and thus within the scope of the present invention. It is also anticipated that the fuse 40 could include additional turns of a preferably light-gauge fusing wire wound onto the same bobbin 50 that is wound with the resistive wire 56 for the heat coil 36. From the foregoing description, it will be appreciated that the method and apparatus in accordance with the present invention protect telephone equipment from excessive voltages and/or currents normally occurring on telephone lines, which are caused by lightning strikes, power line crosses, and currents induced by adjacent power lines. It will also be appreciated that the method and apparatus significantly reduce the cost, size, failure rate, and other disadvantages of known telephone protection modules. From the foregoing description, it will also be appreciated that the method and apparatus in accordance with the present invention protect telecommunication equipment in response to a slight excess current on a telephone line in a relatively short time. It will also be appreciated that the method and apparatus interrupt current flow on a telephone line by shorting the offending supply conductor to ground as opposed to opening the circuit.

From the foregoing description, it will further be appreciated that the method and apparatus in accordance with the present invention protect telecommunication equipment while introducing a minimal amount of resistance between the telephone line and the equipment to be protected. It will also be appreciated that the method and apparatus provide a fast-acting, low-resistance heat coil assembly that can readily be mass-produced using automated equipment.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for protecting electronic equipment coupled to a telephone line, the apparatus comprising:

a heat coil, the heat coil being electrically coupled in series between the electronic equipment and the telephone line; and

a first device, the first device being electrically coupled in parallel across the heat coil, the first device having a first resistance in response to a first current, the first device having a second resistance in response to a second current, the first resistance being less than the second resistance, the first current being less than the second current.

2. An apparatus for protecting electronic equipment coupled to a telephone line as defined by Claim 1 , wherein the first device includes a positive temperature coefficient device.

3. An apparatus for protecting electronic equipment coupled to a telephone line as defined by Claim 1, wherein the first device includes a fuse.

4. An apparatus for protecting electronic equipment coupled to a telephone line as defined by Claim 1 , wherein the first device includes fusing wire.

5. An apparatus for protecting electronic equipment coupled to a telephone line as defined by Claim 1 , wherein the first resistance is about 5 ohms.

6. An apparatus for protecting electronic equipment coupled to a telephone line as defined by Claim 1, wherein the first resistance is about 1.7 ohms.

7. An apparatus for protecting electronic equipment coupled to a telephone line as defined by Claim 1, wherein the heat coil includes:

an elastically biased contact member; a bobbin, the bobbin being secured to the contact member at a first temperature range to prevent relative motion between the bobbin and the contact member, the bobbin and the contact member being movable with respect to one another at a second temperature range; and

a resistive wire, the resistive wire including a conductive core, an insulative coating, a first end and a second end, a plurality of turns of the resistive wire being wound about at least one of the bobbin and the contact member, the resistive wire generating heat at a sufficient rate when conducting current at least as great as a predetermined current value to substantially transition the contact member and the bobbin between the first and second temperature ranges for tripping the heat coil by relative motion between the bobbin and the contact member, the first end of the resistive wire being electrically coupled to the telephone line, the second end of the resistive wire being electrically coupled to the electronic equipment, the contact member contacting a ground plane in response to the relative motion between the bobbin and the contact member, thereby creating a current path from the telephone line to ground, the first device being electrically coupled in parallel across the first end of the resistive wire and the second end of the resistive wire.

8. A method of protecting electronic equipment coupled to a telephone line, the method comprising the steps of:

coupling a heat coil electrically in series between the electronic equipment and the telephone line; and

coupling a first device electrically in parallel across the heat coil, the first device having a first resistance in response to a first current, the first device having a second resistance in response to a second current, the first resistance being less than the second resistance, the first current being less than the second current.

14

SUBSTITUTE SHEET (RULE:26>

9. A method of protecting electronic equipment coupled to a telephone line as defined by Claim 8, wherein the first device includes a positive temperature coefficient device.

10. A method of protecting electronic equipment coupled to a telephone line as defined by Claim 8, wherein the first device includes a fuse.

11. A method of protecting electronic equipment coupled to a telephone line as defined by Claim 8, wherein the first device includes fusing wire.

12. A method of protecting electronic equipment coupled to a telephone line as defined by Claim 8, wherein the heat coil includes:

an elastically biased contact member;

a bobbin, the bobbin being secured to the contact member at a first temperature range to prevent relative motion between the bobbin and the contact member, the bobbin and the contact member being movable with respect on one another at a second temperature range; and

a resistive wire, the resistive wire including a conductive core, an insulative coating, a first end and a second end, a plurality of turns of the resistive wire being wound about at least one of the bobbin and the contact member, the resistive wire generating heat at a sufficient rate when conducting current at least as great as a predetermined current value to substantially transition the contact member and the bobbin between the first and second temperature ranges for tripping the heat coil by relative motion between the bobbin and the contact member, the contact member contacting a ground plane in response to the relative motion between the bobbin and the contact member, thereby creating a current path from the telephone line to ground, the method further including the steps of:

coupling the first end of the resistive wire to the telephone line; coupling the second end of the resistive wire to the electronic equipment; and

coupling the first device in parallel across the first end of the resistive wire and the second end of the resistive wire.