MXPA98009335A - Thermal protection for overvolt suppressors - Google Patents

Abstract

A thermal protective fuse having a cylindrical body with a thermally protective portion disposed substantially adjacent one end of the cylindrical body is mounted very close to the body of a metal oxide varistor, with the thermally sensitive portion of the fuse positioned approximately next to the center of the body of the varistor.

Description

THERMAL PROTECTION FOR OVERVOLTAGE SUPPRESSORS FIELD OF THE INVENTION This invention relates in general to the suppression of transient voltages, and more particularly to an improved thermal fusion assembly for protecting transistor surge protectors of the varistor type, more particularly the type of oxide varistor. metal. BACKGROUND OF THE INVENTION Varistors, more particularly metal oxide varistors, have been used to protect electrical circuits from the harmful effects of transient voltage spikes for many years. Metal oxide varistors, more particularly zinc oxide varistors, have the ability to set the high transient voltages that appear in unconditioned power lines at a low level to protect electrical equipment or devices connected to the line. In the simplest assembly, a metal oxide varistor can be simply connected between the active and neutral conductors of a single-phase electric circuit, in parallel between the line and the load. Transient voltages exceeding the varistor threshold voltage are shunted by the varistor. Although metal oxide varistors have a long life, and have the ability to repeatedly set high transient voltage peaks at a safety level, they fail eventually and ultimately, even if there is no catastrophic failure, the impedance of the varistors of metal oxide decreases to the point of presenting a considerable load, and eventually overheat and fail by emitting steam and fumes. It is known how to provide protection to metal oxide varistors by connecting the varistors through the power line in series with a current limiting fuse and / or a thermal fuse. If the impedance of the varistor decreases to the point where a current greater than the current provided for the current limiting fuse is exceeded, the fuse will open and the varistor will effectively be removed from the circuit. Also, even if the current does not rise to a level sufficient to open the current limiting fuse, if the temperature of the varistor increases beyond the temperature of the thermal fuse, the thermal fuse will open, thereby removing the varistor from the circuit . The thermal fuses used so far to protect electrical circuits against varistor failure are generally cylindrical in shape, and are mounted on the same printed circuit boards on which the varistor is mounted with the cylindrical body of the fuse arranged adjacent to and parallel to the body. of the varistor. While the thermal protective fuse is physically close enough to the varistor, an increase in the temperature of the varistor will increase the temperature of the thermal protective fuse, causing it to open. Although thermal protection fuses of the type used so far to protect electrical circuits against varistor failure have been somewhat effective, varistors can overheat when they fail in a physical position in the varistor body, which is remote from the thermal protective fuse. Figure 1 represents a printed circuit board in which a metal oxide varistor and a cylindrical thermal protective fuse have been mounted. As long as the varistor body is sufficiently heated in a position close to the fuse, the fuse will be effective. However, if superheating occurs in a portion of the body of the varistor remote from the fuse, the varistor and the surrounding areas can be substantially destroyed before the temperature at the position of the fuse increases sufficiently to cause the fuse to open. Accordingly, an object of this invention is to provide a protective thermal assembly for metal oxide varistors that overcomes the aforementioned problems.

Expressed succinctly, and according to a currently preferred aspect of the invention, a thermal protective fuse having a cylindrical body with a thermally protective portion disposed substantially adjacent one end of the cylindrical body is mounted very close to the body of a metal oxide varistor, with the thermally sensitive portion of the fuse positioned approximately next to the center of the body. Varistor body. According to another aspect of this invention, the cylindrical body of the thermally sensitive fuse is disposed substantially vertically or with the thermally sensitive portion on top of the printed circuit board, generally adjacent to the center of the varistor body. Brief Description of the Invention Although the new aspects of the invention are set forth in detail in the appended claims, the invention itself, together with its other objects and advantages, can be easily understood by reference to the following detailed description of a currently preferred embodiment. of the invention taken in conjunction with the accompanying drawings, in which: Figure 1 is a diagram of the combination of a metal oxide varistor and a thermal fuse according to the prior art. Figure 2 is a schematic diagram of a metal oxide varistor together with protective fuses according to this invention. And Figure 3 is a diagrammatic view of a metal oxide varistor and a thermal protective fuse placed next to it according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figure 2, an electrical circuit includes two electrical conductors attached to a power source, designated line for reasons of convenience. The electrical conductor 10 is the active electrical conductor and the electrical conductor 12 is the neutral electrical conductor. It is understood that one or more conductors may also be provided, such as an electrical conductor to ground, and additional varistors may be connected between the active and neutral conductors and ground. A metal oxide varistor 14 is connected in series with a thermally sensitive fuse 16, and a current limiting fuse 18, between the active and neutral conductors. A load, designated schematically by the resistor 20, is also connected between the active and neutral conductors. In practice, the transient voltages appearing in the line that exceed the varistor threshold voltage are suppressed by the metal oxide varistor, while the fuses 16 and 18 remain intact. If the metal oxide varistor 14 fails, as long as the current limit of the fuse 18 or the temperature limit of the fuse 16 is exceeded, one or both of the fuses 16 or 18 will open, thereby removing the varistor from the circuit.

Figure 3 represents the physical placement of the varistor 14 and the fuse 18 on a printed circuit board. The metal oxide varistor 14 has first and second wires 22 and 24 conventionally connected to printed circuit traces on a printed circuit board 26, also of conventional design. A thermally sensitive fuse 18 has first and second wires 28 and 30, which are also connected to traces on the printed circuit board 26 to effect the schematic assembly shown in Fig. 1. The fuse 18 has a generally cylindrical body with a thermally portion. sensitive 32 placed generally on one end of the body. The fuse 18 is disposed with respect to the varistor 14, so that the thermally sensitive portion 32 is positioned generally adjacent the center of the body of the varistor 14. According to a preferred embodiment of this invention, the thermally sensitive fuse is attached to the varistor 14 with a thermal conductive epoxy layer or other high thermal conductivity adhesive, such as Epocap 15144A / Epocure 114B bicomponent epoxy obtainable from Hardeman Division of Harcros Chemicals, Inc., Belleville, NJ Preferably, an epoxy layer is applied not cured to the body of the varistor and the thermally sensitive fuse is placed in the layer with the thermally sensitive portion generally adjacent to the center of the varistor body A thermally sensitive fuse arranged according to this invention provides a considerably improved response to the excessive heating of the varistor body whenever the overheating occurs, placing the The thermally sensitive portion of the fuse next to the center of the varistor body minimizes the sum of the distances between the sensitive portion and the possible points of failure in the body of the resistor. Applicants have found that faults in the metal oxide varistors are detected more reliably and more quickly by the assembly shown in Figure 3 than has been possible so far. Those skilled in the art will recognize that many modifications and changes can be made in the invention, without departing from its true spirit and scope, which is therefore intended to be defined only by the appended claims.

Claims (12)

1. CLAIMS 1. A transient surge suppressor for an electrical circuit that includes: a varistor having a varistor body, and a thermal fuse having a thermally sensitive element disposed within a fuse body, the fuse being mounted with respect to the varistor so that the thermally sensitive element is approximately next to the geometric center of the varistor body.
2. 2. The transient surge suppressor of claim 1, wherein the varistor body generally has a disk shape.
3. 3. The transient surge suppressor of claim 1, wherein the thermal fuse body is generally cylindrical.
4. 4. The transient surge suppressor of claim 3, wherein the thermally sensitive element is located adjacent one end of the cylindrical body of the thermal fuse.
5. The transient surge suppressor of claim 4, wherein the varistor has first and second wires connected to a printed circuit wiring board, and the disk-shaped body is disposed generally perpendicular to the board.
6. 6. The transient surge suppressor of claim 5, wherein the thermal fuse has generally axial first and second wires, and is mounted with an end proximate to the printed circuit wiring board, and with the end where the wire is located. thermally sensitive element, disposed away from the printed circuit wiring board, and generally adjacent to the geometric center of the varistor body.
7. 7. The transient surge suppressor of claim 6, wherein the thermal fuse is oriented generally vertically.
8. The transient surge suppressor of claim 6, wherein the thermal fuse is oriented at an angle between 45 degrees and 90 degrees with respect to the printed circuit wiring board.
9. 9. The transient surge suppressor of claim 1, including a layer of adhesive between the varistor body and the thermal fuse.
10. The transient surge suppressor of claim 9, wherein the adhesive layer includes a layer of high thermal conductivity adhesive.
11. The transient surge suppressor of claim 10, wherein the adhesive includes a thermal conductive epoxy.
12. 12. The transient surge suppressor of claim 10, which includes a high thermal conductivity filler in the adhesive layer.