US2989739A - Blown fuse indicator - Google Patents

Description

June 29, 1961 s, w so 2,989,739

BLOWN FUSE INDICATOR Filed Aug. 14, 1957 FIG. 20

INVENTOR.

- +5Ov EDWARD 3. WILSON MWLM ATTORNEY 2,989,739 BLOWN FUSE INDICATOR Edward S. Wilson, Poughkeepsie, N.Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Aug. 14,1957, Ser. No. 678,211 5 Claims. (Cl. 340-250) This invention relates to electrical apparatus and more particularly to means for indicating the occurrence of a blown fuse in low voltage electrical circuits.

It is common practice to fuse electrical circuits for protection against faults which cause the excessive flow of current, and the electrical fuse commonly used is merely a conductor having a section which melts when excessive current is drawn through the fuse when a fault occurs. In many of the fuses in use today, excessive current causes a section of the fuse to melt away which, in turn, permits a movable mechanical link to operate a fault indicator circuit which may be visual or an audible alarm. One way of indicating that a fuse has blown is by connecting a glow lamp and a series resistor in shunt with the fuse whereupon opening of the fuse circuit permits firing potential to be developed across the neon bulb, and the glow once struck is an indication that the particular fuse is now open. This method is unsatisfactory in low voltage circuits, for example, those used with transistors where the total voltage of the circuit is less than the firing potential of the glow lamp. In other devices of like purpose, the blowing of the fuse frees a mechanical member which closes another circuit thereby providing some indicating device such as a relay coil or a lamp with a suitable voltage. Such circuits are adaptable to low voltage applications but are unsatisfactory nevertheless in many instances due to failure of the mechanical element to move, or once moved, the failure of the mechanical element to provide the contact for completing the indicating circuit. Accordingly, it is the object of the invention to provide an improved fault indicating apparatus.

Another object of this invention is to provide an improved and inexpensive fault indicator circuit for use with a plurality of low voltage sources.

It is a further object of this invention to provide an improved fuse device which employs a moving link member to indicate the occurrence of a fault in a circuit.

It is a still further object of this invention to provide an improved fuse having a movable element which is a current carrying element when the fuse is not blown.

It is an object of this invention to provide an improved indicating circuit in which the indicating circuit is isolated from the work circuit by the melting of the fuse element.

It is another object of this invention to provide a fault \indicating device which is reliable in operation and does not depend upon a circuit connection to be completed when the fuse is blown.

Briefly, in the preferred embodiment of this invention a fuse is employed having three terminals. The fuse element is connected between two terminals while a spring is connected between the weak section of the fuse element and the third terminal. The fuse is so connected that the load current flows through the fuse member via two terminals and the third terminalis connected to a point be tween a glow lamp and a resistor, the latter two devices being connected in series with a potential of sufficient magnitude to fire the glow lamp. When the fuse melts, the spring is freed allowing the potential between resistor and bulb to change and thereby permit full firing potential to be applied to the lamp.

Other objects of the invention will pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode that has been contemplated of applying that principle.

In the drawings: v

FIGURE la is a view of the fuse.

FIGURE lb is a view of the fuse element of the fuse in FIGURE 1a.

FIGURE 2a is a circuit diagram of the fuse employed in the indicating circuit and a low voltage work circuit.

FIGURES 2b and 2c are circuit arrangements showing suitable potentials and polarities of the voltage applied.

In FIGURE la a fuse 10 having terminals 12, 14 and 16 is shown. The fuse element 18 is of a suitable fuse material and is connected across the terminals 12 and 14 with a portion of reduced cross sectional area at the vertex of the fuse material as shown in FIGURE 1b. A cap or cup 20 provides a mounting for the terminals 12 and 14 as well as for the fuse element and may be of any suitable insulating material. A central or body portion 22 is composed of some suitable insulating material and is in the form of a hollow cylinder or tube. The body 22 may be made of glass if it is desired to visually check whether or not the fuse is blown. The terminal 16 may take the form of a cap of suitable conductive material to which one end of a spring 24 is attached and the other end of the spring 24 is connected to the fuse portion 18 under tension. The spring 24 and fuse portion 18 form continuous conducting circuits from pin 12 through cap 16, through terminal 14 through cap 16 and through terminal 12 through terminal 14. When excessive current flows between the two terminals 12 and 14, 'thefusematerial 18 melts providing a separation at the point wherethe spring is connected to the element. When the spring is released, its length is decreased to an extent whereby the circuit connection between terminals 12 and 14 and the cap 16 is broken.

In FIGURE 20, a 1.5 volt D.C. work circuit is shown with a load 26 connected in series with a 1.5 volt battery 27 and with the fuse element 18. Connected to the terminal '16 is a resistor 28 and a glow lamp 30, such as a neon bulb or the like. A pair of 50 volt DC. power supplies 32 are in parallel with the resistor 28 and the glow lamp 30 with a reference point established by a ground or a chassis connection between the power supplies 32. Before the fuse blows, the terminal 16 is at +1.5 volts leaving +515 volts across the neon bulb 30. This voltage is less than the firing potential of the glow lamp so it will not fire. When the fuse blows, the point 16 is isolated from the work circuit since the spring 18 retracts and the voltage at the point 16 rises providing a full volts across the lamp 30. The lamp fires and the resistor 28 limits the current to some suitable value depending upon the rating of the lamp 30.

It is pointed out that for a fuse voltage above 5 volts, the polarity of the neon circuit is important in that the voltages should always be arranged where the voltage in the work circuit is to be subtracted from the voltage across the neon. In those cases, where the fuse voltages exceed 5 volts, the polarity must be proper in order that the firing potential of the bulb 30 is not to be exceeded. FIGURES 2b and 2c illustrate two connections which might be made in those instances where the fuse voltages are above 5 volts. However, the selection of voltages may vary over wide limits and those shown are merely illustrative and are not to be considered as limiting.

While there have been shown, described and pointed out the fundamental novel features of the invention as applied to the preferred embodiment it will be understood that various omissions and substitutions and changes in the form and details of thedevice illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited as indicated by the scope of the following claims.

What is claimed is:

I. In apparatus for indicating the occurrence of a fault which causes an overload in a low voltage circuit, a low voltage power source, a load device in series with said power source, a fuse element connected in series with said load and said low voltage source, a conductive junction, conductive resilient means under tension coupled from said fuse element to said junction, an indicator circuit comprising a glow lamp which ionizes at a firing potential connected to said junction, the connec tion between said conductive junction and said fuse element being adapted to be opened by said resilient means when said fuse element is destroyed, a resistor connected to said junction and a voltage source greater than the firing potential of said lamp connected across said resistor and lamp, and a circuit connection between said low voltage circuit and said indicating circuit for normally establishing a voltage level at said junction less than the firing potential of said lamp.

2. In combination with an electrical supply circuit of low voltage, a glow discharge tube, a current limiting resistor, means connecting said resistor in series with said glow discharge tube, a fuseably responsive connection between said connecting means and said low voltage circuit, a source of potential of a value exceeding the firing potential of said glow discharge tube across said series resistor and discharge tube combination, voltage clamping means efiective at said fuseably responsive connection for holding the voltage across said lamp below said firing potential, and means responsive to the opening of said fuseably responsive connection for removing the clamping means from the connecting means between said resistor and glow discharge tube.

3. In combination, a low voltage circuit having a first voltage source, indicating circuit means including a second source of voltage having a magnitude greater than the magnitude of said first voltage source and a voltage sensitive device operable when the second voltage is applied to said device, a fuseably responsive element in said low voltage circuit, and means including a conductive circuit connection between said fuseably responsive element and said indicating circuit means for normally prohibiting the entirety of the second voltage from acting on said voltage sensitive device, said conductive connection being adapted to be opened when said fuseably responsive element is desetroyed.

4. A claim in accordance with claim 3 wherein said voltage sensitive device is a glow discharge lamp.

5. A claim in accordance with claim 4 wherein said last mentioned means comprises a spring of conductive material connected between said fuseably responsive element and said indicating circuit and under tension in a direction away from said fuseably responsive element.

References Cited in the file of this patent UNITED STATES PATENTS 707,894 Andrews Aug. 26, 1902 921,732 Potter May 18, 1909 1,094,030 Sprong et a]. Apr. 21, 1914 1,757,753 Thiery et al. May 6, 1930 2,663,862 De Anthony Dec. 22, 1953 2,683,201 Miller July 6, 1954 2,717,375 Lubkin Sept. 6, 1955 2,762,884 Van Eyk Sept. 11, 1956 2,774,960 Podell Dec. 18, 1956

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