The present invention relates to safety in electrical devices and means of providing safety in such devices.

The following description will be described with reference to electricity meters but it will appreciated that the safety measure can be applied to any other electrical product.

Generally, electrical products have to be protected from accidental short circuits by means of an overcurrent protection device such as a fuse. Normal fuses are also prone to failures under surge currents and voltages which can result in the fuses needing regular replacement. However, certain electrical products such as electricity meters cannot be built with integrated fuses inside them as they are sealed devices and customers are not expected to open the housing of the electricity meter.

In some cases, thermal fuses are used to deal with this problem. But thermal fuses are not only expensive for this application; they are also unreliable to assemble exactly over potential hot spots. In addition, self healing/resetting fuses are available but such fuses are rather expensive and not desirable for use with electricity meters.

Although general protection fuses are sometimes used outside the electricity meter, such fuses may not provide adequate protection depending upon the nature of the circuit inside the meter. For example, the meter may have a minor fault in the electronic circuit which does not cause large enough current for the external fuse to blow but may be large enough to cause unsafe temperature rise within the meter. Electricity meters and other similar electrical devices can cause damage by burning due to minor short circuits (for example, inter-turn shorts in transformers or failures in power supplies in electronic products).

US 4,661,881 discloses an overload protector for a telephone set. In both embodiments, a resistor is provided on a printed circuit board (PCB) and the resistor is connected to the PCB using plated-through-hole techniques

It is an object of the present invention to provide a low cost way of alleviating the need for a fuse and yet achieve ‘safe failure’ mode that can typically be achieved when a fuse is used.

From one aspect, the present invention comprises a power supply circuit for an electrical device, the circuit comprising a resistor that is surface mounted on the circuit, and being of a predetermined value and weight such that application of an overcurrent to the circuit causes the resistor to become dismounted from the circuit.

Accordingly, the resistor is surface mounted such that the conductive ends are arranged to terminate on the side of the circuit on which the resistor is located. The invention ensures that no fire safety hazard is created. Also it avoids the need for any extra component for protection by using a current limiting resistor, which is present anyway, but in the present invention the resistor is of relatively larger weight compared to conventional current limiting resistors and adapts the method of mounting to a manner not normally associated with larger components. The surface mounting enables easy and guaranteed dislodging of the component.

The surface mounting is achieved typically by a releasable attachment means such as solder which changes physical state when heated.

From another aspect, the present invention provides an electricity utility meter having a housing comprising a power supply circuit for an electrical device, the circuit comprising a resistor mounted on the surface of the circuit, and being of a predetermined value and weight such that application of an overcurrent to the circuit causes the resistor to become dismounted from the circuit by natural gravity.

Embodiments of the invention will now be described by way of example with reference to the drawings in which:

Fig. 1 shows a circuit diagram of a preferred power supply circuit;

Fig.2 shows the a perspective view of a preferred orientation of a resistor used in the power supply circuit of Fig. 1,
Fig. 3 shows a top perspective view of the resistor based on the resistor of Fig. 2;

Typically all electronic products that operate from mains supply have a power supply circuit at the input and a power supply circuit is considered a particular type of circuit where current flow is to be prevented following over heating. Such circuits normally also have a current limiting resistor to prevent inrush currents at switch on and to protect from voltage surges. A simple wire wound resistor is often used for this purpose. In this embodiment, we propose to use this resistance with the additional purpose of safe failure. The wire wound resistor is chosen in such way to be slightly heavy and is mounted on the printed circuit board of the power supply circuit in a surface mount fashion rather than the traditional plated-through-hole techniques used with such large devices. The advantage of this mounting style along with the appropriate choice of the resistor value and weight is that when a short circuit happens and, the resistor gets heated, its solder melts dislodging it from the printed circuit board under gravity action. The mass of the resistor is typically a few tens of grams which is considered relatively large for a current limiting resistor. The mere dislodging of the heavy resistor under gravity prevents further current flow and protects from further heating and damage. Being robust resistors, they not only do not blow under surges, but in fact protect against them.

Fig. 1 shows a circuit diagram 10 of an input part of a power supply circuit for an electricity meter. Only the input part of the power supply circuit is shown and all the components of the power supply circuit 10 have not been described in detail as they are known from a conventional power supply circuit. Typical values of components of the power supply are shown in Fig. 1. A current limiting resistor 11 is provided on the input to power supply circuit 10. The resistor is a wire wound resistor and is attached to a printed circuit board of the power supply circuit. The attachment is achieved by surface mounting the resistor 11 using solder. The orientation of the resistor 11 is chosen to enable the resistor to remain in electrical contact when surface mounted through the use of the solder.

As shown in Fig. 2 and 3, at least part of the first conductive end 12 of the resistor 11 is arranged in an opposite manner to part of the second conductive end 13 of the resistor 11 to enable the resistor 11 to remain upright when surface mounted using the solder.,The first conductive end 12 is bent in different sections 12a,12b,12c which represent the x,y,z planes respectively. Each section 12a,12b,12c is generally perpendicular to the other but it will be appreciated that the invention is not limited to each being exactly perpendicular. The second conductive end 13 of the resistor is also bent in different sections 13a,13b,13c which represent the x,y,z planes respectively. The section 13c is orientated in an opposite direction in the z plane to the section 12c of the first conductive end. The z-plane is generally perpendicular to the longitudinal axis of the resistor 11.

In use, solder (not shown), which is a form of releasable attachment means that changes state when heated, is used to attach the resistor 11 to the printed circuit board (PCB) of the circuit 10. The amount of soldered surface can be set based on the weight of the resistor 11.

The weight of the resistor 11 is such that when a short circuit occurs, this results in the resistor 11 getting heated and the solder melts. The resistor 11 becomes dislodged from the surface of the PCB of the power supply circuit 10 under the action of gravity. The dislodging prevents further current flow and further heating and damage.

The resistor 11 is advantageously used in a power supply circuit of an electrical product such as an electricity meter (not shown). The electricity meter has a housing (not shown) containing the power supply circuit 10 therein. By choosing an appropriate value and weight of resistor 11 it is possible to provide a ‘safe failure’ mode. That is, if the meter has a minor fault in an electronic circuit contained therein which does not cause a large enough current for an external fuse outside the meter housing to blow, the resistor 11 will become dislodged to avoid an unsafe temperature rise within the meter.

CLAIMS

1. A power supply circuit for an electrical device, the circuit comprising a resistor having conductive ends mounted on the surface of the circuit, and being of a predetermined value and weight such that application of an overcurrent to the circuit causes the resistor to become dismounted from the circuit under the action of gravity, wherein the conductive ends of the resistor are arranged to terminate on the side of the circuit on which the resistor is positioned.

2. The power supply circuit according to claim 1 wherein the resistor is mounted at the input of the circuit.

3. The power supply circuit according to claim 1 or 2 wherein the resistor is a wire wound resistor.

4. The power supply circuit according to any preceding claim further comprising releasable attachment means for attaching the resistor to the surface of the circuit.

5. The power supply circuit according to any preceding claim wherein at least one portion of one conductive end of the resistor is bent in one direction and at least one portion of the other conductive end of the resistor is bent in the opposite direction.

6. A power supply circuit substantially as hereinbefore described with reference to the accompanying drawings.

7. A utility meter having a housing comprising a power supply circuit according to any preceding claims, the utility meter comprising means for receiving mains power and means for connecting the mains power to the power supply circuit.