Heater

Abstract: A heater is disclosed comprising a foil heating element 6 embedded in a dielectric sheet 8. A a leaf spring 20 which is curved when relaxed, clamps the embedded element against a surface 18 to be heated. The heater can be mounted on an internal surface of the enclosure. Using a proprietary flexible heating element, the heater has been run at a flux of 3.9 kW.m 2 . The heater is mounted with a flexible dielectric face directly against an internal surface of the enclosure. No machining of the enclosure is needed since the flexible dielectric acts as a thermal gasket.

Description: This invention relates to heaters. The background to the invention will be explained in relation to a particular application. The reader will appreciate, however, that the invention is not limited to that application. Externally mounted enclosures for mobile radio base stations potentially experience a wide range of ambient temperatures. If the internal temperature drops below about 0°C the electronics cease to function. The external ambient temperature can fall to -45°C. At low external ambient temperatures, the heating which arises naturally from the electronics is insufficient to keep the internal temperature sufficiently high. Even at higher temperatures where the heat dissipation in the electronics is sufficient to keep the electronics operative, there can be a problem if a power cut occurs. If the internal temperature falls below that at which the electronics is operative, it will not restart when the power returns. A heater is therefore desired. In order to keep the enclosure desirably small the heater should not be large. It is known from United States patent US 5624750 to provide a heater comprising a foil heating element embedded in a dielectric sheet. The present invention is characterised over the disclosure of US 5624750 in that a leaf spring is provided which is cured when relaxed for clamping the embedded element against a surface to be heated. In the particular application described above, the heater can be mounted on an internal surface of the enclosure. Using a proprietary flexible heating element, we have been able to run the heater at a flux of 3.9 kW.m 2 . The heater is mounted with a flexible dielectric face directly against an internal surface of the enclosure. No machining of the enclosure is needed since the flexible dielectric acts as a thermal gasket. In other conventional uses of such a heating element it is bonded to a surface with double sided self adhesive tape. Such tapes are not operable at the extremes of temperature to which the enclosure may be subject. In order to prevent local hot spots, the heater preferably includes a heat conductive layer against that side of the embedded element, which in use is remote from the surface to be heated. To aid assembly, the heat conductive layer, which may be sheet aluminium, is bonded to the element with double sided adhesive tape. Once the element and heat conductive layer are clamped to the surface of the enclosure, the fact that the double sided tape is inoperative at extreme temperatures is irrelevant. Preferably, the embedded element is in the form of a conventional foil element laminated between two flexible dielectric layers. In order to simplify assembly of the heater on the surface of the enclosure, it preferably includes fastening means including shoulder features which space ends of the spring from the surface to be heated by a distance approximately equal to the thickness of the embedded element and any conductive layer. One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of a heater embodying the invention; Figure 2 is a cross section of a spring, from the heater of Figure 1, when relaxed; Figure 3 is a plan view of the heater of Figure 1; and Figure 4 is a detailed cross section of the heating element of the heater of Figure 1. Referring to the drawings, an enclosure 2 (only part of which is shown) contains radio equipment (not shown) for a mobile telephone base station. The enclosure is formed from two interfitting light metal castings part of one of which is shown in Figure 1. A commercially available trace heating element 4 comprises a Nichrome foil resistor pattern 6, seen only in section in Figure 4, sandwiched between two layers of flexible rubber dielectric 8. A thermal cutout 9 is located inside the heating element 4 and causes a bulge 11 in the lower layer 8 which projects from the heating element 4. The element 4 is bonded by a layer of double sided adhesive tape 10, to an aluminium sheet 12 of similar dimensions, which acts to disperse any potential hot spots in the assembled heater. Two set screws, screwed into the enclosure, provide shoulders 16 spaced a distance d from the internal surface 18 of the enclosure 2. A leaf spring 20 is formed with notches 22 to receive the shanks 24 of the set screws 14 so as to be retained by the heads 28. The heads are positioned so that the underside 30 of the spring is spaced from the surface 18 of the enclosure 2 by a distance approximately equal to the combined thickness of the heating element and the sheet 12. The bulge 11 is received by a recess 13 in the surface 18 thereby locating the heating element 4. As is shown in Figure 2, the leaf spring 20 is curved when relaxed. At its end remote from the notches 22, the leaf spring 20 has a step providing a shoulder 32 leading to a tang 34. The step is approximately the size as the combined thickness of the heating element 4. When the tang is fastened to the surface 18 of the enclosure 2 by a screw 36, the leaf spring clamps the heating element with a uniform load against the surface 18 of the enclosure 2. The provision of shoulders 16 and 32 ensure that the spring is not over tightened on the ends of the heating element 4.

Claims: A heater comprising a foil heating element (4) embedded in a dielectric sheet (8) characterised in that a leaf spring (20) is provided which is curved when relaxed for clamping the embedded element (4) against a surface (18) to be heated. A heater as claimed in claim 1, including a heat conductive layer (12) against that side of the embedded element (4), which in use is remote from the surface to be heated. A heater as claimed in claim 1 or 2, wherein the embedded element (4) is in the form of a foil element (4) laminated between two dielectric layers (8). A heater as claimed in any preceding claim, including fastening means (14) including shoulder features (16) which space ends of the spring (20) from the surface (18) to be heated by a distance approximately equal to the thickness of the embedded element and any conductive layer. A heater as claimed in any preceding claim, wherein the spring (20) is formed with an integral concave projection (11) to serve as a cable clamp. A heater as claimed in any preceding claim for use on an enclosure for electronics. An enclosure (2) for electronics comprising a heater according to any preceding claim.

Inventors: GATES WILLIAM GEORGE, KEEGAN MARTIN MICHAEL MARK, GATES, WILLIAM GEORGE, KEEGAN, MARTIN MICHAEL MARK
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 1997
Main classification: H05B3
IPC codes: H05B3