GB2064927A

GB2064927A - Heatable mirror

Info

Publication number: GB2064927A
Application number: GB8033288A
Authority: GB
Original assignee: NKF Groep BV
Current assignee: NKF Groep BV
Priority date: 1979-10-18
Filing date: 1980-10-15
Publication date: 1981-06-17
Also published as: DE2942155A1; IE802142L; IE50167B1; GB2064927B; FR2467737A1; FR2467737B1

Abstract

In order readily to obtain a rapid and uniform heat generation and discharge, a PTC resistance body 42 is arranged as a heating and control element between the mirror glass 34 and a heat distribution plate 37. There is an air gap 37b between the PTC resistance body 42 and the mirror glass 34. The PTC resistance body 42 transfers its heat to the mirror glass 34, indirectly by conduction via the heat distribution plate 37 and directly by heat radiation. The invention may be used in particular for mirrors which are exposed to the weather, such as outside rear view mirrors of motor vehicles. <IMAGE>

Description

SPECIFICATION Heatable mirror The invention relates to a heatable mirror, such as a rear view mirror for motor vehicles, the glass of which is heated by a heating element comprising a PTC resistance body, the back of the mirror glass being provided with a heat distribution plate which is connected to the PTC resistance body over a primary contact area which is spaced from the mirror glass, and the plate is connected in a heat-conducting manner to the mirror glass over a secondary contact area which is connected to the primary contact area via an intermediate connecting portion of the plate. Such a mirror is described in United Kingdom Patent Application 8022322.

It has been found that when the embodiment proposed in the above-mentioned application is used it is difficult to obtain a uniform distribution of the heat over the mirror glass. The heat losses due to the transfer of heat to the housing located at the rear, and consequently to the environment, are considerable if no additional heat-insulating measures are taken.

It is an object of the invention to make still better use of the quantity of heat delivered by the PTC resistance body for heating the glass and at least to reduce the requirement for said additional heatinsulating measures.

According to the invention, this is accomplished for a mirror of the type described in the opening paragraph in that the PTC resistance body is arranged in the space between the mirror glass and the primary contact area of the heat distribution plate and that there is an air gap between the PTC resistance body and the mirror glass.

This new arrangement particularly accomplishes that the PTC resistance body is brought closer to the mirror glass to be heated. However, there remains an air cushion between the resistance body and the glass to prevent local over-heating of the glass. In a practical embodiment in which there is a layer of only approximately 1 mm of air between the PTC resistance body and the glass, the direct radiant heat of the PTC resistance body also contributes to the heating of the glass. As is also the case for the constructions eariier described, the heat of contact is transferred to the glass by conduction via the heat distribution plate. Since, however, the heat distribution plate is now arranged between the PTC resistance body and the housing located at the back of the mirror, less radiant heat is transferred to the housing, so that heat losses are reduced.

Especially for the case where one or more areas of the heat distribution plate are in the form of a dish, the PTC resistance body is surrounded by the heat distribution plate and the mirror glass and consequently directly transfers its radiant heat to the mirror glass and to a surrounding portion of the heat distribution plate. The heat distribution plate then also functions as a reflector for the radiant heat which reaches it. In such a case the measures described in connection with the prior construction for heat insulation of the housing may be dispensed with.

To give passage to an electric supply conductor, the heat distribution plate may have either a feedthrough aperture in the intermediate connecting portion or a channel formed in the plate in the secondary contact area, or a combination of both these features.

An embodiment of the invention will now be further explained with reference to the accompanying drawing, in which Figure 1 is a cross-sectional view on the line l-l in Figure 2 of an outside rear view mirror embodying the invention, and Figure2 is an elevational view in the direction, indicated by means of arrow S in Figure 1 of the back of the mirror glass, the housing which contains the mechanical and electrical connections having been omitted.

Mirror 31 has a housing 32 made of a suitable material, for example a plastics material or sheet metal. Housing 32 is dish-shaped and is provided with an inwardly curving edge 33. The housing 32 incorporates the mirror glass 34, a circular seal 35 consisting of rubber or such-like being placed between the mirror glass 34 and the edge 33.

A heat distribution plate 37 made of metal is fastened to the back 36 of the mirror glass 34 by means of adhesive tape. This heat distribution plate has a central dish-shaped recess 37a by which a central, primary contact area 38, an adjoining frustoconical connecting portion 39, and a secondary contact area 40 fastened to the back 36, are obtained.

The areas 38 and 40 extend parallel to the glass 34, while the connecting portion 39 extends conically at an angle of approximately 45" to the glass 34.

Aflat, round, PTC resistance body 42 is soldered (see soldered connection 43) to the side 41 of the heat distribution plate 37 which faces the glass 34 in such a way as to form an air gap 37b of approximately 1 mm thick between the PTC resistance body and the glass 34. Current is applied primarily via an electric conductor 44, to which the PTC resistance body 42 is connected by means of the soldered connection 45. The conductor 44 is passed through a feed-through aperture 37c in the frusto-conical connecting portion 39 to the current source, not shown, of the motor vehicle; to the ignition switch in particular. The second electrical connection to the PTC resistance body 42 is made by means of a soldered connection 46, from which a conductor 17 leads to a soldered or crimped connection 48 having a contact washer 49.

The contact washer 49 is pushed onto the shaft of a threaded bolt 50, which has been passed through a hole 51 of the housing 32. The mirror 31 is secured to the vehicle body by means of the bolt 50. The conductor 44 is passed through a bore in the bolt 50 to the current source of the vehicle, while the conductor 47 is electrically connected to chassis of the vehicle body via the bolt 50 and the contact washer 49, respectively.

Figure 2 shows by means of a dot-and-dash line an alternative path for the conductor44through the heat distribution plate 37 to the soldered connection 45. At that side of the heat distribution plate 37 which faces the glass 34 a ridge 37d in the plate 37 extends from the recess 37a across the secondary contact area 40 to a feed-through aperture 37e to form a channel through which the conductor 44 can be led out. A third possibility, not shown, is for the ridge 37dto extend as far as the outer edge of the contact area 40.

The mirror heating system functions as follows.

When the vehicle is started by means of the ignition switch, the mirror heating is switched on simultaneously. The PTC-resistor 42 consumes approximately 25W until it reaches a temperature of approximately 100C, the temperature at the back of the mirror then being approximately 50"C. Then the heating automatically switches to a lower power consumption of approximately 5 W. This range may be denoted the stand-by range. Rain, frost, fog and such-like produce decrease in temperature as they withdraw energy due to evaporation, which cools PTC-resistor 42 and causes it to switch back to its higher power consumption. The electric heating energy then supplied causes the mirror to clear again within a short period of time. In motor vehicles a PTC-resistance element may be used which has a resistance in the range from approximately 3 to 6 Ohm at temperatures below about 100"C and a resistance of about 100 Ohm at higher temperatures.

The switching temperature of the PTC-resistor is about 100 C.

Although the above-described embodiment relates to a heatable mirror having a single PTCresistor and a single heat distribution plate, other embodiments may of course include a plurality of such PTC-resistors mounted on one or more said heat distribution plates, the number of each largely depending on the area of the mirror concerned.

Claims

1. A heatable mirror, such as a rear view mirror for motor vehicles, the glass of which is heated by a heating element comprising a PTC resistance body, the back of the mirror glass being provided with a heat distribution plate which is connected to the PTC resistance body in at least one primary contact area which is spaced from the mirror glass, and the plate is connected in a heat-conducting manner to the mirror glass over a secondary contact area which is connected to the primary contact area by an intermediate connecting portion of the plate, characterized in that the PTC resistance body is arranged in the space between the mirror glass and the primary contact area of the heat distribution plate and that there is an air gap between the PTC resistance body and the mirror glass.

2. A mirror as claimed in Claim 1, in which the heat distribution plate is provided with at least one dish-shaped recess, characterized in that the heat distribution plate has a feed-through aperture in the intermediate connecting member.

3. A mirror as claimed in Claim 1, characterized in that the heat distribution plate has a ridge which extends over at least part of the secondary contact area to form a channel between the glass and the plate through which an electric cable can be led to the PTC resistance body.

4. A mirror as claimed in Claim 3, characterized in that the channel extends to the outermost edge of the plate.

5. A mirror as claimed in Claim 3, characterized in that the channel extends part way to the outermost edge of the plate, the plate being provided with an aperture through which the electric cable can be led in to the channel.

6. A mirror as claimed in any previous Claim having a plurality of PTC resistance bodies and a corresponding plurality of said primary contact areas arranged on one or more heat distribution plates.

7. A heatable mirror substantially as hereinbefore described with reference to the accompanying drawings.