GB2039173A

GB2039173A - Switching device for an electrical rear window heater of a motor vehicle

Info

Publication number: GB2039173A
Application number: GB7940278A
Authority: GB
Original assignee: Bayerische Motoren Werke AG
Current assignee: Bayerische Motoren Werke AG
Priority date: 1978-11-23
Filing date: 1979-11-21
Publication date: 1980-07-30
Also published as: DE2850730C2; DE2850730A1; FR2442152A2; GB2039173B; IT1126339B; IT7927422A0; FR2442152B2

Abstract

A switching device for a motor vehicle automatically switches the elements 1, 2 of an electric rear window heater in dependence upon the setting of the vehicle heating or air-conditioning system, whereby the heater develops an output of high power above a given setting of the system and one of low power below this setting. The latter condition causes switches 10, 10' to be respectively closed and opened to connect elements 1, 2 in series. When above the setting both switches 10, 10' are closed and relay 6 is energised to change over contacts 4, 5 and so connect elements 1, 2 in parallel. During this condition a bimetal switch 8 periodically opens and closes to cycle the elements 1, 2 between high and low power. Switches 10, 10' are operated in dependence on the air volume adjuster of the system (e.g. duct apertures and fan speed) and are both open to de-energise elements 1, 2 when the setting is below a lower value. <IMAGE>

Description

SPECIFICATION Switching device for an electrical rear window heater of a motor vehicle This invention relates to a switching device for a motor vehicle which automatically switches an electric rear window heater in dependence upon the setting of a heating or air-conditioning system.

In our co-pending Patent Application No.

49855/77 (Serial No. ), filed on 30th November 1977, there is described a switching arrangement which makes a separate manual actuation of the rear window heater unnecessary, so as to stop misting-up of the rear window automatically as far as possible.

In a further development of this idea the present invention aims to avoid misting-up of the rear window even under extremely adverse weather conditions.

With this aim in view, the present invention is directed to a switching device as set out in the opening paragraph of the present Specification, in which the rear window heater receives an output of high power above a given setting of the heating and air-conditioning system and one of low power below this setting.

The present invention represents an additional correspondence between the power output of the heating or air-conditioning system and the power output of the rear window heater. Under extremely adverse weather conditions the heating or air-conditioning system operates at high power and, at the same time, a generally severe misting-up of the rear window is particularly likely. Thus, the automatic switching-on of the rear window heater to receive high power keeps the rear window free from misting-up even under these conditions. Below the threshold setting of the heating or air-conditioning system at which this switching occurs, the switching of the rear window heater to low power is generally adequate to prevent misting-up of the rear window.Thus, traffic safety is increased both by taking strain off the vehicle user, since he has fewer manual switches to operate, and by the reliable prevention of misting-up of the rear window under all weather conditions.

In order to keep the demand on the vehicle electrical grid system by the rear window heater relatively small, it is preferable to arrange for the rear window heater to be switched to low power after a given time following the switching on to high power.

Advantageously, provision is made for switching the heater back to high power some time after the switching to low power. This ensures a renewed heating up of the rear window heater at high power if operation of the heating or air-conditioning system at high power continues, with an associated continuing high probability of severe misting-up of the rear window.

A bimetallic switch is a particularly simple and reliable switch for effecting this switching to low power and back to high power.

A rear window heater construction particularly simple in terms of structure and switching is one comprising two or more heated parts connected in parallel for high power operation and in series for low power operation.

An example of a switching device for a rear window heater of a motor vehicle in accordance with the present invention is illustrated in the accompanying drawing, which is a circuit diagram of the switching device, for effecting the switching of a rear window heater in dependence upon the operation of a heating or air-conditioning system.

An electric heater for a rear window of a motor vehicle (not shown) comprises two bands 1 and 2 of heating filaments, for heating respective areas of the rear window, with an equal number of heating filaments 1' and 2', of which the first heating band 1 is, on one side, always connected to earth and, on the other side, via a lead 3, to the second heating band 2. Opposite ends of this second heating band 2 are connected respectively to switching contacts 4 and 5 of a switching relay 6. In an inoperative position the switching contacts 4 and 5 are connected to a lead 7 and to a terminal having no potential applied to it, respectively, while in an operative position they are connected to earth and to the lead 7 respectively.

The excitation coil 6' of the switching relay 6 together with a bimetallic switch 8 is located in a lead 7' parallel to the lead 7. Leads 7 and 7' are connected to a lead 9 which is connected to an ignition switch (not illustrated) at 15 and, therefore, when the ignition switch is closed, to the positive pole (+) of the vehicle grid system. The parallel leads 7 and 7' contain respective switches 10 and 1 0' which are operated in dependence upon an air volume adjuster, for example in the form of a sliding-member and guide track arrangement, which regulates air duct apertures and a fan motor in a heating or airconditioning system. The switches 10 and 10' are closed when the fan motor has a normal and a high power input respectively, and the fan is operating at a normal and a high delivery rate of, for example, 20% and 80% of its maximum, respectively.

The lead 9 contains a switch 11 which is operated in dependence upon an air distribution adjustment member for regulating various air exits (not shown) when the windscreen is charged by air. The latter regulation may also be effected by means of a sliding member and guide track arrangement. Where applicable, the lead also contains a switch 12 which is closed when a temperature selector (not shown) is moved beyond a central adjust ment.

If the delivery rate of the fan is less than 20% of its maximum value, being the threshold value for closure of the switch 10, then both leads 7 and 7' are open, and the heating bands 1 and 2 are switched off.

If the delivery rate of the fan is increased to a value lying somewhere between 20% and 80% of the maximum value, being the threshold values for closure of the switches 10 and 10' respectively, then the switch 10 is closed while the switch 10' remains open. Provided air is being directed at the windscreen, i.e. the switch 11 is closed, and provided the temperature selector is between its central and maximum positions, i.e. the switch 12 is closed, then current passes through the heating bands 1 and 2 in series via the lead 7 and the switching contact 4. With conditions calling for this average setting of the heating or airconditioning system obtained by the adjustment of the air volume adjuster, the air distribution adjustment member and the temperature selector, the rear window would very probably mist up were it not heated.

If the high power operation of the heating or air-conditioning system persists for some time, then the bimetallic switch 8 interrupts the lead 7'. The excitation coil 6' is therby deenergised, and by means of the switching contacts 4 and 5 causes the heating bands 1 and 2 to be reconnected in series. If the heating or air-conditioning system continues to operate at high power, then after a certain cooling period has elapsed the bimetallic switch 8 again closes the lead 7', thus reconnecting the heating bands in parallel for a certain period of time.

In the case, for example, of an automatically controlled air-conditioning system, the switches 10 and 10' in the leads 7 and 7' may be regulated by a servomotor for a temperature mixing flap. The lead 9 may then be permanently closed. This is an alternative to coupling the switches 10, 10', 11 and 12 regulating the parallel and series connection of the heating bands 1 and 2 to an air volume adjuster, an air distribution adjustment member and a temperature selector. With this alternative mode of operation the switch 10' is closed when the heating or the air-conditioning system uses more than a given high heating power and the switch 10 is opened when it uses less than a given low heating power. The switches 10 and 10' can again be actuated by a sliding-member regulating mechanism coupled with the servomotor.

With at least one set programme (for example with an air-conditioning compressor switched off) the switch 10 may be short-circuited.

Under these circumstances both heating bands are connected in series over the airconditioning system range from zero power up to the high heating power setting.

Instead of the illustrated embodiment with two heating bands, the rear window heater can consist of at least three heating bands. To give high and low heat these are partly or wholly switched in parallel and in series respectively. Making the assumption that with high and low power heating all the heating filaments of the heating bands are switched on to give the same respective power output, and that the electrical resistance R of the heating filaments is the same, then the total heating power L is given by

with V the operating voltage, p the number of series-connected bands of parallel heating filaments and nthe equal number of heating filaments in each band. This heating power L is distributed uniformly over the heating filaments. For a given high heating power L, this enables the low heating power to be adjusted in a pre-determined manner, for example, by a corresponding allocation of the heating filaments to different heating bands.

Claims

1. A switching device for a motor vehicle which automatically switches an electric rear window heater in dependence upon the setting of a heating or air-conditioning system, in which the rear window heater receives an output of high power above a given setting of the heating and air-conditioning system and one of low power below this setting.

2. A switching device according to claim 1, in which the rear window heater, when receiving high power, is switched to low power after a given period of time.

3. A switching device according to claim 2, in which the rear window heater is switched back to high power some time after being switched to low power.

4. A switching device according to claim 2 or 3, in which a bimetallic switch is arranged to perform the switching between high and low power.

5. A switching device according to any preceding claim, in which the rear window heater comprises two or more parts, and in which the two parts are connected in parallel mode for receiving high power and in series mode for receiving low power.

6. A switching device for a motor vehicle, which automatically switches an electric rear window heater in dependence upon the setting of a heating or air-conditioning system, substantially as described herein with reference to the accompanying drawing.