Road Vehicle Rear Window Heater Circuits

Abstract

A road vehicle rear window heater circuit is operated by a manually operable switch urged to an open position. Closure of the switch charges a capacitor, which controls a bi-stable trigger circuit determining the length of time for which the heater remains on.

Description

This invention relates to road vehicle rear window heater circuits.

A circuit according to the invention includes a manually operable switch which is resiliently urged to an open position, a capacitor which is charged upon momentary closure of the switch, and a bistable trigger circuit which when the capacitor is charged is driven from a first state to a second state in which it causes the heater to be energised, the trigger circuit returning to said first state and de-energising the heater when the capacitor has discharged to a predetermined level.

The use of a trigger circuit has the advantage that the periods of which the heater is energised can be accurately determined, because the trigger circuit reverts to its first state quickly when the capacitor has discharged to the appropriate level. Preferably, the trigger circuit is a Schmitt trigger circuit.

The accompanying drawing is a circuit diagram illustrating one example of the invention.

Referring to the drawing, the battery 11 of a road vehicle has its negative terminal connected to a supply line 12 which is earthed, and its positive terminal connected through the vehicle ignition switch 13 and a diode 14 in series to a supply line 15. The junction of the ignition switch 13 and diode 14 is connected through a manually operable switch 16 and a capacitor 17 in series to the line 12, and the junction of the switch 16 and capacitor 17 is connected through a resistor 18 which may be variable or pre-set to the base of an n-p-n transistor 19.

The transistor 19 has its collector connected to the line 15 through a resistor 21, and the collector is further connected through a resistor 22 to the base of an n-p-n transistor 23 which has its base connected through a resistor 24 to the line 12, and its collector connected through a resistor 25 to the line 15. The emitters of the transistors 19 and 23 are connected through a resistor 26 to the line 12.

The collector of the transistor 23 is connected through a resistor 27 to the base of an n-p-n transistor 28 having its emitter connected to the line 12 and its collector connected to the line 15 through the winding 29 of a relay, the winding 29 being bridged by a diode 31 and serving when energised to close a normally open contact 32 which is connected to the positive battery terminal through the ignition switch 13, and to the line 12 through the rear window heater 33 of the rear window 35 of the vehicle. The heater 33 is bridged by a warning lamp 34 which conveniently is combined with the switch 16.

The switch 16 is resiliently urged to an open position, but on closing the switch momentarily, the capacitor 17 charges instantaneously to the supply voltage and the Schmitt trigger circuit constituted by the transistors 19 and 23 and their associated components assumes a state in which the transistor 19 is on and the transistor 23 is off. Current flows through the resistors 25 and 27 to turn on the transistor 28, energising the winding 29 to close the contact 32 and energise the heater 33. The lamp 34 is illuminated to indicate to the driver that the heater 33 is in operation.

The capacitor 17 discharges through the resistor 18, the base-emitter of the transistor 19 and the resistor 26, and the resistor 18 is adjusted or pre-set to provide a substantial delay, for example five minutes. However, when the capacitor has discharged to a predetermined level, the Schmitt trigger circuit switches rapidly to its alternative state in which the transistor 19 is off and the transistor 23 is on. As soon as this happens, current flowing through the resistor 25 is diverted by way of the transistor 23, the resistor 26 having a load value relative to the resistor 25, so that there is a substantial voltage swing at the collector of the transistor 23. Conduction of the transistor 23 causes the transistor 28 to turn off, de-energising the winding 29 so that the heater 33 is switched off and the lamp 34 is extinguished.

The purpose of the diode 14 is to protect the transistors from damage resulting from reverse polarity transients caused by inductive ignition loads.

Claims

I claim:

1. A road vehicle rear window heater circuit, which comprises:

a rear window of a road vehicle having rear window heating means associated therewith;

a battery connected to said rear window heating means for supplying electrical energy thereto;

a first switch connected in series between said battery and said rear window heating means; and

timing circuit means connected between said battery and said rear window heating means for selectively actuating said rear window heating means for a predetermined time interval after the closure of said first switch, said timing circuit means comprising:

a second switch which is resiliently urged to an open position;

a capacitor connected to said battery through said second switch and said first switch, said capacitor being charged upon momentary closure of said second switch;

a bistable trigger circuit comprising first and second transistors connected in a Schmitt trigger configuration, the base of said first transistor connected to receive the charge from said capacitor to initially be rendered conductive while said second transistor is rendered non-conductive;

a third transistor connected across said second transistor, said third transistor being initially driven into conduction by a connection through said first switch to said battery;

a relay having a coil, and a pair of contacts connected between said first switch and said heating means, said coil and contacts being actuated by the output from said third transistor for energizing said rear window heating means;

said first transistor being rendered non-conductive after the discharge of said capacitor to thereby render said second transistor conductive and said third transistor non-conductive to thereby deactivate said relay and disconnect said rear window heating means from said battery.