U.S. patent number 3,858,027 [Application Number 05/338,221] was granted by the patent office on 1974-12-31 for road vehicle rear window heater circuits.
This patent grant is currently assigned to The Lucas Electrical Company Limited. Invention is credited to Edward Graham Phillips.
United States Patent |
3,858,027 |
Phillips |
December 31, 1974 |
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.
Inventors: |
Phillips; Edward Graham (Colne,
EN) |
Assignee: |
The Lucas Electrical Company
Limited (Birmingham, EN)
|
Family
ID: |
10070338 |
Appl.
No.: |
05/338,221 |
Filed: |
March 5, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Apr 7, 1972 [GB] |
|
|
16055/72 |
|
Current U.S.
Class: |
219/203; 219/492;
219/522; 219/501; 361/196 |
Current CPC
Class: |
H05B
3/84 (20130101); H05B 1/0236 (20130101); H05B
2203/035 (20130101) |
Current International
Class: |
H05B
3/84 (20060101); H05b 001/02 () |
Field of
Search: |
;219/490,507,501,203,492,522 ;307/293 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Shaw; Clifford C.
Attorney, Agent or Firm: Holman & Stern
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.
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.
* * * * *