Note: Descriptions are shown in the official language in which they were submitted.
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THERMOSTAT CONTROL SYSTEM FOR AN E~ECTRIC KETTLE
This invention relates to an electric kettle
for boiling water. In particular it relates to a control
circuit that turns the kettle off relative to water in
the kettle beginning to boil.
In a boil and off electric kettle it is known
to locate a heat sensor, which forms part of a control
circuit for the kettle, either in the kettle handle or
in an electrical housing adjacent the handle. In either
location, the heat sensor is positioned in close
proximity to an opening in the upper shell portion of
the kettle so as to ~detect" the presence of steam which
is generated when water in the kettle begins to boil.
When the heat sensor detects the steam, it activates the
control circuitry of the kettle which turns the kettle
off. This is usually accomplished by opening a pair of
contacts which are connected in series with the heating
element of the kettle. The water level in the kettle
generally has to be above the opening where the spout
meets the kettle shell to ensure a pressure build-up of
steam within the kettle. After the generation of steam
within the kettle ceases and the heat sensor cools to
a temperature at which it can again function, then and
only then can the kettle be reset for operation. Thus,
the term "boil and off" when used to describe an
electric kettle means an electric kettle that automati-
cally shuts itself off sometime after water contained
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in the kettle begins to boil.
A disadvantage common to most boil and off
electric kettles is the lengthy period of time that is
required to allow the heat sensor to cool to a
temperature at which it can again ~unction and thus permit
the control circuitry of the kettle to be reset. It can
be appreciated that in certain circumstances the lengthy
time delay, which may be in the order of a few minutes,
can ~e a nuisance. Ano,her disadvantage occurs when a
small amount of water is to be boiled and the water
level is below the opening where the spout meets the kettle
shell. When this occurs the steam generated when the
water boils escapes out of the spout resulting in long
boiling times before the kettle turns off.
lS It is therefore an object of the present
invention to provide in a boil and off electric kettle
a control circuit that requires a relatively short
period of time for the heat sensor to cool to a temper-
ature at which it can again function permitting the
control circuit of the kettle to ~e reset.
It is another object of the present invention
- to provide a boil and off electric kettle that can be
filled through its spout and not have to rely on the
pressure build-up of steam in the kettle to turn it off.
Briefly, the present invention provides in an
electric ketcle a control circuit that includes a main
thermoresponsive switching device connected in series
with a main resistance heating means. The main switching
device senses the temperature of the main heating means,
which heats water contained in the kettle. Connected
to the main device is a bias heating means which raises
the temperature of the main switching device above that
normally attained by it when the main heating alone is on.
An auxiliary thermoresponsive switching device electrically
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connected to the bias heating means controls the passase
of current therethrough. During the heating of water in
the kettle the auxiliary switching device precludes current
from flowing through the bias heating means. The auxiliary
device indirectly senses the temperature of the water by
sensing the temperature of a shell portion of the kettle.
As the water is heated it reaches a predetermined temper-
ature value below the boiling temperature of water which
causes the auxiliary switching device to effectively
energize the bias heating means. The temperature of the
main switching device then rapidly rises to a pre-
determined temperature value above that sensed when boiling
water.At this latter predetermined temperature value the
main switching device open-circuits. When the main device
open-circuits, the heating means causes to heat the water
in the kettle. The two predetermined temperature values and
increased rate of temperature rise of the main switching
device, due to the bias heating means being in circuit
therewith, are chosen such that during the time elapsed
between the auxiliary device permitting current flow
through the bias heating means and the main device
becoming open-circuited, the water in the kettle will
come to the boil.
One advantage in the control circuit lies in
the ability of the main switching device to rapidly cool
down to a temperature at which it can again function. This
is possible because the predetermined temperature value
at which the main switching device open-circuits is above
the boiling temperature of the water and above the
temperature of the heating means. After the main heating
means is turned off, the water in the kettle rapidly cools
the main heating means which in turn cools the main
switching device to a reset condition. Another advantage is
the kettle can be spout filled because the control circuit
is not dependent upon a pressure build-up of steam.
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In one embodiment it is envisaged that the
bias heating means and the auxiliary switching device are
electrically connected in mutual series relation. The series
arrangement is electrically connected in parallel with the
main switching device and the main heating means. In this
embodiment the auxiliary switching device will normally
be open-circuited and will close when it senses the lower
one of the predetermined temperature values. In another
embodimemt, the preferred one, it is envisaged that the
bias heating means and the auxiliary switching device are
electrically connected in mutual parallel arrangement. ~his
parallel arrangement is electrically connected in series
with the main switching device. In this preferred embod-
iment the auxiliary device is normally close-circuited and
will become open-circuited when it senses the lower one of
the predetermined temperature values.
In accordance with a broad aspect of the preser..
invention there is provided in a boil and off electric
kettle a control circuit therefor comprising: a resistance
main heating means; a main thermoresponsive switching
device connected in heat transfer relation with and
electrically in series with the main heating means, the main
switching device switching from a closed condition permitting
electrical current to flow through the heating means to an
open condition precluding current from flowing through the
main heating means when the main switching device senses a
first predetermined value; a bias resistance heating means
connected in electrical and heat transfer relation with
the main switching device for raising the temperature of the
main device to the first predetermined value when the bias
heating means is effectively energized; and, an auxiliary
thermoresponsive switching device connected in electrical
relation with the main device and the bias heating means, the
auxiliary device being connected in heat transfer relation
with a shell portion of the kettle and switching from a first
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condition precluding effective energization of the bias
heating means to a second condition providing effective
energization of the bias heating means when the
auxiliary switching device senses a second predetermined
temperature value from the shell portion.
For a better understanding of the nature and
objects of the present invention reference may be had by
way of example, to the accompanying diagrammatic drawings
in which:
Figure 1 is a plan view of a boil and off
electric kettle as disclosed herein;
Figure 2 is a bottom view taken from direction
2-2 of Figure 1 showing the preferred embodiment of the
control circuit of the kettle;
Figure 3 is a schematic drawing of the preferred
electrical circuit shown in Figure 2;
Figure 4 is a graph showing the temperature
vs. time relations of the auxiliary and main thermostats
in relation to the temperature rise of water contained
in the kettle; and
Figure 5 is a schematic drawing of an alternate
control circuit for the invention.
- Referring to Figure 1 there is shown a boil
and off electric kettle 10 having a shell portion 12,
a combined nandle and spout 14 and a base cover 16.
The kettle 10 is provided with an outlet 18 to which an
electric plug ~not shown) may be connected.
Referring now to Figures 2 and 3, outlet 18 is
electrically connected to the interior of kettle 10 by
30 line terminals 20, 24 and ground terminal 22. Ground
terminal 22 is connected by wire 22a to post 26 so as
to ground the ground terminal 22 to the inner bottom
19 of the kettle shell 12. Ground terminal 22 is employed
in the kettle 10 for use in a 240 volt system and may be
eliminated to adapt the kettle for use in a 120 volt
system. Line terminal 24 is connected by wire 24a to
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connection point F, which comprises a post 25
extending through inner bottom 19 connected to the
heating means. The heating means comprises a
sheathed heating element 28 (shown in Figure 3).
Line terminal 20 is connected by wire 20A to connection
point A.
An auxiliary thermoresponsive switching
device comprising auxiliary thermostat 30 is
electrically connected by link arms 32 and 34 between
connection points A and C. Link arm 32 is connected
to a first contact of thermostat 30 and link arm
34 is connected to a second contact of thermostat
30. Auxiliary thermostat 30 is connected in heat
transfer relation at 31 with a portion of shell 12
which is inner bottom 19. Thermostat 30 indirectly
senses the temperature of the water in the kettle
10 by sensing the temperature of the inner bottom
19 .
Also connected to connection point C is
a first input terminal 36 of a main thermoresponsive
switching device which comprises main thermostat
38. The first input terminal 36 is electrically
connected to a first contact of thermostat 38. A
bias heating means comprising a bias heater 40
forms the second input terminal for thermostat 38
and is electrically connected between the first
contact of thermostat 38 and connection point A.
The bias heater 40 comprises a metal strip element having
a tip portion welded to the first contact of thermostat
38 in heat transfer relation with thermostat 38. The
second contact of thermostat 38 is electrically connected
by output terminal 42~to connection point D. Bar 44
interconnects connections points D and E. Connection
point E comprises a post 46 extending through inner
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bottom 19 connected to heating element 28. Thus, main
thermostat 38 is electrically connected in series with
heating element 28. The main thermostat 38 senses by
conduction the temperature of heating element 28 through
a stud 47 brazed to heating element 28. Stud 47 connects
thermostat 38 in heat transfer relation with heating
element 28. Auxiliary thermostat 30 and bias heater 40
are electrically connected in series with main thermostat
38 and in parallel to one another.
In alternate embodiments the main and auxiliary
thermoresponsive switching devices may comprise devices
other than thermostats such as, for example, thermistors
or other forms of electronic devices.
A lamp 48 is shown connected by wires 50,52
between connection points A,D. When the contacts of main
thermostat 38 open, lamp 48 turns on giving a visual
indication that the kettle 10 has turned off. Lamp 48
has a high resistance in circuit therewith so as to limit
the current flowing through heating element 28 when the
contacts of main thermostat 38 open.
A manually operated reset mechanism 54 is
provided to reset the contacts of main thermostat 38 to
a closed position after the contacts have opened. Reset
mechanism 54 may be activated by depressing reset
button 54a.
Referring now to Figures 3 and 4 the operation
of the control circuit will now be described. The contacts
of auxiliary thermostat 30 are set to open when
thermostat 30 senses a predetermined tempeature value
slightly less than the boiling temperature of water.
Main thermostat 38, which senses the temperature~of heating
element 28, is set to cpen its contacts at a predetermined
temperature value above the boiling temperature of water.
When the contacts of thermostats 30,38 are closed, the
thermostats are $aid to be in their closed position and
when the contacts are open, the thermostats are said to
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be in their open position. The auxiliary thermostat 30
is chosen such that it has a thermal time constant which
allows it to closely match the rate of temperature rise
of water in the kettle. Because the auxiliary thermostat
senses the temperature of the shell and the shell
experiences heat losses, the auxiliary thermostat does
not sense the actual water temperature. The main
thermostat 38 senses the actual temperature of the
heating element 28 which is usually slightly above the
temperature of the water in the kettle. However the
temperature of the heating element 28 does not rise to
the predetermined temperature at which main thermostat
38 open-circuits. Bias heater 40, when activated, pushes
the temperature of the main thermostat 38 rapidly up to
the predetermined temperature at which thermostat 38
open-circuits.
Assuming the thermostats 30,38 are in their
closed positions as shown in Figure 3 and electrical
current is supplied to the kettle terminals 20,24, then
electrical current flows through thermostats 30, 38 and
heating element 28. Current flowing through resistance
heating element 28 generates heat which warms the
water in the kettle. Because auxiliary thermostat 30 is
in its closed position electrical current is substantially
precluded from passing through bias heater 40. Thus
the rate of temperature rise of water, the main
thermostat 38, and auxiliary thermostat 30 will
approximate that respectively shown by curves 98, 100 and
102 over time period Pl. It should be understood that
when the auxiliary thermostat 30 is in its closed position
the main thermostat 38 is precluded from rising in
temperature to its predetermined temperature at which
main thermostat 38 open-circuits. This is because main
thermostat 38 senses by conduction only the temperature
of the heating element 28 which is always slightly above
the temperature of water in the kettle. When a
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predetermined temperature Tl, which is less than the
boiling temperature of water, is sensed by auxiliary
thermostat 30, thermostat 30 switches from its closed
to its open position thereby permitting electrical
current to flow through bias heater 40. The time rate
of temperature rise of main thermostat 38 is then
increased as shown by the portion of curve 100 over
time period P2. It is to be noted that during time
period P2 water represented by curve 98 reaches its
boiling temperature Tw. The main thermostat continues
to rise rapidly in temperature until it reaches pre-
determined temperature T2 which is above the boiling
temperature Tw and the temperature of the heating
element 28. When main thermostat 38 reaches temperature
T2 it switches from its closed to its open position there-
by turning the kettle off. The temperature of main
thermostat 38 rapidly decreases as shown in curve 100
over period P4 because the thermostat 38 senses the
temperature of heating element 28 which rapidly cools to
the temperature of the water after the kettle is switched
off. Period P4 has a time duration in the order of about
10 to 15 seconds. Period P3, the time duration in
which the kettle remains on after the water boils, has
a time duration in the order of 10 to 90 seconds.
The duration of period P3 is dependent upon the volume
of water in the kettle.
Referring now to Figure 5, an alternate
embodiment for the present invention is shown.
Auxiliary thermostat 100 is electrically connected in
series with bias heating element 102. The series
arrangement of auxiliary thermostat 100 and bias heating
element 102 is shown connected across line terminals
108,110. The series arrangement is also connected in
parallel with the series combination of main thermostat
104 and heating element 106. In Figure S, auxiliary
thermostat 100 is shown in its open position and main
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thermostat 104 in its closed position. In operation,
current passes through closed thermostat 104 and heating
element 106. When the temperature sensed by the auxiliary
thermostat 100 reaches a predetermined value, auxiliary
S thermostat 100 closes permitting current to flow through
bias heating element 102. As a result, heat is radiated
from element 102 to thermostat 104 causing the temperature
of thermostat 104 to rise rapidly to a predetermined
temperature at which temperature thermostat 104 opens,
10 precluding current flow through element 106.
It will be appreciated that alternate
embodiments will be readily apparent to a man skilled in
the art in light of the present disclosure.
Accordingly the present invention is to be limited only
15 to that which is claimed in the accompanying claims.
One particular alternate embodiment may
comprise locating the auxiliary thermostat in the
handle of the kettle so as to sense the temperature of
the top portion of the kettle shell. Such an
20 arrangement is primarily influenced by the presence of
steam in the kettle and is relatively insensitive to
changes in water volume.
