Abstrict
The present invention relates to controllable electric heaters,
in particular heaters employing tungsten halogen lamps and suitable
for use in cookers of the type having a ceramic hob below which
are placed the heaters. It is an object of the present invention
to provide a controllable electric heater employing two tungsten
halogen lamps and other components of an inexpensive nature and
which allows six different power settings to be obtained.
Claims
What is claimed is:
1. A controllable AC electric heating assembly comprising:
(a) an electric heating element group having a free connection
point and a common connection point;
(b) a resistor group connected at one end to said electric heating
element group common connection point and having at an opposite
end a free connection point;
(c) first and second half-wave rectifiers connected in reverse
polarity at an assembly common point, and each said rectifier having
a free connection point;
(d) a pair of terminals for connection to an external AC power
source, one said terminal being connected to said assembly common
point and the other said terminal being a free terminal; and
(e) a switch means connected to said electric heating element group
free connection point, said resistor group free connection point,
said rectifier free connection points, and said free terminal and
having:
(i) a first configuration in which said electric heating element
and said resistor group are connected in series with one of said
half-wave rectifiers so that said electric heating element group
and said resistor group are energized by half-wave rectified AC
voltage;
(ii) a second configuration in which said resistor group is connected
in series with one of said half-wave rectifiers and said electric
heating element group is connected in series with the other of said
half-wave rectifiers, so that said resistor group and said electric
heating element group are individually energized by a half-wave
rectified AC voltage; and
(iii) a third configuration in which said half-wave rectifiers
are each bypassed, and said electric heating element group and said
resistor group are connected in series so that said electric heating
element group and said resistor group are energized by a full wave
AC voltage.
2. The electric heating assembly of claim 1, wherein said witching
means is further connected to said electric heating element group
common connection point and has:
(iv) a fourth configuration in which said half-wave rectifiers
are each bypassed, and said electric heating element group and said
resistor group are connected together in parallel so that an undivided
full wave AC voltage is supplied to said electric heating element
group.
3. The electric heating assembly of claim 2, wherein said electric
heating element group includes two series-connected heating elements
having a tap point located therebetween, and said resistor group
includes two series-connected resistors having a tap point located
therebetween and said switch means is connected to said electric
heating element tap point and said resistor group tap point and
further includes:
(v) a fifth configuration in which said heating elements are connecting
in parallel and said heating elements are connected in series with
one of said half-wave rectifiers to form a first circuit branch
and in which the other of said half-wave rectifiers is connected
in series with one of said resistors to form a second circuit branch
which is connected in parallel with said first circuit branch whereby
said one resistor and said parallel combination of heating elements
are each energized by an undivided half-wave rectified AC voltage;
and
(vi) a sixth configuration in said half-wave rectifiers are bypassed
and one of said resistors and said electric heating elements are
connected in parallel and are energized by undivided full wave AC
voltage.
4. The electric heating assembly of claim 1, wherein said electric
heating element group consists of at least one tungsten-halogen
lamp.
5. The controllable electric heating assembly of claim 3, wherein
each of said heating elements is a tungsten-halogen lamp and the
resistance of each said lamp is between 1.5 and 2.5 times the resistance
of each said resistor, whereby the respective heat output values
obtained from said six configuration of said switching means are
in a geometric progression.
Description The present invention relates to controllable electric heaters,
in particular heaters employing tungsten halogen lamps and suitable
for use in cookers of the type having a ceramic hob below which
are placed the heaters.
Heaters comprising tungsten halogen lamps are very suitable for
use with ceramic hob cookers. However, for such use it is necessary
that the heat output should be controllable, either continuously,
or at a substantial number of different heating levels. To enable
this to be done it has generally been necessary either to use three
or four halogen lamps in combination with suitable switching circuits,
or to use phase controllers or to resort to the use of energy regulators.
The use of four lamps is expensive, phase control is restricted
to below 200 watts, and energy regulators discard the beneficial
visual properties of tungsten halogen heaters.
It is an object of the present invention to overcome or alleviate
the above disadvantages.
Accordingly to the invention provides an AC electric heater circuit
comprising at least one half-wave rectifier, at least one resistor
and at least one electric heating element and multiple pole switching
means connected to said resistor, half-wave rectifier and heating
element in such a manner that in use, the heat output of the heater
circuit can be varied by switching between at least the following
states:
(i) at least one electric heating element in series with at least
said resistor,
(ii) at least one electric heating element in series with at least
said half-wave rectifier and energized by half-wave rectified D.C.
(iii) at least one electric heating element energized at full power
by substantially the full A.C. voltage across the input terminals
of the heater circuit.
Preferably the circuit comprises two such electric heating elements,
wherein said electric heating elements are tungsten halogen lamps
and said multiple-pole switching means is so connected that in use,
the heat output of the heater circuit can be varied by switching
between at least the following additional states:
(iv) two tungsten-halogen lamps in series with at least said resistor,
(v) two tungsten-halogen lamps in series and energized at the free
terminals of their series combination by substantially the full
AC voltage across the input terminals of the heater circuit,
(vi) two tungsten-halogen lamps in parallel and either the individual
tungsten-halogen lamps being connected in series with respective
half-wave rectifiers or the parallel combination of tungsten halogen
lamps being connected in series with a common half-wave rectifier
whereby in either case they are energized by half-wave rectified
D.C.,
(vii) whereby a total of at least six different power settings
can be obtained.
In a switched heating element for a cooker it is desirable that
the different power levels should be in approximate geometric progression.
Accordingly in a preferred embodiment the resistance of each tungsten-halogen
lamp at its nominal rated power is between 1.5 x and 2.5 x the resistance
of each resistor, which enables the required approximately geometric
progression to be obtained.
Furthermore it is also desirable that at the lowest setting, corresponding
to the power level required for proper simmering, the radiation
emitted by the lamps should be visible through the ceramic cooker
top and at the highest setting the power of the heater unit is the
maximum which can be safely transmitted by the cooker top material.
As will be seen below, with a choice of preferred component values
for the circuit components of the present invention these further
objects can also be attained.
In preferred embodiments, switching between certain circuit configurations
is accomplished by making and breaking contact between the opposite
poles of respective diodes, the other poles of the diodes being
connected together so that when contact is made, they form a reverse
parallel combination with substantially no impedance, and, when
contact is broken they are independently in series with separate
branches of the circuit.
The invention, which is defined in the claims appended hereto,
will be further explained by way of example only with reference
to the accompanying drawings in which:
FIG. 1 is a circuit diagram of a controllable electric heater according
to the invention suitable for use in a ceramic hob cooker;
FIG. 1A is a variant of the circuit of FIG. 1;
FIGS. 2-1 to 2-6 are simplified equivalent circuits of the arrangement
of FIG. 1 and FIG. 1a at various switch settings,
FIGS. 3, 4, 4A, 5 and 6 show modifications of the circuit of FIG.
1;
FIG. 6-1 is a simplified equivalent circuit of one setting of the
circuit of FIG. 6; and
FIG. 7 is another simplified version of FIG. 6
The controllable electric heating of this invention is described
with reference to FIG. 1. The controllable electric heater has an
electric heating element group that includes two series connected
heating elements 3 and 4, typically in the form of halogen lamps.
A resistor group, comprising a pair of series connected resistors
5 and 6, si connected at one end to the electric heating element
group at a heating element common point 20 so that heating element
4 and resistor 5 are connected together. A pair of half-wave rectifiers,
represented by diodes 7 and 8, are connected together in reverse
polarity to a heater common point 22. An energizing voltage is applied
from an external power source (not illustrated) to the electric
heater through a pair of supply terminals 1 and 2. Supply terminal
2 is a common terminal connected to the heater common point 22.
A control switch assembly, comprising eight sets of switch contacts,
A-H, controls the radiated heat by regulating the voltage applied
to the heating elements 3 and 4. Switch contact A connects supply
terminal 1 to a heating element free point 24 which is adjacent
the free end of heating element 3. Switch contact B connects supply
terminal 1 to the heating element common point 20. Switch contact
C connects a resistor group free point 26, which is adjacent the
free end of resistor 6, to the heater common point 22. Switch contact
D connects the resistor group free point 26 to a free point 28 adjacent
the free end of half-wave rectifier 8. Switch contact E connects
free point 28 of diode 8 to a resistor group tap point 30 between
resistors 5 and 6. Switch contact F connects a heating element group
tap point 32 between heating elements 3 and 4 to a free point 34
adjacent the free end of half-wave rectifier 7. Switch contact G
connects heating element free point 24 to free point 34 of half-wave
rectifier 7. Switch contact H connects free point 34 of half-wave
rectifier 7 to the heater common point 22. In FIG. 1, conductors
are shown separating the actual points of points of connection to
the heat common point 22 and the free point 34 of diode 7. Electrically
however, heater common point 22 and free point 34 of diode 7 can
each be considered a single point and are so identified. As described
hereinafter, the control switch assembly selectively makes and breaks
the connections established by switches contacts A-H to regulate
the voltage applied to the heating element group in order to regulate
the production of heat.
The lamps 3, 4 and the ohmic elements 5 and 6 are connected end
to end, and by closing the switch contacts A and D the four elements
may be connected in series with each other and with the half-wave
rectifier 8 across the input terminals 1 and 2. This constitutes
the lowest switch setting and is shown in FIG. 2-1. At this setting
the filaments of the lamps 3 and 4 glow sufficiently brightly to
be visible through the ceramic hob of the cooker.
For the next higher setting (FIG. 2-2) is obtained by closing contact
C and opening contact D, which effectively shorts out the half wave
rectifier 8 thereby approximately doubling the total power delivered
by the heater.
The next higher setting contact A and C are opened, and contacts
B,D and G are closed, and this establishes connections providing
two circuit branches, one through the lamps 3 and 4 and the rectifier
7, and the other through the resistors 5 and 6 and the rectifier
8. This, again, approximately doubles the power output. The equivalent
circuit for this setting is shown in FIG. 2-3.
At the next higher setting switch contacts C and H are also closed,
short-circuiting the two half-wave rectifiers 7 and 8 and thereby
applying full-wave power to the two circuit branches further increasing
the power output from the heater. The equivalent circuit at this
stage is shown in FIG. 2 - 4.
At the fifth switch setting, contacts C,D,G and H are opened, and
A,B,E and F are closed, and this establishes a circuit having two
main branches, one of which comprises the lamps 3 and 4 in parallel
with each other and in series with the half-wave rectifier 7, and
the other comprises the resistor 5 in series with the half-wave
rectifier 8. The equivalent circuit is shown in FIG. 2 - 5.
Finally, to obtain the highest power setting, switch contacts C,D
and H are additionally closed, effectively short-circuiting the
two rectifiers 7 and 8 and again doubling the heat output from the
heater.
The following table shows the contact settings in the above six
positions of the switch. In the table zeros represent open contacts,
and ones represent closed contacts. The contacts are all associated
with a single rotary gauged switch, which enables the heat output
to be raised sequentially by rotating a suitable control knob. |