Radiant electric heaters

A radiant electric heater (10) has two coiled resistance wire heating elements (22,24) each connected to one terminal (N) of a power supply via respective rectifiers. These rectifiers are each made up of two like-poled arms of a bridge rectifier (34) connected in parallel, and they are arranged to allow current through one heating element (22) on positive-going half-cycles of the power supply waveform, and through the other heating element (24) on negative-going half-cycles. The elements are rated for continuous power dissipation under these circumstances. A PTC thermistor (36) is connected between the ends of the heating elements connected to the rectifiers. Optionally an NTC thermistor (40) can be connected in series with the PTC thermistor. Upon initial energization the PTC thermistor is a near short-circuit, so current flows through both heating elements on both polarity half-cycles, dissipating twice their rated power. The elements increase in temperature more quickly than if they were initially energized at only their rated power, so the visible response of the elements to energization is faster. Meanwhile the PTC thermistor increases in resistance owing to self-heating, thereby removing the short-circuit after a few seconds, reducing the power dissipation in the elements to its normal level and protecting them from excessive operating temperatures.

FIELD OF THE INVENTION 
This invention relates to radiant electric heaters. 
BACKGROUND OF THE INVENTION 
Radiant electric heaters are known in which an element of coiled bare 
electric resistance wire is supported on a layer of thermal insulation 
material compacted in a metal support dish. Such heaters are described, 
for example, in GB 1 580 909, and are incorporated in glass-ceramic smooth 
top cookers. Although these operate satisfactorily, a perceived 
disadvantage is that they take a relatively long time, of the order of 20 
to 30 seconds, to respond visibly to changes in temperature control 
settings, in particular when they are first energized in the cold state. 
This delay can be reduced by using a thinner wire which thus runs at a 
higher temperature; however the overall operating life of such elements 
may be reduced and the response time is still of the order of 8 to 10 
seconds. 
Another kind of radiant electric heater, described in EP 0 117 346, 
incorporates infra-red lamp heating elements having tungsten filaments in 
a fused silica envelope containing a halogen atmosphere. Such heaters have 
an almost instantaneous response, of the order of 1 second or less. 
However, because of the pronounced positive temperature coefficient of 
resistance of tungsten their cold resistance is much less than their hot 
resistance. Consequently there is a high surge current when they are first 
energized, leading to problems in conforming with electricity utility 
regulations on disturbance to electricity supplies. Furthermore, such 
heating elements are substantially more costly than bare wire elements. 
One solution that has been suggested to the problem of slow response of 
electric resistance wire heaters is to energize the wire heating element 
at a higher power than its normal operating power for a short period after 
it is first energized and until it has reached its normal operating 
temperature. However, this technique also has difficulties associated with 
it. Thus, in one implementation (GB 2 199 706), a complex and expensive 
electronic control circuit is required. In addition, it is necessary to 
ensure that if the heater is de-energized and then re-energized while it 
is still warm, the period of higher-power operation is shorter than if the 
element is completely cold. Otherwise the element will be operated at 
excessive power while hot and will overheat, thereby reducing its 
operating life. This is particularly important in the case of heaters 
controlled by cyclic energy regulators, in which the energization of the 
heater is repeatedly interrupted to provide an adjustable average level of 
energization. 
It is an object of this invention to provide a radiant electric heater with 
a relatively fast response, of the order of about 5 seconds or less, which 
alleviates some of these problems. 
SUMMARY OF THE INVENTION 
According to one aspect of this invention there is provided a radiant 
electric heater comprising first and second resistive heating elements 
arranged to be coupled to one terminal of an electric supply via 
respective, oppositely-poled rectifiers, and a positive temperature 
coefficient thermistor coupled between the ends of the heating elements 
connected to the respective rectifiers. 
Preferably the elements have approximately equal resistances, in order to 
minimize any d.c. component in the current drawn from the power supply. 
A negative temperature coefficient thermistor may be connected in series 
with said positive temperature coefficient thermistor, in order to limit 
any initial current surge when the heater is energized. 
The rectifiers can conveniently each comprise two like-poled arms of a 
bridge rectifier connected in parallel. This simplifies mounting, 
connection and insulation, and may limit cost. 
The rectifiers and thermistor may be mounted in the vicinity of a control 
device for regulating the power dissipated by the heater, such as a cyclic 
energy regulator. This simplifies their mounting and wiring, avoids 
exposing the rectifiers and thermistor to temperatures above their 
operating limits and also provides an appropriate thermal environment for 
correct operation of the thermistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, a radiant electric heater 10 has a container in 
the form of a metal dish 12 with an upstanding rim 14 and containing a 
layer of electrical and thermal insulating material 16. This material is 
for example a microporous insulation which comprises a highly-dispersed 
silica powder, such as silica aerogel or pyrolytic (fumed) silica, mixed 
with ceramic fibre reinforcement, titanium dioxide opacifier and a small 
quantity of alumina powder to resist shrinkage, and which is compressed 
into the dish 12. A ring-shaped wall 18 of ceramic fibre extends around 
the inside of the rim 14 of the dish 12, on top of the layer 16 and 
protruding slightly above the edge of the rim 14. When installed in a 
glass ceramic top cooker the wall 18 is pressed against the underside of a 
glass ceramic cooking surface, shown in dashed outline at 20 in FIG. 2, 
the heater 10 being held in position by a spring or other mounting device 
(not shown). Prior to installation the wall 18 is retained in position by 
staples extending into the layer 16. 
The layer 16 supports two coiled bare resistance wire heating elements 22 
and 24 which are laid out in inter-penetrating serpentine configurations 
of generally concentric circles. Such an arrangement provides an 
aesthetically pleasing appearance, with each element seeming to extend 
over most of the heated area, whilst at the same time accommodating the 
required lengths of wire and promoting uniform heat distribution. The 
coiled elements 22 and 24 are secured to the layer 16 by, for example, 
staples held by friction in the insulating material of the layer 16, or by 
gluing to the layer 16 or to stakes inserted therein. The ends of the wire 
heating elements 22 and 24 are coupled to an electrical connector block 26 
mounted at the edge of the dish 12, one end of each element being coupled 
to a common connector and the other ends being coupled to individual 
connectors. 
As is customary with heaters for glass ceramic top cookers, a temperature 
sensitive rod limiter 28 is provided with its probe 30 extending across 
the heater 10 above the elements 22 and 24. This probe typically comprises 
a fused silica tube containing a metal rod. A snap-action switch 32 
controlled by the probe 30 is connected in series with the elements 22 and 
24 at their common connection, as is also shown in FIG. 3, and is itself 
coupled at terminal L to the live line of a power supply. 
The remaining ends of the elements 22 and 24 are coupled via the connector 
26 to the negative and positive terminals respectively of a bridge 
rectifier 34 (though this polarity may be reversed). This rectifier is 
rated in accordance with the supply voltage and power rating of the 
heating elements 22 and 24; for example at 600 V, 17 A, assuming the 
elements 22 and 24 are rated for a continuous power dissipation of 850 W 
each on a 240 V supply. The a.c. terminals of the rectifier 34 are 
connected together, and via terminal N to the neutral line of the power 
supply. 
A positive temperature coefficient (PTC) thermistor 36, rated at 265 V, 20 
A maximum, is connected between the ends of the heating elements 22 and 24 
which are coupled to the bridge rectifier 34. This thermistor, which is 
typically made of barium titanate, has a resistance/temperature 
characteristic as shown in FIG. 4. Suitable thermistors are available for 
example from Siemens of West Germany. 
The power supply via the terminals L and N is controlled by the user with a 
conventional control device 38, such as a cyclic energy regulator or 
multi-position switch (shown schematically in FIG. 3). such devices are 
normally mounted in a control box adjacent the glass ceramic cooking 
surface, and the rectifier 34 and thermistor 36 can conveniently be 
located in the same box. In this way the maximum temperature specification 
of the rectifier and thermistor can be respected, and the thermistor is 
kept in an environment which permits it to heat up and cool down as 
necessary. 
When the heater 10 is energized in the cold condition, the thermistor 36 is 
in its low resistance state and thus virtually short-circuits together the 
ends of the elements 22 and 24 coupled to the bridge rectifier 34. 
Consequently electric current from the a.c. supply can flow through both 
elements during half-cycles of either polarity. The heating elements are 
rated so that they are temporarily over-driven in this state, resulting in 
a rapid temperature rise in response to the commencement of energization. 
Consequently the element becomes visibly incandescent more quickly than if 
it were energized at its rated power level. 
However, the current flowing through the thermistor causes it to be 
self-heated, resulting in an increase in its resistance, effectively 
removing the short-circuit between the heating elements 22 and 24 after a 
few seconds (typically 4 to 5 seconds). This leaves these elements 
connected in series with a respective half of the bridge rectifier 34. As 
a result, each heatinq element now passes current on only the 
positive-going or negative-going half-cycles respectively, thereby halving 
the power dissipated in it. The elements are designed to dissipate their 
continuous rated power in this mode. Because current is still drawn from 
the supply on each half-cycle, there is little or no direct current 
component in this current; the resistances of the two elements 22 and 24 
are preferably matched as closely as possible to minimize any such d.c. 
component. 
When the heater 10 is de-energized, the thermistor 36 will retain heat for 
a short period of time. Thus, if the heater 10 is re-energized while the 
heating elements 22 and 24 are still warm (so the time to reach 
incandescent temperature is shorter), the thermistor 36 will reach its 
high temperature state more quickly, thereby protecting the elements 22 
and 24 against operation at excessively high temperatures. 
The matching between the time taken for the heating elements 22 and 24 to 
reach incandescence and the change in state of the thermistor 36 from low 
resistance to high resistance can be adjusted if necessary by adding 
thermistors in parallel with the thermistor 36. However, for large-scale 
production it is envisaged that a thermistor having appropriate 
characteristics for use with a specific heater would be procured. 
FIG. 5 shows two modifications to the circuit of FIG. 3, which may be used 
separately or together. A negative temperature coefficient (NTC) 
thermistor 40 is connected in series with the PTC thermistor 36 between 
the heating elements 22 and 24. This NTC thermistor has characteristics 
chosen so that it heats up, and thus drops to a very low resistance, in a 
period of the order of a second. This has the advantage of reducing any 
initial current surge that may otherwise occur when the elements 22 and 24 
are completely cold. Consequently improved conformance with power supply 
disturbance regulations can be provided. 
As also shown in FIG. 5, the bridge rectifier 34 may be replaced by two 
individual diode rectifiers 42 and 44, one each in series with a 
respective heating element 22 and 24 and arranged with opposite poles 
connected towards the live terminal L, so as to pass a.c. half-cycles of 
opposite polarity. It can be seen that the bridge rectifier 34 in FIG. 3 
is connected so that it has two like-poled arms connected in parallel on 
each side, thereby producing the same electrical circuit action as the 
individual rectifiers 42 and 44 in FIG. 5. The bridge rectifier 34 has the 
advantage that its use can simplify mounting, insulation and connection of 
the thermistor and the rectifying components in the circuit.