WO2010131050A1

WO2010131050A1 - Toasters

Info

Publication number: WO2010131050A1
Application number: PCT/GB2010/050791
Authority: WO
Inventors: Stephen Anthony Sayers; Mark Andrew Gagiano; Timothy David Moore
Original assignee: Sagentia Ltd
Priority date: 2009-05-15
Filing date: 2010-05-14
Publication date: 2010-11-18
Also published as: GB0908395D0; WO2010131017A3; GB2470292B; WO2010131017A2; WO2010131051A2; GB201008087D0; GB201008053D0; WO2010131051A3; GB2470292A; GB2470471A; WO2010131050A9

Abstract

This invention relates to electrical apparatus for the thermal processing of food, in particular to improvements in food toaster design. Thus we describe a food toaster having a pair of substantially transparent sides through which toasted food is visible, each said substantially transparent side comprising: a substantially transparent substrate (104) bearing an electrical heating element (102); an inner substantially transparent panel (104); and an outer substantially transparent panel (106); wherein said inner substantially transparent panel is located between said substantially transparent substrate and said outer substantially transparent panel such that, in use, toasted food is visible by a user through said outer and inner panels and said substrate.

Description

Toasters

FIELD OF THE INVENTION

This invention relates to electrical apparatus for the thermal processing of food, in particular to improvements in food toaster design.

BACKGROUND TO THE INVENTION

Background prior art can be found in the following documents:

Further background prior art can be found in WO2007/ 140652 (which describes a toaster with film-on-glass heating and groups of electrodes in pairs which divide the film into many heating zones); and also in GB2,367,482 (and US equivalent US6,543,337); AU652608B; WO2009/128673; and FR2783689.

There is, however, a need for improvements in toaster design, in particular to achieve satisfactory performance and safety.

SUMMARY OF THE INVENTION

The invention is set out in the independent claims.

According to a first aspect of the invention there is therefore provided a food toaster having a pair of substantially transparent sides through which toasted food is visible, each said substantially transparent side comprising: a substantially transparent substrate bearing an electrical heating element; an inner substantially transparent panel; and an outer substantially transparent panel; wherein said inner substantially transparent panel is located between said substantially transparent substrate and said outer substantially transparent panel such that, in use, toasted food is visible by a user through said outer and inner panels and said substrate.

Embodiments of the above described arrangement provide a thermal and safety barrier; preferably the heating element is on a face of the substrate facing towards the face of the inner panel, to restrict user access, and preferably this substrate and inner panel between them define a substantially sealed chamber to reduce heat loss. It will be appreciated that in general a majority, but not all of the side of the food toaster will be transparent-preferably most of the side is transparent to provide good visibility of the food inside, but in general at least end portions of the toaster will not be transparent.

In embodiments the transparent outer panel defines one or more external side walls of the food toaster. In some embodiments the inner and outer panels and substrate are hinged at the bottom to enable the toaster to be opened, for example for cleaning; in other embodiments the outer panel may be lifted away from the toaster so that the inner panel and substrate may then be hinged down. In embodiments the outer panel may comprise a substantially transparent sheath around the toaster, more particularly defining a pair of opposite, substantially transparent sides of the toaster and a pair of end walls of the toaster (which may also be transparent). In this way the outer panel may define an outer "cool wall" for the toaster.

In embodiments, as previously mentioned, a side wall of the toaster may be hinged down to facilitate access to the food-receiving space. In embodiments the hinging action may also enable access to a crumb-receiving space, in embodiments a chamber within a base or hinge/pivot at the bottom of the toaster. In embodiments the hinging action may also wipe crumbs from an inner base of the food-receiving space into the crumb chamber or tray, for example by arranging for an inner base of the food- receiving space to be mechanically connected to the side walls so that when the side wall is hinged downwards the inner base is moved to open an aperture into the crumb- receiving chamber, preferably a surface of the inner base being wiped or scraped as the inner base is moved, in order to wipe crumbs into the crumb chamber/tray.

In embodiments a hinging side wall may have a portion which extends at least partially over the top of the toaster, more particularly over the food receiving space. In embodiments such a toaster is configured such that, in operation, the top of the food receiving space is fully covered; this then defines a (vented) chamber.

In some preferred embodiments the hinging of the side of the toaster is inhibited by a thermal lock, in particular a mechanical thermal lock, for example operated by a bimetallic element, to restrict access to the interior of the toaster whilst it is still hot (a mechanical arrangement being preferable to an electrical arrangement as operating when unpowered). Preferred embodiments of the toaster also include a thermal warning indicator, preferably comprising a symbol on a transparent portion of the side which is substantially invisible when the substrate is not hot. Such an arrangement may be implemented using a thermochromic material, or by using a symbol etched into the transparent material to scatter light when illuminated.

Both substrates are substantially transparent, and both bear a thin film coating to provide the electrical heating element. One or both such film coatings may comprise a semi-conducting oxide, more particularly doped tin oxide, for example antimony tin oxide, fluorine-doped tin oxide or some other substantially transparent conducting doped oxide material.

In embodiments one or both of the substrates may be configured to provide a pattern of browning on the food, for example by means of a pattern of bumps or undulations.

In some preferred embodiments a base portion of the toaster, more particularly a region immediately beneath the food receiving space, is provided with a set of air vents so that, in use, air flows in a substantially single direction from the base towards an upper opening of the food-receiving space. The air vents may, in embodiments, take the form of a set of parallel slots extending transversally across the short width of the food-receiving space, spaced at intervals along the (interior) base of the food- receiving space along the long edge of the space. This facilitates a generally parallel air flow and helps to achieve a relatively even temperature horizontally.

Preferred embodiments also include a system for achieving an approximately even vertical temperature distribution in particular by configuring the substrates to achieve this. Thus in some embodiments the substrates are angled outwards towards the upper region of the space. Additionally or alternatively the film coatings on one or both substrates may be dividing into a plurality of generally horizontal bands configured to be electrically heated such that in use a lower band becomes hotter than an upper band. This may be achieved by selection of the physical dimensions of the bands, for example, by defining one or both of the width of a band and the thickness of a coating of electrical resistance material of the band, and optionally by choosing difference materials for the band (although this less preferable). It will be appreciated that although in embodiments the bands are generally horizontal, they may undulate and/or have a shape at one or both ends of a band to compensate for end-effects of the heating process, in particular to compensate for temperature variations caused by end effects at the end edges of the substrates.

To achieve even toasting it is preferable to hold the food approximately in a central position between a pair of substrates bearing heating elements. Thus preferred embodiments of the toaster incorporate a food centering mechanism, in particular with two configurations, first open configuration to receive food, and the second, closed to configuration in which portions of the mechanism close on the food one or either side, preferably substantially symmetrically. Thus in some embodiments the mechanism may comprise a wire frame defining a pair of wings biased apart and hinged at an apex (the "hinge" may simply comprise a bend in one or more wires of the frame). Then motion of the frame downwards into the toaster can be used to close the wings onto the food by means of a camming action against an internal surface of the toaster.

In other embodiments the mechanism comprises a pair of pivotally mounted supports each able to rotate about a respective axis. The supports may comprise, for example, discs or portions of discs. One or both sides of a cage or frame for the food are pivotally mounted on the supports, away from an axis about which the supports rotate. In this way rotation of the supports moves one or both of the sides; if the sides are pivotally mounted on opposite sides of the axes about which these supports rotate, rotation of the supports moves the sides towards one another in a substantially symmetric fashion. Preferable biasing means is included to bias the sides of the cage or frame closed; conveniently such biasing means may comprise a spring acting to provide a rotational force on one or both of the supports.

In embodiments the electrical heating element, in particular where it comprises a thin film, for example a layer of semiconducting material, may itself be used as a temperature sensor. In this case a signal maybe modulated onto the electrical power, typically DC or low-frequency AC, supplying the heating element, to enable this signal to be detected by demodulation. For example a higher frequency AC signal than a frequency of an AC current providing power for heating the heating element may be employed. Additionally or alternatively a region of the film maybe defined to be dedicated to temperature sensing and being provided with at least one separate electrode connection (optionally sharing one electrode connection with the heating element).

In another aspect the invention provides food toaster having a pair of transparent sides with a pair of substrates each bearing an electrical heating element, and defining a toasting space for said food between said substrates, the food toaster further comprising at least one side door, hinged at the bottom to enable the door to be opened to enable access to said toasting space, and wherein a portion of said door extends over the top of said toasting space such that said space is at least partially enclosed during operation of the food toaster.

In embodiments a pair of side doors maybe provided on opposite sides of the toaster, closing above the toaster, preferably to define a substantially completely enclosed (but vented) space in which food may be toasted. In embodiments a pair of doors may be provided on each side of the toaster. In embodiments an outer "cool" wall panel of cover may be provided around or above the top of the toasting space to further protect a user from the heat.

In a food toaster as described above an electrical power control device or switch maybe incorporated into a layer of (transparent) semiconducting material forming the heating element itself. For example by applying a gate electrode over an insulating layer on a portion of the semiconducting layer an FET (Field Effect Transistor) switch maybe fabricated. Such a device may be fabricated in a dedicated, separately defined region of the semiconducting layer or may be incorporated into the heating element, for example extending along the length of an electrode connection to the heating element.

The device may comprise a diode, in particular a diode using a metal- semiconductor junction. Alternatively p-type and n-type doped regions of the layer maybe employed to fabricate a bipolar transistor. Alternatively, as previously described, an insulated gate FET (or junction FET) maybe fabricated. In general the power controlled semiconducting device comprises an FET, bipolar transistor, IGBT, thyristor, SCR rectifier, TRIAC, or other device. Suitable materials include, but are not limited to, tin oxide, for example doped with antimony or fluorine, indium tin oxide, and silicon carbide.

Thus in a further aspect the invention provides a food toaster having a pair of transparent sides with substrates each bearing an electrical heating element, and defining a toasting space for said food between said substrates, the food toaster further comprising one or more of: i) a set of air vents in a base portion of said toaster opening into a toasting space for said food to, in operation, provide an air flow through said toasting space in which air flows in substantially a single direction from said base towards an upper, food-receiving opening of said space;

ii) a configuration wherein said substrates are configured to heat said food more at a lower region of said space that at an upper region of said space;

iii) a food centering mechanism to receive and hold food to be toasted substantially in a central position between said pair of substrates, and wherein said food centering mechanism has two configurations, a first, open configuration in which said mechanism is open to receive food, and a second, closed configuration in which positions or said mechanism close on said food from either side;

iv) a configuration wherein at least one of said substrates is substantially transparent, and a thermal warning indicator to indicate when said substrate is not at an elevated temperature, wherein said thermal warning indicator comprises a symbol displayed on said on said transparent side which is substantially invisible when said substrate is not at said elevated temperature;

v) a configuration wherein at least one of said substrates is hingeably mounted to enable the substrate to be pivoted to fluctuate access to a toasting space for said food, and a thermally operated mechanical lock to inhibit said pivoting when said substrate is at an elevated temperature; vi) a substrate bearing a layer of semiconducting material comprising said heating element, and wherein a said food toaster includes a temperature sensor comprising a portion of said layer of semiconducting material.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a schematic illustration of a vertical cross section through an embodiment of a toaster according to the invention;

Figure 2 illustrates an undesired air convection flow within a toaster;

Figure 3 illustrates a toaster configured to provide an improved convection air flow;

Figure 4 illustrates one embodiment of a centering mechanism for a toaster;

Figure 5 illustrates angled heating panels configured to provide an improved, more even vertical temperature distribution;

Figure 6 illustrates an example electrode configuration for a thin film heating element, configured to enable power to be applied to selected combinations of electrodes in order to be able to selectively control heating of different regions of a thin film coated substrate;

Figure 7 shows a temperature distribution for the a thin film heating element of Figure 6;

Figure 8 illustrates a film coating heating element divided into a plurality of horizontal bands for heating control; Figure 9 illustrates an example of a thermal lock mechanism in combination with a hinged side panel;

Figure 10 illustrates a first example of an elevated temperature warning indicator;

Figure 11 illustrates a second example of an elevated temperature warning indicator;

Figures 12a and 12b illustrate alternative configurations for a heat guard outer wall of a toaster;

Figure 13 shows opening of the side of a toaster after removal of an outer heat guard;

Figure 14 illustrates a self-cleaning device in which crumbs are wiped into a crumb chamber;

Figures 15a and 15b show, respectively, patterned transparent substrate configurations for providing a patterned browning effect;

Figures 16a-e, show, respectively, a wire cage, operation of a food sensoring mechanism employing the wire cage, and alternative configurations of the wire cage;

Figure 17 illustrates heating element power control schemes;

Figure 18 illustrates temperatures of the toaster in substantially continuous operation;

Figure 19 shows one preferred example of food (bread) placement, illustrating air flow;

Figures 20a, b illustrate use of a thermochromic ink as a temperature indicator; and

Figure 21 illustrates venting in a hingeable side panel of a toaster. DETAILED DESCRPTION OF PREFERRED EMBODIMENTS

Referring first to Figure 1, we will describe a glass toaster 100. The glass toaster makes use of clear thin film resistive coating 102 on a glass substrate 104 as the heating element to provide the mechanism for browning toast.

Benefits have been identified when using thin film technology as a heating mechanism, these include even browning and clear visibility of the toasting process. While the temperature of the coating operates above the caramelisation temperature (200⁰C), the main method of toasting is through conduction and convection rather than radiation. Hence, in order to achieve these benefits, certain performance and safety requirements should be realised:

Three pane system

A three pane / panel system is used so as to achieve cool wall touch temperatures on the outer wall. This system is designed to optimise efficiency at the same time as reducing energy loss. Figure 1 illustrates the cross section through panels. The inner glass panel 104 is coated with a clear resistive coating. The second panel 106 provides both a thermal and safety barrier. It is sealed so as to reduce heat loss and at the same time prevent user access to the live surface. The outer wall, third panel 108 provides a final thermal barrier, which induces a convection draft that draws in cool air and can keep the outer wall temperature within desired/required standards.

Convection optimisation

Given that the toasting mechanism is based upon conduction and convection, it is important that the convection in the toaster is correct. Failure to create a laminar convection flow will result in toast being burnt in the middle or sides as illustrated in Figure 2. This can be improved by the introduction of parallel flow vents 110 on the base of the toaster, as shown in Figure 3. This generates a horizontally even temperature. The vertical temperature can then be made even by adjusting the coating temperature - described later. Centring mechanism

Using thin film technology as the heating mechanism it is preferable for the bread slice to be uniformly centred to achieve an even browning. A centring mechanism 112 (Figure 4) should preferably not only centre the bread slice symmetrically between the two thin film heating panels but should preferably maintain the bread slice in a vertical plain inhibiting the bread slice from bending or twisting. Irregular spacing between the heat glass surface and the surface of the bread slice will result in uneven browning.

In order to achieve the above requirement a mechanism using a parallelogram concept has been devised. This allows a cage mechanism 114 to close onto the bread slice equally from both sides and maintains the bread slice in an almost perfect vertical plane prevent the slice from bending under its own weight. A spring mechanism 116 allows for appropriate pressure to be applied to the bread on loading. The mechanism is shown in Figure 4.

The cage itself comprises a metal frame, preferably the same size as the toaster frame with thin steel wire (piano wire) stretched between the frame. The thin wire reduces or prevents shadowing on the bread.

Even Browning

With evenly coated thin film heating panels in a vertical plane it is found that the top edge of the panel tends to get hotter than the bottom edge through convection. The result of this is that the bread slice browns quicker at the top than at the bottom and does not achieve and even toasting appearance. To alleviate this, the features described below are applied.

Angle panels

The thin film heating panels are angled so that they are further apart at the top than at the bottom, as shown in Figure 5. This compensates for the excessive heat build up at the top of the panels allowing the top of the bread slice to toast / brown at the same speed and time as the bottom creating an even toasted slice.

A disadvantage of this method is the energy efficiency is perhaps less than desired, since a lot of warm air escapes from the top of the toaster.

Thin Film Coating patterns to achieve heat distribution

Though different means of manipulation the thin film coating pattern and placement of bus bars (electrodes) 118 we are able to heat different areas of the coated glass panel. Figure 6 shows a means of directing heat into the bottom corners. This can be achieved by switching power from the two main bus bars, 1 and 4, to the other bus bars 2 and 3. This can be done in different combinations to obtain the desired result: Switching between 1 and 4, 1 and 2, 3 and 4, 2 and 4 and 1 and 3.

Figure 7 shows how by manipulating the bus bars on the edges of the panels one can increase the temperature on the sides of the panels. It was found that there was excessive heat loss on the side of the heating panels resulting in the centre of the panel having a higher temperature than that of the sides. By simply reducing the length of the bus bar and leaving the thin film coating at its original dimension hot spots are created at the edge of the bus bar from increased current density. The increase heat at the edges of the panel compensates for the losses experienced and contributes to move even supply of heat to the bread slice.

Figure 8 shows how on can manipulate the heat distribution by simply dividing the thin film resistive coating into different band widths 120 across the length of the coated panel between the two bus bars. By increasing the width of each band 120 (and/or changing the coating resistance, for example the coating thickness) one reduces the resistance for that band between the two bus bars. With a reduction in resistance there would be an associated increase in power and hence increase in heat output. Having wider bands at the bottom of the panel than at the top will produce higher heat output at the bottom of the panel than at the top. This will compensate the heat increase at the top edge of the panel from convection. In embodiment the resistivity across all bands may be the same in Ohms / square for the different resistance of each band (the benefit of this is that these bands can be coating is a single process and would not require a separate process for each individual band).

Thermal lock

In order to clean the glass panels one preferably needs to gain access to the glass surfaces that are closest to the bread slice. To make this area accessible the one side of the toaster is hinged. From a safety stand point there is a desire to prevent the user from opening / hinging the side of the toaster while the unit is still hot. An electronic solution can be created but is relatively complex and expensive, and has the ability to be over ridden when disconnected from mains power. By using a bimetallic lock 122, the toaster can be unplugged from mains electricity without overriding the locking mechanism. The mechanism incorporates bimetallic lever that actuates a (sliding) pin 124, locking or unlocking the hinging panels when hot or cold respectively by providing a stop to inhibit movement of a rotating portion 126 of a hinge 128 carrying the panel(s). Also, as illustrated, a sliding crumb tray may act as locking mechanism for the pivoting panels to be cleaned: For example, push crumb tray to release (5mm) or fully pullout for crumb tray empting.

Thermal warning

When the glass panels are hot, there is no visible indication that they are warm. Hence, it is desirable to show the consumer that they are still warm. Thermal indication can been shown in a number of ways, including: Thermochromic inks placed/printed on the sides of the toaster, however these degrade with use and UV light. An LED back light 132 warning using lensing 130, such that the light is not visible when hot - Figure 10. LED edge lighting and etched glass 134 to scatter the light - Figure 11.

Energy efficiency and safety doors

A device for toasting bread using clear glass panels with an active clear coated heating substrate has been described. Because many conventional toasters rely on the toast rising out of the top of the product, this area loses much of the rising heat and energy straight out of the device. A requirement therefore arose for a means to improve the efficiency in a toaster whilst simultaneously allowing safe and easy access to the toasting bread.

Thus in one aspect the invention provides a toaster with two glass sides and a pivoting door that opens from the side of the toaster device. In embodiments the or each access door wraps around the full side and over the top of the toaster to the mid point. On a two slice toaster there will preferably be two doors on both sides of the device, thus enabling a substantially fully enclosed environment when closed. In preferred embodiments venting is provided to allow some hot air to be evacuated at convenient points. Preferably this will allow the chamber, where the bread is toasting, to retain much of the previously lost heat and therefore add to the energy efficiency (heat loss). Once both doors are closed the opportunity to access hot and electrical parts internally may thus be substantially reduced as compared with other toasters.

In embodiments items for toasting that are normally shallower in profile, like crumpets and buns etc can be fully accessed when the door is fully opened over 90 degrees unlike many toasters that do not allow access of such food products (they reside below the access point of toast and tend to sit within the toasting unit). This also enhances the safety advantages of this door access as consumers might otherwise place something into the dangerous areas of a toaster to access such food products below the exit slot. Preferably, once opened, any power is cut to the heated glass panels and therefore providing a safer environment for the user.

In embodiments this principle can also allow the doors to automatically ('pop') open when required browning levels are reached allowing an immediate cooling and power off to the heated glass panels. A browning level may be determined by any of a range of methods including temperature, time, a combination of these, or by use of an ionising sensor (as described in GB2,367,482). When the door is fully open the user is well away from any hot zones generated from the warm air rising. In embodiments the device is not able to be activated until both doors are closed and complete the electrical circuit. Thermal sensor

Typical thin film coatings are (intrinsic) semiconductors. For example, SiC and tin oxide are both semiconductors with large band gaps (typically ~3.2eV). By doping the semiconductor can be made to be n-type or p-type. Typically, impurities make the thin film an n-type semiconductor. For example, ATO (antimony tin oxide) is an n- type semiconductor. However p-type semiconductors can also be produced.

Typical thin film materials hence have a reversible resistance - temperature characteristic and thus the heating element itself can be used as a thermal sensor to measure the temperature of the heating element or substrate. Alternatively, a separate area of thin film which does not constitute part of the heating element, but placed on the same substrate close to the element can be used to measure the temperature using a separate low voltage/low current circuit. The area can be manufactured using a masking process when the main heating element is being created.

It is preferred to detect the resistance change using a low voltage / low current so that the sensitivity is improved and the semiconductor is not saturated, hence a separate area for thermal detection is preferred rather than using the bulk element itself. Should the bulk element need to be used, then a high frequency signal multiplexed on to the DC or low frequency AC bias can be used to detect variation in resistance, without the requirement to measure high voltages or currents.

Switching mechanism

In embodiments the heating elements are required to be switched on and off. This may achieved using a manual switch, a relay or a solid state switching device, generally separated from the heating element itself. However this can add extra cost to the overall system.

Hence, it is desired to create a system by which the heating element switch is included within the heating element. Given that the thin film technology is a semiconductor, it is possible to create at the same time as the heating element different types of semiconductor switch or rectifier. In particular, one can produce a FET device by overlaying a thin insulator, such as mica or silicon dioxide on top so an area of the thin film element (typically where the current enters or leaves the element). On top of the insulator a metallisation layer is created to which a voltage can be applied to switch the element. Further devices are possible: for example, at the metal - thin film junction a Schottky diode is created, further using n-type and p-type variants of silicon carbide or tin oxide it is possible to create a rectifying diode or bipolar transistor. Because the material can withstand high temperatures, there is no need for a heat sink and any heat losses are directly used in the heater, thus increasing efficiency as well as reducing cost. Many of these devices can be transparent and hence can be used within the toaster to switch the elements to provide different heating levels and control.

Further embodiments

Referring to Figure 12a, this shows one example of an outer "cool wall" to restrict user access to hot surfaces of the toaster (in the illustration, front and back surfaces, referred to earlier as side surfaces). As illustrated, preferably the wall is removeable for cleaning; the front sealed glass panel maybe pivoted open, again for cleaning; the rear wall may be fixed upright. Figure 12b illustrates an alternative example of a cool wall which maybe released and lifted off for cleaning.

Figure 13 illustrates an example pivot mechanism in operation; Figure 14 shows details of how the pivoting action enables retained crumbs to be wiped into a crumb tray. Preferably, as illustrated, the bread is lifted above the lowest heating point on the glass. In embodiments the crumb tray may be pulled out longitudinally for emptying.

Figures 15a and 15b show examples of glass substrates with bumps or ridges/ripples, for example formed by moulding. These help the centring and provide an even offset of the bread slice from the substrate. Figure 16a shows an example cage and Figure 16b illustrates operation of the cage, closing side wings to hold bread as the apex of the cage is inserted into the base of the toaster. Figures 16c-e illustrate example forms of the cage; in embodiments the cage is coloured or coated white so that it does not appear to be a heating element thereby breaking the illusion that the glass is the toasting substrate. Either proportional or on/off power control may be employed, but on-off control is preferred as this is faster; an example element power is 110OW. the distance of the toast to the element effects toasting time; direct contact enables comparable toast time to a standard toaster at 840W. Figure 17 illustrates the effect of power control on temperature over time at different locations within an example toaster. Figure 18 shows temperatures at the top of the glass substrate (upper trace), the top of the outer panel (middle trays) and at the middle of the outer panel (lower trace). The temperature control was set for 350⁰C, regulated at the edge of the glass panel; the top edge of the outer panel stabilised at 100⁰C; increasing the distance between the outer panels substantially reduces the outer temperature.

Figure 19 illustrates that it is preferable to lift the bread slices so as not to block the air vents that promote a more uniform convection cell.

Figures 20a and 20b illustrate the use of thermochromic inks for temperature indication.

Figure 21 illustrates the use of venting to keep the outer surface of the toaster cool, although this is less preferable as it provides an opening for dirt, crumb and dust ingress.

In embodiments the elements may be switched by TRIAC switching, with differential choke filtering (for example, a differential mode filter of 18mH) for EMC capability.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS:

1. A food toaster having a pair of substantially transparent sides through which toasted food is visible, each said substantially transparent side comprising: a substantially transparent substrate bearing an electrical heating element; an inner substantially transparent panel; and an outer substantially transparent panel; wherein said inner substantially transparent panel is located between said substantially transparent substrate and said outer substantially transparent panel such that, in use, toasted food is visible by a user through said outer and inner panels and said substrate.

2. A food toaster as claimed in claim 1 wherein said heating element is disposed on a face of said substrate facing a face of said inner panel.

3. A food toaster as claimed in claim 1 or 2 wherein said substrate and said inner panel define opposite faces of a substantially sealed chamber.

4. A food toaster as claimed in claim 1, 2 or 3 wherein said outer panel defines an external side wall of said food toaster.

5. A food toaster as claimed in claim 1, 2 or 3 at least wherein said substrate, and said inner panel are hingeably mounted to enable said substrate and said inner panel to be pivoted to open said transparent side to facilitate access to a toasting space for said food.

6. A food toaster as claimed in claim 5 further comprising a thermally operated mechanical lock to inhibit said pivoting when said substrate is at an elevated temperature.

7. A food toaster as claimed in any preceding claim further comprising a thermal warning indicator to indicate when said substrate is not at an elevated temperature, wherein said thermal warning indicator comprises a symbol displayed on said transparent side which is substantially invisible when said substrate is not at said elevated temperature.

8. A food toaster as claimed in any preceding claim wherein one or both of said substantially transparent substrate and said electrical heating element is configured with bumps and/or ridges to assist in providing an even offset of the bread slice from the substrate.

9. A food toaster as claimed in any preceding claim further comprising a set of air vents in a base portion of said toaster opening at a toasting space for said food to, in operation, provide an air flow through said toasting space in which air flows in substantially a single direction from said base towards an upper, food-receiving opening of said space.

10. A food toaster as claimed in any preceding claim further comprising a food centering mechanism to receive and hold food to be toasted substantially in a central position between said substrates, and wherein said food centering mechanism has two configurations, a first, open configuration in which said mechanism is open to receive food, and a second, closed configuration in which positions or said mechanism close on said food from either side.

11. A food toaster as claimed in claim 10 wherein said mechanism comprises a wire frame defining a pair of wings biased apart and hinged at an apex, and wherein motion of said frame downwards into said toaster closes said wings by means of a camming action against a surface of said toaster.

12. A food toaster as claimed in claim 10 wherein said mechanism comprises a pair of pivotally mounted supports each able to rotate about a respective axis, the supports carrying at least one side of a cage or frame for said food, said side of said cage or frame being pivotally mounted to each of said supports at a position spaced away from said respective axes such that rotation of said supports translates said at least one side of said cage or frame in a direction substantially normal to said side of said cage or frame towards a second side of said cage or frame.

13. A food toaster as claimed in claim 12 wherein said second side is pivotally mounted to each of said supports such that rotation of said supports moves each of said sides towards the other.

14. A food toaster having a pair of substantially transparent sides through which toasted food is visible, the toaster further comprising a set of air vents in a base portion of said toaster opening at a toasting space for said food to, in operation, provide an air flow through said toasting space in which air flows in substantially a single direction from said base towards an upper, food-receiving opening of said space.

15. A food toaster having a pair of substantially transparent sides through which toasted food is visible, the toaster having a pair of substrates each bearing an electrical heating element, the toaster further comprising a food centring mechanism to receive and hold food to be toasted substantially in a central position between said pair of substrates, and wherein said food centring mechanism has two configurations, a first, open configuration in which said mechanism is open to receive food, and a second, closed configuration in which positions or said mechanism close on said food from either side, wherein said mechanism comprises a pair of pivotally mounted supports each able to rotate about a respective axis, the supports carrying at least one side of a cage or frame for said food, said side of said cage or frame being pivotally mounted to each of said supports at a position spaced away from said respective axes such that rotation of said supports translates said at least one side of said cage or frame in a direction substantially normal to said side of said cage or frame towards a second side of said cage or frame

16. A food toaster as claimed in any preceding claim wherein said substrates defining a toasting space for said food between said substrates, and wherein said substrates are configured to heat said food more at a lower region of said space that at an upper region of said space.

17. A food toaster as claimed in claim 16 wherein said substrates are angled towards said upper region of said space.

18. A food toaster as claimed in claim 16 or 17 wherein a said electrical heating element comprises a film coating on said substrate, and wherein said film coating is electrically divided into a plurality of generally horizontal bands, said bands being configured to be electrically heated such that, in use, a lower said band becomes hotter than an upper said band.

19 A food toaster as claimed in any preceding claim having at least one side door, hinged at the bottom to enable the door to be opened to enable access to said toasting space, and wherein a portion of said door extends over the top of said toasting space such that said space is at least partially enclosed during operation of the food toaster.

20. A food toaster as claimed in any preceding claim wherein said substrate bears a layer of semiconducting material comprising said heating element, and wherein said food toaster includes a temperature sensor comprising a portion of said layer of semiconducting material.

21. A food toaster having a pair of substrates each bearing an electrical heating element, and defining a toasting space for said food between said substrates, the food toaster further comprising at least one side door, hinged at the bottom to enable the door to be opened to enable access to said toasting space, and wherein a portion of said door extends over the top of said toasting space such that said space is at least partially enclosed during operation of the food toaster.

22. A food toaster as claimed in claim 21 wherein, in operation said top of said toasting space is substantially completely enclosed by said one or more side doors.

23. A food toaster as claimed in any preceding claim wherein said electrical heating element comprises a layer of semiconducting material on said substrate; the apparatus further comprising: an electrical power input; and an electrical power control device electrically connected between said electrical power input and said electrical heating element; wherein said electrical power control device is a semiconductor device; and wherein at least a portion of said semiconductor device comprises a portion of said layer of semiconducting material.

24. A food toaster as claimed in claim 23 wherein said portion of said layer of semiconducting material is electrically connected in series with a portion of said layer of semiconducting material defining said electrical heating element.

25. A food toaster as claimed in claim 24 wherein said portion of said layer of semiconducting material of said semiconductor device and said portion of said layer of semiconducting material defining said electrical heating element comprise a substantially continuous region of said layer of semiconducting material.

26. A food toaster as claimed in claim 23, 24 or 25 wherein said electrical heating element has at least one electrode, and wherein said electrical power control device is fabricated in a said portion of said layer of semiconducting material adjacent said electrode.

27. A food toaster as claimed in any one of claims 23 to 26 wherein said electrical power control device has a metal electrode connection and comprises a diode formed between said metal electrode and said layer of semiconducting material.

28. A food toaster as claimed in any one of claims 23 to 27 wherein said layer of semiconducting material includes both p-type and n-type doped regions and wherein said electrical power control device comprises a bipolar transistor.

29. A food toaster as claimed in any one of claims 23 to 26 further comprising a gate electrode over said portion of said layer of semiconducting material of said semiconductor device, and an insulator between said gate electrode and said layer of semi conductivity material, and wherein said electrical power has a metal electrode connection and comprises a field effect transistor.

30. A food toaster as claimed in any one of claims 23 to 29 wherein said layer of semiconducting material comprises tin oxide or silicon carbide.

31. A food toaster having a pair of substantially transparent sides through which toasted food is visible, and having a pair of substrates each bearing an electrical heating element, and defining a toasting space for said food between said substrates, the food toaster further comprising one or more of: i) a set of air vents in a base portion of said toaster opening into a toasting space for said food to, in operation, provide an air flow through said toasting space in which air flows in substantially a single direction from said base towards an upper, food-receiving opening of said space;

v) a configuration wherein at least one of said substrates is hingeably mounted to enable the substrate to be pivoted to fluctuate access to a toasting space for said food, and a thermally operated mechanical lock to inhibit said pivoting when said substrate is at an elevated temperature;

vi) a substrate bearing a layer of semiconducting material comprising said heating element, and wherein a said food toaster includes a temperature sensor comprising a portion of said layer of semiconducting material.