The present invention relates to an overheating alarm, and in particular to a device, which, located adjacent to a hot object, measures radiated infra-red rays, arranged to transmit a warning signal and/or interrupt the power supply when an object under surveillance becomes exceptionally warm, i.e. when a predetermined temperature level is exceeded. In an embodiment having a battery power supply, the overheating alarm can be arranged in a housing, for example attached to a wall in the premises where surveillance is to be carried out.

Fires caused by overheating, for example by an overheated hotplate on a kitchen cooker, are relatively rare. The result is normally only a damaged hotplate or a melted down cooking vessel. However, particularly elderly people and their relatives are often worried about the consequences, and many people worry during travel and when on holiday, due to uncertainity whether or not the cooker was switched off when the home was left. In order to reduce these risks, the manufacturers of cookers can arrange each hotplate individually temperature controlled, or alternatively arrange the hotplates with an overheating fuse, arranged to interrupt the power supply when a predetermined temperature level is exceeded. Such an overheating fuse can in _. its simpliest form comprise of a melting fuse, provided that same can be easily replaced. Temperature indication by means of thermo elements, arranged in each monitored hotplate, can obviously also be used, if such monitoring is arranged to result in suitable actions when a predetermined temperature level is exceeded. However, cookers often lack protection against overheating, and often only one hotplate is arranged temperature controlled. Also for other purposes, it is desirable to accomplish surveillance against overheating, for example electrical motors, electrically heated radiators and other types of objects.

The object of the present invention is to disclose an overheating alarm, which can be arranged at a distance from the object to be placed under surveillance, arranged to transmit alarm and/or interrupt the power supply when the temperature of the object exceeds a predetermined temperature level .

The overheating alarm according to the present invention is intended for surveillance of heat emitting objects, for example hotplates, electric cookers or similar, and is mainly characterised in that emission of heat from an object under surveillance is being monitored by an IR-detector, arranged located at a distance from the object under surveillance, arranged to transmit a warning signal and/or interrupt the power supply to same when the temperature level for the object under surveillance exceeds a predetermined maximum temperature level.

As further characteristic features can be mentioned, that the IR- detector is arranged with a highpass filter, which by means of absorption or reflection reduces or prevents radiation having shorter wavelengths than 1 - 3 um from influencing the detector element included in the IR-detector, and that the IR-detector is arranged with a short- pass filter, arranged to reduce radiation having longer wavelengths than 3 ^m.

An example of an embodiment of an overheating alarm according to the present invention is described below with reference to the accompanying drawings, in which:-

Fig. 1 shows a perspective view of a cooker and associated kitchen fan, disclosing an embodiment attached to the lower plane of the kitchen fan, intended to monitor the hotplates of the cooker;

Fig. 2 shows a diagram with regard to radiated power from a hotplate as a function of the temperature of same;

Fig. 3 shows a diagram with regard to spectral dispersion of radiated effectt different temperatures;

Fig. 4 shows a diagram of the temperature of hotplates as a function of time, indicating with a continous line all hotplates at maximum power

(7000 ), with a broken line a smaller and outer hotplate at maximum power (1500 ), and with dotted line all hotplates with the temperature o restricted to 300 C, and with the power supply interrupted after 10 minutes;

Fig. 5 shows how the temperature and output signal of a certain detector element varies with time when the hotplates emit power according to Fig. 4, and with the detector located on the wall behind the cooker, and with the output voltage over 1 Mohm from a bridge having 9 V feeding voltage; and

Fig. 6 shows a circuit diagram of an embodiment of an overheating alarm according to the present invention.

The shown embodiment, which is more fully described below, is intended to be used for surveillance of electrical hotplates at a cooker, but the overheating alarm can obviously also be used for other applications.

The overheating alarm according to the present invention measures IR- radiation from a monitored object, e.g. a hotplate, and warns and/or interrupts the power supply to the monitored object when the temperature of same exceeds a predetermined temperature level. The technical background is well known, and devices for measuring without physical contact have been known for a long period of time. In an ardometer for example, emitted heat is focused by means of a lens against a thermo element, thereby heating same, and resulting voltage is measured by means of a miHivoltmeter. IR-target searching devices are also previously known, which can detect warm objects at a distance of several kilometers. However, any embodiment intended for the field of application according to the present invention is previously not known.

Technical background

All bodies transmit heat or temperature rays, which, as visible light, is a form of electro-magnetical radiation. The warmer a body becomes, the more temperature radiation is transmitted. The radiation effect from a solid body or liquid is contiously dispersed over all vawelengths o o according to Plancks formula. Emission at 300 (27 C) is maintained substantially within the range 5-15 urn, Fig. 3.

With increased temperatures follows a displacement of radiation towards shorter wavelengths - according to Wiens law of displacement stating that the product of temperature (K) and wavelength (yum) is maintained

constant - 2986. The total effect - radiation over al l wavel engths - is decided by Stefan Bol tzmann ^' s formul a.

Emitted heat is absorbed and refl ected in a fashion simil ar to visible l ight. However, the human eye can only regi ster radiation wi thin the wavelength range 0,45 - 0,65 yum. Shoul d the eye instead act sensitively in the so cal led medium IR-range of 2-6 ^ιm, the kitchen woul d be l it up by the heated hotpl ates which woul d appear as strong photo l amps. If their strength of l ight and colour tone coul d be regi stered with a meter having the sensitivity of the human eye, one coul d stipulate that a warning signal shoul d be given only when a dark blue l amp within the viewing fiel d was fit with high intensity, independent of any other lamps having other colours, even i f these were l it up with higher intensity.

A warm hotplate has a very high emission effect. Fig. 2 shows the effect of hotplates in three conventional sizes emitted as a function of temperature.

The spectral dispersion is shown in Fig. 3. As shown, the wavelength in which maximum emission is achieved is reduced with increased temperature, from approximately 10 μm at room temperature to o approximately 3 yum at 700 C. This fact, in combination with the several times increased power obtained, makes it possible to fulfill the above stipulations.

If it is particularly desirable to monitor the temperature range o 900-1000 , i.e. when the hotplate becomes a dark red tone of colour, it is obvious than one should shield emission having shorter wavelengths than approximately 1 yum. This object can be achieved by means of a so called highpass filter. This will be non-transparent to conventional light. Furthermore, it is desirable to filter away emission having larger wavelengths than approximately 3 jum, shortpass filtering. The emission from hotplates having a normal temperature is thereby restricted. To conclude, a narrow band-pass filter would be required.

The short-pass filter can be omitted, provided that the sensitivity of the detector is reduced at a corresponding wavelength.

A number of detectors can be used to transfer the incoming radiation into for example an electrical signal. They are divided into two main groups. Photon (photo) or quantum detectors utilize the changes in electrical properties which arise in semiconductor materials during IR-radiation. By a suitable choice of material and degree of doping, maximum sensitivity can be obtained for for a certain wavelength/tem¬ perature. Any temperature changes in the detector can be regarded as nonimportant. Amongst others, the following detectors operate according to this principle.

Photoconductive detectors. Incoming photons against a semiconductive material can lift up electrones in the conductive band - the photo¬ electric effect. As a result, the resistance of the material is changed. When one applies a voltage over the material, incoming photons are detected by altered current. Mainly used to detect rapid changes.

Photovoltaic detectors. These very sensitive detectors generate a voltage when made subject to photon radiation below a certain restricting wavelength. Bias voltage is not required.

Thermal detectors are basically temperature indicating and result in a monitorable output signal when the temperature of the detector element is changed.

Thermistors. These comprise of semiconductor material, having a resistance strongly dependent of the temperature of the element. Thermistors may have a positive temperature coefficient, PTC- thermistors, with a resistive body consisting of sintered oxides. NTC-thermistors are manufactured in a basically corresponding way, but from other oxides, which results in a negative temperature coefficient. At room temperature, the change in resistance is 2-5% per degree C.

Thermo elements. If two materials, e.g. wires, are soldered together at one end portion, and the others are located at a temperature differing from the point of interconnection, a thermo electrical voltage source is created feeding a current through the circuit, and having a magnitude in relation to the temperature difference. A combination of iron and constantan would for example result in 52 yjV/C at room temperature. The

voltage can be increased by connection of several elements in series.

Pyroelectrical detectors. A pyroelectrical chrystal has spontaneous polarisation or charge concentration, which is strongly dependent of the temperature. A thin layer of such a material between two electrodes form a condensator, the charge of which is a function of temperature. Heating of the element alters the charge and produces a voltage over the electrodes proportional to the radiation. Pyroelectrical detectors are also very sensitive, but are best suited for monitoring rapid changes.

Pneumatical detectors. Pressure changes in a gasfilled chamber are registered in these.

The high effect makes it possible to use all types av IR-detectors in an alarm device for the intended field of use. Suitable photondetectors are lead sulphide, PbS, and lead selenium, PbSe, both being photo conductive semiconductor detectors influenced by IR-radiation without requiring direct heating. Without additional cooling, their maximum sensitivity o for temperatures resides in the region of 600 C, being rapidly reduced for both higher and lower temperatures, which eliminates the need for filters. Such a warning device could in principle be used anywhere.

In the first embodiment tried, two thermistors, being cheap NTC- resistors, were used. These are sensitive over the entire wavelength range of interest. By locating both detectors in a common closed housing, compensation was made for changes in room temperature. However, one thermistor was protected against direct heat radiation, whereby the temperature of same was somewhat lower and thereby having higher resistance, resulting in a differential load applied to the bridge circuit.

As a bandpass filter, conventional glass and a dark coloured plastics film was used. In order to ensure that the temperature difference only is related to differences in incoming radiation, it is important that the glass filter used to screen IR-radiation having low frequencies can be "seen" by both thermistors, in order to compensate for heating of the- glass. The best function is achieved utilizing two glass plates, separated from each other by means of an air filled space. The inside

glass will thereby screen emitted heat from the outer, the temperature of which is increased due to emission of heat and e.g. steam from the cooking vessels. Furthermore, both thermistors must be located within the same housing, which must be manufactured from a material having good heat transmission properties, e.g. aluminium.

The temperature increase achieved in the detector made subject to radiation becomes a function of the difference between absorbed heat radiation and the heat emitted by convection and the heat conducted by the connection wires. To maintain heat conduction at a minimum, the wires leading away from the thermistors must have small cross-section and be heat insulated. Concentration of radiation, e.g. by means of a semi-spherical glass member, may be required if the detector is to be located at a longer distance away from the hotplate. Also in this case, the glass will serve as a shortpass filter, screening radiation from bodies having a lower temperature.

Technical description of embodiment

In two experimental devices tested, a so called industrial circuit from National Semiconductor denominated 1801 was been used, which was primarily designed for use in fire warning devices of ionization type. The electrical circuit is shown in Fig. 6. The circuit is connected to a 9 V battery and includes two voltage stabilizers of which one at pin two, having an output level of 5,9 V, is used to feed the bridge circuit. Same comprises of thermistors Tl and T2, and resistors RI and R2. The last mentioned is connected in series with a variable resistor R3, which is used to unbalance the bridge, whereby the output voltage obtained via the resistor R4 to the comparator can be unbalanced in the region of 100 V. The comparator will monitor when the voltage at the negative input, pin 4, becomes higher than the negative, pin 5, and then trigger via an OR-circuit the final stage, driving a suitable buzzer. The circuit also includes a voltage sensing device, causing a short alarm signal each other minute when the battery is becoming discharged and should be replaced. The level is decided by means of the resistors R10 and Rll and should be choosen in such a way, that a warning signal is obtained when the voltage becomes lower than approximately 8 V. This voltage is sufficient for continous operation of the warning device

during several months.

When an overheating alarm occurs, the capacitor Cl is discharged via R8 and the diode Dl, thereby offsetting the bridge and reducing the voltage applied to pin 4, and interrupting the alarm signal. With the components choosen, a short signal is achieved with intervals of 15 seconds. However, this interval is reduced in proportion to absorbed radiation, i.e. the alarm frequency is successively increased if the hotplate is not disconnected.

In order to silence the alarm while the temperature of the hotplate is being reduced, there is a "Keep quiet" button, which discharges the capacitor C4 offsetting the bridge, and thus interrupting the signal. However, the capacitor is slowly charged again via the resistor R9 and after approximately 5 minutes, the alarm has regained normal sensitivity. The warning signal can thus not be interrupted permanently as long as the detector is located adjacent to the cooker.

The relatively large capacitors C2 and C3 reduce sensitivity for disturbances, for instance as occuring when the fan is switched on or off.

The number of components can be reduced while maintaining the function unchanged if NS 1801 is replaced by a more modern circuit, e.g. CA3164E from RCA. This circuit also includes an output facilitating connection in parallel of up to 20 overheating alarms. In the same connecting circuit can also be included most battery operated fire warning alarms, today being installed in most homes. This is of particular value in service apartments, since a large number of warning devices of both types can be connected in parallel to a common warner, e.g. located with the supervisor, or connected to an external alarm.

The overheating alarm can also be arranged including a smoke detector, in order to accomplish alarm during smoke formation, since formation of smoke in certain cases preceed exceeding of a predetermined temperature level. Detectors of standard type can be integrated with the same bridge coupling, thus resulting in alarm both for smoke generation and overheating.

Furthermore, the alarm apparatus can either receive a voltage supply from a battery, or alternatively be connected to a suitable electrical circuit at the monitored object, e.g. a signal lamp which is lit when the monitored object is switched on. It is obviously also possible to perform the connection in such a way, that surveillance is maintained continously, independent of whether the monitored object s switched on or not.

During practical tests performed with the embodiment shown in Fig. 6, hotplates have been used as heat emitting and monitored objects, and with the detector located at a distance of 0,36 from the hotplates. Good temperature monitoring has been confirmed, and the detector was not noticably influenced by heat emitted from the hotplates. Said distance can be further varied within broad limits, while maintaining the good functional properties.

Finally, it is also possible to arrange the detector with suitable logic functions, e.g. by connection of a micro processor, in order to accomplish an intermittent warning signal related to earlier temperature changes and the temperature derivate.

The present invention can obviously be further modified within the scope of the inventive thought and the following claims, and is thus in no way restricted to the examples of embodiments shown and described, for which hotplates and cookers have been choosen as examples of monitored objects.