The invention relates to a safety cut-off device for a gas-heated hob comprising a thermocouple and a thermoelectric generator as well as a safety solenoid valve.

In order to prevent the flow of gas in a situation where there is no flame several flame detecting mechanisms are used. A fully electronic kind comprising an ionization detector and suitable high impedance amplifiers would be attractive, but the proximity of this kind of electronic components to the very hot surfaces near gas flames is detrimental to their lifespan, and furthermore such circuits generally need batteries. One type uses a thermocouple which gives off a voltage when it it is heated by the lit flame, and this voltage is sufficient to deliver a holding current for a solenoid safety valve in the gas supply. A system of this kind may have a push-button to perform the initial mechanical actuation of the solenid valve whereby the gas flows and may be lit, possibly by electronic means, and when the thermocouple has been heated, it sustains the open position of the safety valve.

In EP 0 635 680 it is known to use a series connection of safety valves to increase the security, and one thermocouple functions as above described and a neighbouring thermeoelectric generator (a series connection of several thermojunctions) supplies an activation and holding current for a separate solenoid valve in series with the first mentioned.

These solutions are efficient, however they suffer from the great disadvantage that the time constant of the thermoelectric voltage generators is large which means that they will supply holding currents for many seconds after a flame has disappeared, irrespective of the reason for the disappearance of the flame. This means that unburnt gas escapes for many seconds which will create an explosion hazard when the admixture of air is suitable. Also, first turning-on of the main

flame is relatively slow this way.

It is the purpose of the invention to improve the functioning of a flame safety device based on thermoelectricity, thereby retaining the advantages of a sturdy principle but avoiding the long time delays involved. This is obtained in a safety cut-off device according to the invention which is particular in that the thermoelectric generator provides the supply voltage for a transistor amplifier controlling the holding current for the safety solenoid valve, said transistor amplifier being controlled by the thermoelectric voltage from the thermocouple in order to cut off the holding current at a predetermined thermoelectric voltage.

A preferred embodiment is particular in that the transistor amplifier is a DC coupled common emitter amplifier with the collector connected to the thermeoelectric generator, the emitter to the solenoid, and the base to the thermocouple.

An advantageous embodiment further comprises a battery source for providing a short-term actuating voltage for the safety valve.

A further advantageous embodiment uses the battery source to power a spark ignition electrode at the hob.

The invention will be further described with reference to the drawing, in which Fig. 1 shows the voltages available in thermoelectric junctions and generators, Fig. 2 shows a circuit diagram of an amplifier according to the invention, and Fig. 3 shows the general layout and connections of a device according to the invention.

Fig. 1 shows the voltages generated in a thermoelectric junction and in a generator consisting of a series connection of such junctions. It will be seen that the time constant of the series connection is less than the time constant of the single junction, but that the actual voltage of the single junction has an

overshoot before it settles at a stable, temperature dependent voltage. The holding voltage of a magnet used in a safety device is low (although the current may be fairly high), and it is shown in relation to the other voltages present in such appliances. It will similarly be seen that the time constant of the cooling down after a source of heat has disappeared is considerable, and about 6 times as large as the heating-up time constant.

Hence a safety cut-out based on the current generated by the voltage of a thermo-junction would let a fair amount of gas escape before the cut-out is effective.

In Fig. 2 is shown a simple emitter-follower type amplifier in which a thermoelectric generator 1 supplies the drive voltage and the sensing thermojunction 2 provides the input signal to a transistor 3 amplifier.

The solenoid 4 of a magnetic safety valve is connected to the emitter of the transistor 3. A silicon transistor will tolerate operating temperatures of ca. 150oC which is ample in practice. The voltages shown graphicaly in Fig. 1 are shown in the schematic, and it will be understood that suitable dimensioning of the base resistor, based on the voltage drops inherent in transistor operation and a knowledge of the resistance of the solenoid for the magnetic valve, will result in a control of the current from the emitter through the solenoid. In this matter a cut-off is obtained when the voltage from the thermojunction has fallen only a little with respect to the stable high-temperature condition.

In Fig. 1 is shown that the gas would continue to flow about 6 times longer, if the safety valve did not release until the voltage had fallen by the thermal time constant alone.

In Fig. 3 is shown the lay-out"below the top"of a gas hob fitted with electronic ignition of the gas and a safety device according to the invention. A gas burner is generally indicated by the grid at 5, and an ignition electrode 6 is connected to a high-voltage generator 7

supplied by a battery 8 which is remote from the gas burner. Ignition is controlled by the gas valve 9 which is combined with a safety cut-out of the magnetic type.

Ordinarily, this valve is actuated by the gas valve during and immediately after ignition, until sufficient holding voltage is available from a thermoelectric source. Such sources are indicated as a thermojunction 10 and a thermoelectric generator 11. As described above, the thermovoltages generated are fed to an amplifier 12 which is connected to the solenoid in the safety valve 13.

As will be seen from Fig. 1, the voltages of the thermogenerator and the thermojunction very quickly increase above the holding voltage of the solenoid, and it is not an undue strain on the ignition battery to supply the higher actuation voltage and the solenoid current during the brief period just after ingnition, until the transistor amplifier can take over. This avoids the combination with a mechanical actuation directly from the valve 9.