BACKGROUND ART There are two common forms of electronic ignition used in gas cooking appliances.

The first is direct spark ignition, which uses an electronic control unit connected to an electrode placed in the gas path. The control generates a high voltage pulse that is passed through the electrode causing a spark to jump from the electrode tip to the gas burner thus igniting the gas. A further advance in the use of direct spark ignition is a technique known as flame rectification whereby a sinusoidal AC voltage is applied to the electrode. If there is a flame present there is an ionization effect associated with the flame which causes a small current to flow through the electrode to ground. This current can then be used to detect the presence of the flame and thus it is possible to control the ignition cycle so that the unit only sparks when a flame is not detected. This is commonly called auto re-ignition.

Whilst direct spark ignition has proven to be reliable and effective it does have certain drawbacks: it causes electrical interference (EMI), is audibly loud and uses very high voltages to generate sparks. In particular, the audible operation makes it poorly suited to applications utilising low gas flow levels, with the accompanying high probability of the flame being extinguished and operation of the re-igniter being required, such as simmer elements on gas cookers.

An alternative is hot surface ignition, which uses an electrically heated element in the gas path to light the gas. An individual hot surface element is usually provided for each burner. The hot surface igniter is on the whole time the cook top burners are being used, to immediately re-ignite the flame in case of flame failure. The disadvantage is that each hot surface element is a high current device that requires significant power to heat. Further, hot surface igniters often fail in an unacceptably short time. This is due to thermal stress caused by the element being on the whole time the burner is in use.

An object of the present invention is to provide a solution to some of the problems encountered with the current methods of ignition, or at least to provide a useful alternative.

DISCLOSURE OF INVENTION In one form of the invention, it may be said to reside in a hot surface re-igniter control including detector means adapted to provide a"no flame detected"flame detection signal when no flame at a selected gas burner is detected, and element control means adapted to control an electrical resistance heating element, said element being arranged to ignite a gas burner when a supply of electrical current is applied to the element, said element control means being adapted to apply said supply of electrical current to the element while the detector means continues to effect a"no flame detected"flame detection signal and the control is in an operative mode.

In preference, the detector means is adapted to use flame rectification to detect the presence of a flame.

The use of flame rectification with a hot surface igniter has been found to have the drawback that while the hot surface igniter is hot it emits electrons, due to a thermo- electric diode effect. This provides a carrier for a current between the hot surface igniter and a surrounding earthed protective guard or an earthed burner independently of the ionization effect of the flame. As a result the generation of the "no flame detected"signal is prematurely terminated.

In preference, therefore, the element control means includes means to select and effect a minimum time period for the supply of electrical current to flow to the electrical resistance heating element.

In preference, in the alternative, there are at least two detector means and associated element control means, each associated with and responsive to, a respective gas burner, such that each electrical resistance heating element is heated only when required to ignite the associated gas burner.

In the alternative there is provided independent ignition and sensing (using flame rectification) for each of at least two burners using a hot surface igniter element

associated with each burner as an ignition source, such that each hot surface igniter element is heated only when required.

This has the effect of reducing power requirements for a system during cooking and resolving problems associated with ignition systems described previously.

In a further form the invention may be said to reside in a hot surface re-igniter control including detector means adapted to provide a"flame detected"flame detection signal when a flame at a selected gas burner is detected, and element control means adapted to control an electrical resistance heating element, said element being arranged to ignite a gas burner when a supply of electrical current is applied to the element, said element control means being adapted to interrupt said supply of electrical current to the element while the detector means continues to effect a "flame detected"flame detection signal and the control is in an operative mode.

In a further form the invention may be said to reside in an installation with a hot surface re-igniter control where there is included at least one electrical resistance heating element adapted to ignite a gas burner, detector means adapted to provide a"no flame detected"flame detection signal when a flame at a selected burner is not detected, and element control means arranged to apply a supply of electrical current to the electrical resistance heating element upon the detector means effecting a"no flame detected"flame detection signal while the burner is in an operative mode.

In a yet further form, the invention may be embodied in a method of effecting ignition of gas burners, the method including the steps, when in an operative mode, of detecting the existence of a flame, and effecting an application of a supply of electrical current to the electrical resistance heating element upon detector means detecting absence of flame.

In preference, the detection of the burning status is effected by using flame rectification.

In preference there are provided at least two electrical resistance heating elements with associated detector means as characterised above with common element control means adapted to separately operate each element in response to the signal from the associated detector means.

In a further form the invention can be said to reside in a method of effecting ignition of gas burners the method including the steps, when in an operative mode, of detecting the existence of a flame, and effecting an application of a heating supply of current to the electrical resistance heating element upon detector means detecting absence of flame.

In preference this method is further characterised in that the detection of the burning status is effected by using flame rectification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 is a layout diagram of a cooktop system made in accordance with the invention, and FIG 2 is a block diagram of the embodiment of the system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG 1 is a basic layout diagram of a cooktop system made in accordance with this invention.

The cooktop shown in FIG 1 has six burners 108-113 placed on a panel. Each burner is fitted with a hot surface igniter 102-107 connected to a hot surface re- igniter control unit 100. There is also one electronically controlled in-line main gas valve 101 which controls the overall supply of gas to the cooktop. Gas line conduits conduct gas from the mains gas supply to the burners 108-113 under the control of the in-line gas cocks 120-125, that are the user switch controls 114-119 for the cooktop. The unit has six-burners 223-228 cooktop but clearly other number of burners are possible.

FIG2. The unit consists of the following functional blocks : Main block Hot Surface Re-igniter (HSR) 200 controller provides the overall control system containing Sub-blocks :

Flame detection circuits 201-206, provide"flame detected"or"no flame detected"signals to the control means Output power switches 207-212, provides the power distribution control to the hot surface igniter elements 217-222 Main power supply 213, provides necessary voltage and power to all block. It consists of +V and-V low power voltage source for the unit controller, +12Vdc for hot surface igniter, output switches 207-212 and controller switch for the switch mode power supply (SMPS) 214.

Switch mode power supply (SMPS) 215 with high frequency transformer output isolating transformer 216, provides power to hot surface igniters 217-222 and their output switches 207-212.

Switch input circuits 231-236 that connect to the gas cock switches 237- 242 Hot surface igniters 207-212, have electrical resistance heating elements that heat and ignite gas, and also provide the flame sense electrodes.

The HSR controller 200 circuit is permanently powered from a low current power supply. A low current voltage source +V powers the internal logic/micro-control whilst the system is in stand by. In this mode all the gas cock/switches 237-242 are in OFF positions and the hot surface igniters 217-222 are in the OFF state.

As each hot surface igniter 217-222 requires 30 to 40W power to ensure quick and reliable ignition of gas and because all the channels are independent and therefore could be switched on simultaneously the switch mode power supply 215 for the igniters must be capable of providing up to 200-250W power to all the hot surface igniter elements 217-222. To achieve this a special switch mode power supply (SMPS) 215 is used. This SMPS 215 is controlled by an SMPS switch 214 from the main power unit 213 and is an AC to AC step down high frequency converter that transforms the mains electrical supply, in this case 220/240VAC 50/60Hz into a 24VAC 30/40Khz electrical supply for the igniters. In alternative embodiments, the electrical supply to the igniters may be any suitable voltage, in particular, 12V AC or 12V DC or 24V DC. Low voltages are employed for the igniters because of the risk that, in a fault condition, the igniter supply may become connected to the

cooktop body, causing an electrocution hazard. In an embodiment where this risk is acceptable, or is eliminated by other means, the igniters may be supplied with current at mains voltage. This voltage is provided to each hot surface igniter element 217-222 from an individual galvanic isolated secondary winding of the SMPS transformer output 216. The control of these high power low voltage outputs is done by relays 207-211 to provide the necessary ON/OFF control for the hot surface igniter heating power distribution. The normally open type relay switches 207-211 are controlled by the main control 200 and only need powering during the ON state so the power consumption during the OFF state is very small.

The hot surface igniter being electrically conductive also functions as the flame sense electrode and is electrically connected to the flame sense circuits 201-206.

The flame sense circuit relies on a technique known as flame rectification whereby a sinusoidal AC voltage is applied to the electrode. If there is a flame present there is an ionization effect associated with the flame which allows a small current to flow through the electrode to ground. This flame sense current flows between the igniter and the burner which is earthed.

When a gas cock/switch 237-242 is in the ON position and the unit senses the absence of flame current through any one of the flame sensing circuits 201-206 it switches on the SMPS 214, 215 and also engages the appropriate one of the relays 207-211 to drive the power to the corresponding one of the hot surface igniter elements 217-222. As the hot surface igniter heats it emits electrons, due to the thermo-electric diode effect (much like a hot cathode vacuum valve). This provides a carrier for a current between the hot surface igniter and the surrounding earthed protective guard or the earthed burner independently of the ionization effect of the flame. As a result a false flame detection signal is generated. In order to avoid causing the unit to switch off the current to the hot surface igniter element before the flame has been established the initial flame sense detection is delayed so that the ignition phase is extended to ensure reliable ignition of the gas. Once a flame is established the control unit 200 switches off the SMPS power 215 to the hot surface igniter element 217-222 and switches the channel into flame sensing mode. As an additional safety feature if the unit does not sense flame within a specified time limit the gas valve control 229 will deactivate the main gas valve 230 and shut down gas to all the burners 223-228.

In an alternative embodiment (not illustrated) the flame sense and flame ignition systems are electrically separate. This avoids the need for the delay of the initial flame sensing operation.

The main control 200 can be realised using standard analogue and digital logic devices or an alternative method is to use a micro controller with software code to perform the same task.

The overall realisation of the invention is a new device called a hot surface re- ignition control (HSR). This invention provides independent ignition and sensing (using flame rectification) of each burner using a hot surface igniter element. It does so in such a manner that it is only heated when required, thus reducing the power requirements for the system during cooking significantly and resolving the problems associated with ignition systems described previously.