GB2027536A - Improvements in gas-fired appliances - Google Patents

Abstract

A gas-fired appliance, such as a gas cooker, has a clock (16) which after a predetermined time provides a signal which produces a voltage of opposite polarity to that produced by a flame detector (14) to close a gas supply valve (3). The clock can be set to switch on the gas supply to a burner (1) at a first selected time, and at a second selected time either to switch off the gas supply to the burner or to reduce the level of the gas flow, for keeping cooked food warm. The setting of a manual switch (39) determines whether the gas supply is shut off or is merely restricted at the second selected time. A capacitor (22) is charged up after the first selected time, and at the second selected time a switch arm (19) controlled by the clock moves to discharge the capacitor either through a solenoid (11) energised by the flame detector (14) in order to de-energise that solenoid (11) to switch off the gas supply, or through a relay (24) which energises a solenoid (10) to restrict the gas supply. <IMAGE>

Description

SPECIFICATION Improvements in gas-fired appliances This invention relates to gas-fired appliances, particularly, but not exclusively, gas-fired domestic cookers, of the kind comprising a burner, a valve through which gas is supplied to the burner, electrically controlled means for operating the valve, a thermoelectric flame sensing device responsive to a flame at the burner and electrically connected to the electrically controlled means to hold the valve open in the presence of a burner flame.

The invention is particularly concerned with the use of a clock in a gas-fired appliance of the kind set forth which can be set to stop the gas supply to the burner at a selected time. Usually the clock will also be capable of being set to start the gas supply at an earlier time.

One possible way to shut off the gas supply to the burner at the selected time would be to arrange a switch in the connection from the thermo-electric sensing device to the electrically controlled means such that the switch is opened at the selected time to close the valve. In practice this is difficult to achieve owing to the relatively high resistance of available switches as compared with the internal impedance of the thermo-electric sensing device. A typical sensing device has an impedance of 0.016 ohm whereas a solid state switch has an impedance of typically 10 ohm. Even if a mercury wetted switch were to be used it would be necessary to use at least two in parallel, and these are very costly.

According to the invention a gas-fired appliance of the kind set forth comprises a clock which can be set to provide a signal at a selected time, and a voltage unit which is responsive to the signal to produce a voltage of opposite polarity to that produced by the flame sensing device which cancels out the effect of the current from the flame sensing device in order to close the valve.

The clock is preferably an electrically driven clock.

The voltage unit is preferably connected in series with the flame sensing device.

Conveniently the voltage unit comprises a charge storage device, and means are provided for charging the charge storage device prior to the selected time, and a first switch is operated by said signal to discharge the charge storage device at the selected time.

A resistor is preferably connected in series with the charge storage device to control the rate of discharge of the charge storage device and thereby ensure that when the current from the sensing device has been cancelled sufficient time elapses for the electrically controlled means to overcome any inertia and close the valve before the net current becomes reversed and reaches a substantial value.

It is desirable in domestic cookers controlled by a clock for the food to be kept warm in the period following the cooking period by maintaining the burner flame but at a reduced gas flow.

In order to provide such a "warm period", the appliance preferably includes restrictor means for restricting the supply of gas to the burner, further electrically controlled means for operating the restrictor means, and a second switch connected to the output of said first switch and capable of being set to connect the voltage unit to the further electrically controlled means at the selected time instead of to the first electrically controlled means, so that the supply of gas to the burner is restricted but not shut off at the selected time.

The further electrically controlled means is conveniently held energised by a self-holding relay.

The valve may have a normally closed valve member which is actuated by independentiy energised primary and secondary solenoids, the lastmentioned of which is energised by the current produced by the flame sensing device. The primary solenoid may be arranged to be operated by a switch and/or the clock for the initial supply of gas to the burner for ignition. That solenoid may be operated for just a short period, for example for approximate lythirtyseconds. Upon operation of the primary solenoid the valve member is opened and a restricted supply of gas passes to the burner sufficient for ignition of the burner flame. The heat of the ignited flame causes the flame sensing device to produce the electrical current which operates the secondary solenoid.When the primary solenoid is subsequently de-energised the valve member is held open by the secondary solenoid and the supply of gas to the burner is increased to support a full flame at the burner. If for any reason the burner has not been ignited by the time that the primary solenoid is de-energised, the valve will close and stop the supply of gas to the burner because the secondary solenoid will not have been made operative by the flame sensing device.

The restricted supply of gas to the burner when the primary solenoid is energised may be achieved by providing a restrictor which is caused to be moved by the solenoid into engagement with and close partially a port through which gas entering the valve has to pass to reach a main outlet controlled by the valve member.

Suitable valves of this kind are disclosed in our U.K.Patent Application Nos. 26213/77 and 16171/78 (Specification No. 2,100,747).

A control circuit for a gas-fired domestic cooker in accordance with the invention will now be described by way of example only with reference to the accompanying drawing.

A burner 1 of an oven is supplied with natural gas, or other gas such as propane or butane of comparable or higher calorific value, by a pipe 2 connected to the outlet of a valve assembly 3 of which the inlet is connected by a pipe 4 to a gas tap 5 connected to a gas supply 6. Gas tap 5 is provided with electrical contacts arranged to connect a mains live input line 7 to a line 8 connected to the primary of a transformer 9 when the gas tap 5 is turned on by the user.

Valve assembly 3 is preferably that described with reference to Figure 3 in the Specification of our co-pending U.K. Patent Application serial number 2 001 747A (No. 7832288). A normally closed valve member, not shown, of assembly 3 is actuable by independently energisable primary and secondary solenoids 10 and 11 respectively to supply gas to burner 1. When the primary solenoid 10 is energised a restrictor, not shown, is caused to be moved by the solenoid into engagement with and partially close a port, not shown, through which gas entering the valve assembly has to pass to reach a main outlet port controlled by the valve member.The primary solenoid 10 is arranged, as will be explained hereafter, to be energised to give the initial supply of gas to the burner 1 for ignition by an electrode 12 connected to a high voltage unit 13, which constitutes an ignition energisation means. Upon operation of the primary solenoid the valve member of valve assembly 3 is opened and a restricted supply of gas passes to the burner 1 sufficient for ignition. The heat of the ignited flame causes a thermo-electric flame sensing device 14 electrically connected by lead 15 to the secondary solenoid 11 to produce an electric current which energises the secondary solenoid 11.

When the primary solenoid 10 is subsequently de-energised the valve member is held open by the secondary solenoid 11 and the supply of gas to the burner is increased to support a full flame at the burner. If for any reason the burner has not been ignited by the time that the primary solenoid is de-energised, the valve will close and stop the supply of gas to the burner 1 because the secondary solenoid will not have been energised by the flame sensing device.

An electrically driven timer clock 16 can be manually set to initiate ignition of the burner 1 at a first selected time and then to switch off the burner 1, orto reduce the gas supply to the burner 1, at a second selected time. Clock 16 is provided with three ganged switch arms 17, 18 and 19 of which arm 18 is connected to the output of a full-wave rectifier circuit 20 connected to a conventional 12 volt RMS secondary winding of transformer 9.

At the first selected time set on the clock 16 the switch arms are moved downwards, and the arm 18 is connected to a supply line 21 for a charge storage device 22 in the form of an electrolytic capacitor of 2200pF and for an ignition timer circuit 23 which is arranged to energise a relay 24forthe ignition period of 90 seconds. A resistor 48 is connected in the input to timer circuit 23 to prevent back-feed of charge to supply line 21.

The ignition timer circuit 23 comprises a type 555 integrated circuit timer unit 25 of which the voltage supply from lead 21 is stabilised by a zenor diode 26, an electrolytic capacitor 27 of 1 OuF connected across the diode 26, and by a small capacitor 28 which deals with any pulses. The capacitor 22, in addition to its main function of de-energising the secondary sole noid 11 at the end of the timed cooking period, provides protection against supply disruptions. The trigger terminal TR and the reset terminal R of the timer unit 25 are connected to the junction between a resistor 29 and a capacitor 30 which are connected in series between the supply and earth. The resistor 29 and capacitor 30 ensure triggering of the timer unit 25 simultaneously with the switching on of the supply to the timer by the switch arm 18.

The output terminal O/P is connected through a diode 31 to the relay 24, and the discharge terminal DIS, together with the threshold terminal TH, are connected to an RC network consisting of a resistor 32 and capacitor 33, the values of which determine the length of the period timed by the timer circuit 23.

Relay 24 is provided with two relay contacts 24a and 24b. The relay contact 24a is connected to the mains supply line 8 leading from the gas tap 5 and on energisation of relay 24 moves downwards to connect a terminal 34to the line 8. Terminal 34 is connected firstly to a second full-wave rectifier circuit 35 and secondly to switch arm 17 by way of a lead 36. On energisation of relay 24 the rectifier circuit 35 provides a current to the primary solenoid 10 to provide the restricted gas supply to burner 1 during the ignition period, and the high voltage unit 13 is energised by way of switch arm 17, which constitutes a third switch, and a lead 37 to cause sparking between electrode 12 and burner 1.

Switch arm 19 constitutes the first switch and is arranged to discharge capacitor 22 when the second selected time is reached, at the end of the timed cooking period. The arm 19 is connected by a lead 38 to the switch arm of a manually operable switch 39 which controls an automatic hold function. The switch 39 constitutes a second switch.

In a first position of the switch arm of switch 39 the lead 38 is connected to a lead 40 which is connected through a resistor 41 with the secondary solenoid 11, and in a second position the switch arm of switch 39 is connected to a lead 46 which is connectable by means of a manually operable switch 42, which performs a manual hold function, with the relay 24 by way of a lead 43 and a diode 44.

If the automatic hold switch 39 has been set by the user to connect switch arm 19 with the secondary solenoid 11, then at the end of the timed cooking period the capacitor 22 is discharged through the resistor 41 and the secondary solenoid 11 in a direction opposing the current produced by the flame sensing device 14 so as to de-energise the solenoid 11 and shut off the gas supply to burner 1.

If, however, the automatic hold switch 39 has been set by the user in its automatic hold position in which lead 38 is connected with lead 46, and the manual hold switch 42 has been set by the user in the off-position in which leads 46 and 43 are connected, then at the end of the timed cooking period the capacitor 22 is discharged through the relay 24 by way of leads 38,46,43 and diode 44.

Energisation of relay 24 in this way will result in closure of relay contact 24b, which has a self-holding action on the relay 24, the relay being maintained energised by way of switch arm 18, a lead 45, relay contact 24b, switch 42, lead 43 and diode 44.

Energisation of relay 24 also results in energisation of the primary solenoid 10 by way of relay contact 24a and the rectifier circuit 35, and energisation of primary solenoid 10 brings the restrictor of valve assembly 3 into effect to restrict the gas supply to burner 1 to a low level for maintaining the food in the oven warm until the user returns to switch off the gas tap 5.

A first electrically controlled means for operating the valve of valve assembly 3 is constituted by secondary solenoid 11, and a further electrically controlled means for operating the valve is constituted by the primary solenoid 10 together with rectifier circuit 35 and relay 24.

If the switch arm of the manual hold switch 42 is put in the on-position by the user during the timed cooking period, lead 43 is connected with a lead 47 to energise the relay 24 through switch arm 18, with the effect that the primary solenoid is energised to restrict the gas supply. Thus if the user returns to the oven during the timed cooking period and decides that cooking is completed, the user can turn switch 42 to the on-position to restrict the gas supply to keep the food warm.

If such a manual hold function is not required lead 47 and switch 42 may be dispensed with, lead 46 then being connected permanently to lead 43, and if an automatic hold function is not required then switch 39 may be dispensed with, leads 38 and 40 then being permanently connected together.

The choice of the values of capacitor 22 and resistor 41 needs to be carefully made. It is essential that sufficient charge be available in capacitor 22 at the end of the timed period to de-energise solenoid 11, but it is desirable to keep the costs, and therefore the value, of capacitor 22 as small as possible. The value of resistor 41 is therefore chosen such that the rate of discharge of capacitor 22 is sufficiently slow to enable the armature of secondary solenoid 11 to react to close the valve when the net current through the solenoid 11 passes through zero. In this case resistor 41 is a one ohm resistor, the impedance of secondary solenoid 11 being 0.016 ohm.

Terminals 49 and 50 which connect with the primary coil of transformer 9 are connected respectively to mains neutral input and to a neutral terminal, not shown, on the high voltage unit 13.

The operation of the circuit will now be summarised. After putting the food to be cooked in the oven the user sets on the clock 16 the time at which the burner is to come on, and the time corresponding to the end of the cooking period. The user sets the automatic hold switch 39 to the on or off-position in dependence upon whether the food is to be kept warm at the end of the cooking period. If it is, then the user must also set the manual hold switch 42 to the off-position. The user switches on the gas tap 5, and of course sets the usual thermostatic controller, not shown, which controls the gas supply to pipe 6 in dependence upon the oven temperature.

Initially the switch arms 17, 18 and 19 of the clock 16 are in the neutral position shown and relay 24 is de-energised. When the first time set on the clock is reached the switch arms 17, 18 and 19 are moved down-wards so that lead 21 is made live through switch arm 18 to energise and trigger simultaneously the operation of the timer unit 25. The relay 24 is therefore simultaneously energised by the timer unit 25 and is maintained energised by the timer unit for 90 seconds. Energisation of relay 24 results in energisation of the primary solenoid 10 by way of relay contact 24a, and in energisation of the high voltage supply 13 to the ignition electrode 12 by way of relay contact 24a, and in energisation of the high voltage supply 13 to the ignition electrode 12 by way of relay contact 24a and switch arm 17.As previously explained, the burner will either be alight at the end of the ignition period, and the secondary solenoid 11 energised, or in the event of a fault the burner will be unlit and the gas supply shut off. Assuming that the burner is alight, then the clock reaches the second set time, at the end of the desired cooking period, the switch arms 17, 18 and 19 move to their upward positions to disconnect line 21 from the transformer 9 and to connect capacitor 22 to lead 38 by way of switch arm 19. As previously explained, this will either result in de-energisation of secondary solenoid 11 and shut off the gas supply to burner 1 if the auto hold switch 39 was set to the off-position, or energisation of relay 24 if the switch 39 was set to the on-position.If switch 39 was on, then energisation of relay 24 will result in energisation of first solenoid 10 to restrict gas flow to the burner to keep the food warm, but switch arm 17 will be disconnected from lead 37 to de-energise the high voltage spark unit 13.

It is desirable not to energise the spark ignition device 12 when the primary solenoid 10 has been energised for keeping the food warm. If the spark ignition device were to be continually energised during this period it would have a restricted life, but in some circumstances this may be acceptable. In the circuit described the clock 16 is provided with the switch arm 17 solely for this purpose, but in other embodiments the de-energisation of the ignition during this period could be effected by other means, such as an additional relay.

The transformer 9 may conveniently be combined with the drive mechanism for the clock 16, the secondary winding of the transformer comprising an additional coil provided on the winding of the clock motor.

Claims (9)

1. A gas-fired appliance of the kind set forth and comprising a clock which can be set to provide a signal at a selected time, and a voltage unit which is responsive to the signal to produce a voltage of opposite polarity to that produced by the flame sensing device which cancels out the effect of the current from the flame sensing device in order to close the valve.

2. A gas-fired appliance as claimed in claim 1 in which the voltage unit comprises a charge storage device, means for charging the charge storage device, and a first switch operated by said signal to connect the charge storage device in series with the flame sensing device and the electrically controlled means.

3. A gas-fired appliance as claimed in claim 2 in which the charge storage device comprises an electrolytic capacitor.

4. A gas-fired appliance as claimed in claim 2 or claim 3 in which a resistor is connected in series with the charge storage device.

5. A gas-fired appliance as claimed in any of the preceding claims including restrictor means for restricting the supply of gas to the burner, further electrically controlled means for operating the restrictor means, and a second switch connected to the output of said first switch and capable of being set to connect the voltage unit to the further electrically controlled means at the selected times instead of to the first electrically controlled means, so that the supply of gas to the burner is restricted but not shut off at the selected time,

6. A gas-fired appliance as claimed in claim 5 and comprising ignition means, and ignition energisation means for energising the ignition means, the ignition energisation means being connected to the restrictor means such that the gas supply is restricted when the ignition means is energised.

7. A gas-fired appliance as claimed in claim 6 in which the ignition energisation means comprises a relay which is responsive to a timing circuit and also to the second switch, and further comprising a third switch which is operated by the clock at the selected time to inhibit operation of the ignition means by the relay.

8. A gas-fired appliance as claimed in any one of claims 5 to 7 in which the first and further electrically controlled means respectively comprise independently energisable secondary and primary solenoids.

9. A gas-fired aplianceofthe kind set forth and substantially as described with reference to the accompanying drawings.