GB1594349A - Combined spark ignition and flame detection apparatus for a gas burner - Google Patents

Description

(54) COMBINED SPARK IGNITION AND FLAME DETECTION APPARATUS FOR A GAS BURNER (71) We, PARKINSON COWAN AP PLIANCES LIMITED, a British Company of Stechford, Birmingham, B33 9AD, do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a combined spark ignition and flame detection apparatus for a gas burner and has as an object to provide such an apparatus in a convenient form.

Apparatus in accordance with the invention comprises the combination of electrode means, a spark generating circuit including a step-up transformer having the electrode means connected to its secondary winding, a.c. supply for applying an a.c. signal across the electrode means, a detection capacitor connected in circuit with said a.c. supply and the electrode means whereby the detection capacitor acquires a charge when there is a flame in the region of the electrode means as a result of flame rectification, an f.e.t. switch device controlled by the capacitor and arranged to prevent operation of the spark generating circuit when a flame is detected and a surge arresting device connected across the secondary winding and the electrode means to protect the f.e.t. from voltage spikes created in the transformer secondary winding during the production of a spark.

The present invention will now be described in more detail by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of an example of a combined spark ignition and flame detection apparatus embodying the present invention.

Figure 2 is a circuit diagram showing the apparatus of Figure 1 modified so as to provide a variable delay between flame extinction and re-establishment of ignition sparks; and Figure 3 is a circuit diagram of another example of a combined spark ignition and flame detection apparatus embodying the present invention.

The circuit shown in Figure 1 includes a mains transformer 10 having its primary connected to the live L and neutral N mains supply rails and intended primarily for isolation purposes rather than for voltage transformation.

For spark ignition there is a half-wave rectified d.c. supply from the transformer secondary provided by a diode D,. The load on the transformer is balanced by a second diode D2 in series with a resistor R, across the secondary winding. The diode D, is connected by a resistor R2 to one side of an ignition capacitor C, the other side of which is connected by the primary winding of a step-up transformer 11 to an earth rail 12.

The earth rail 12 is connected to the main earth supply rail E. A spark gap device 13 is connected between the junction of resistor R2 with capacitor Cl and the rail- 12 and the ignition circuit works in well known manner. The capacitor C, charges relatively slowly via the resistor R. until the voltage on it exceeds the break down voltage of the spark gap device which discharges the capacitor quickly providing a pulse to the primary winding which generates a high voltage impulse in the transformer secondary winding.

The secondary winding of the transformer 11 has one end connected by an electrode means 14 to earth, such electrode terminating adjacent a part of the burner to be ignited and monitored. The ignition circuit is completed by a surge arresting device 15 connecting the opposite end of the secondary winding of the transformer II to earth.

The device 15 may be cold cathode tube device such as a simple neon tube or a zener diode, or a solid state surge arrestor.

The flame detection arrangement used in the apparatus makes use of a phenomenon known as flame rectification. As a result of this phenomenon, if an electrode pair which is immersed in a flame is connected in series with an a.c. source, a resistor and a capacitor, a direct potential will build up on the capacitor caused by the flame acting as a diode.

To this end the said opposite end of the secondary winding of the transformer 11 is connected via a resistor R3 to the secondary winding of the transformer 10 to apply a.c.

continuously to the electrodes 14 via the resistor R3 and the secondary winding of the transformer 11. A further resistor R4 connects the junction of resistor R3 with the secondary winding of transformer 11, to one side of a detection capacitor C. the other side of which is grounded to rail 23. The capacitor C2 acquires a negative voltage on its said one side when there is a flame present. The load on the transformer 10 is balanced by resistor R, and diode D2 to prevent the capacitor C2 from acquiring a charge when a flame is not present.

Said one side of the capacitor C2 is connected by a high value (1OMQ) resistor R5 to the gate of an n-channel field effect transistor Q, with its source grounded to rail 12. A capacitor C3 and a very high value (100 MQ) resistor R6 are connected in parallel between the gate of the f.e.t. Q, and the rail 12 so as together with resistor R5 and capacitor C2 to filter and damp voltage spikes in the wave form applied to the junction of resistors RX and R4 by the secondary of transformer 11 and thus prevent these spikes from damaging the f.e.t.

Q,. Also, to prevent voltage spikes being applied to the gate of the f.e.t. Q,, low inductance type capacitors are used for capacitors C, and C.

The main protection to the f.e.t. Q, against voltage spikes generated in the secondary of the tranformer 11 is provided by the device 15 which conducts when it reaches its striking or breakdown voltage and thus absorbs most of the voltage spike.

It is the provision of the device 15 which permits a single electrode means to be used for both ignition and flame sensing.

An f.e.t. is used to sense the charge on the capacitor C2 as the flame rectification current is in the microamp region and an f.e.t. provides the required high input impedance. The drain of f.e.t. Q, is connected to the earth rail by a capacitor C4. which prevents voltage spikes from appearing across the drain and source of the f.e.t.

The drain of the f.e.t. is also connected to the junction of two resistors R7, Rs, connected in series between the cathode of a diode D3 and the anode of a diode D4. The anode of the diode D3 is connected to the secondary of the transformer 10 and the cathode of the diode D4 is connected to the base of an n.p.n. transistor Q2 which has its emitter grounded to rail 12 and its collector connected by a diode D5 to the junction of the resistor R, and the capacitor Cl so that when the transistor Q2 is conductive the capacitor Q, cannot charge to the breakdown voltage of the spark gap.

A resistor R9 is connected across the device 15 to provide a leakage path to limit the sensitivity of the detector to flame and reduce the time delay between flame extinction and generation of sparks.

In the absence of a flame there is no charge on the capacitor C2 so that the f.e.t.

Q1 has a relatively low drain source resistance tending to hold the transistor Q2 turned off. When a flame is present the capacitor C2 charges negatively and the operating point of the f.e.t. goes into the so called "pinch-off" region where its resistance rises sharply. Transistor Q2 then turns on and prevents further sparks.

The re-establishment of sparks after the disappearance of the flame is delayed by the time taken for the charge on capacitor C2 to leak away through resistor R5 to R6 in series and resistors R4 and R9 in series. A delay of between 2 and 3 seconds is considered to be acceptable in the presence of a flickering flame, but resistor R9 ensures the reestablishment of sparks before a dangerous build up of gas can occur.

The modification shown in Figure 2 includes a variable delay circuit comprising a p.n.p. transistor QS with its base connected to the rail 12 its collector connected to the gate of the f.e.t. Q, and its emitter connected by a variable resistor R, > , to the drain of the f.e.t. Qi. a further resistor Rl connecting the emitter of the transistor Q3 to the rail 12. In use the transistor Q conducts to provide negative feedback to the gate as Q, and limit the voltage build up on C2. Adjustment of the resistor R, > , will vary the voltage stored on capacitor C2 and hence the time taken for the latter to discharge on flame extinction. The resistor R9 is omitted as this is rendered unnecessary by the variable delay circuit.

Turning now to Figure 3, it will be seen that the circuit shown includes all the elements of Figure 2. It will also be noted that the transformer 11 of Figures 1 and 2 has been replaced by a transformer 111 having an additional secondary winding of, say, 3 turns only. The additional secondary winding provides output signals to a solenoid valve control circuit which has been added to the circuit.

Two solenoids are used, namely a main solenoid 20 which provides the main on-off gas control and a solenoid 21 which provides high/low flame control. The solenoid 2() is connected across a bridge rectifier 22 encrgised by the live and neutral mains conductor. The solenoid 21 is connected between the cathode of a diode D5, which has its anode connected to the live rail. and the anode of a thyristor T1 which has its cathode connected to the neutral rail, a resistor Rl2 being interposed between the solenoid 21 and the anode of the thyristor T,. A capacitor C5 is connected across the solenoid 21. The gate of the thyristor T1 is connected by a resistor R13 to the neutral rail and by a resistor R14 to the anode of a zener diode D6 which has its cathode connected by a capacitor C6 to the neutral rail.

A resistor Rl5 connects the cathode of zener diode D6 to the anode of a thyristor T2 which has its cathode connected to the neutral rail and which also has the additional secondary winding of the transformer 111 connected across its gate-cathode. A resistor R,6 connects the anode of the thyristor T2 to the cathode of the diode D5.

The solenoid 20 is always energised when the circuit is connected to the mains. The solenoid 21, however, is only energised if there is a sufficient voltage on the capacitor C6 to breakdown the zener diode D6 and fire the thyristor T1. The resistor R16 is chosen so that it takes several seconds after power has been applied to the circuit for the capacitor C6 to charge such a voltage and the thyristor T2 conducts to discharge the capacitor several times each second during sparking (i.e. when no flame is present).

Thus during sparking the solenoid 21 is not energised but becomes energised to permit the full gas flow to the burner a few seconds after sparking ceases. In the event of the flame becoming extinguished by a cause other than a protracted removal of the gas supply, e.g. by a draught or by a very brief interruption of the gas supply, sparking will commence as explained above, after a delay set by resistor R,(,. During this delay full gas flow continues so that R", should be set to give a delay short enough to ensure that the amount of gas released into the space into which the burner fires is not sufficient to create an explosive mixture. The solenoid 21 will be re-energised when the flame is re-established.

If the gas supply to the burner is cut off for a period, the sparking will take place and the solenoid 21 will be de-energised. When the gas supply is restored, the burner will go through its normal starting sequence on low flame and then switch to high flame.

In a modification of the circuit shown in Figure 3, the variable delay circuit is replaced with the resistor R9 of Figure 1.

WHAT WE CLAIM IS: 1. A combined spark ignition and flame detection apparatus for a gas burner comprising the combination of electrode means, a spark generating circuit including a stepup transformer having the electrode means connected to its secondary winding, a.c.

supply for supplying an a.c. signal across the electrode means, a detection capacitor connected in circuit with said a.c. supply and the electrode means whereby the detection capacitor acquires a charge when there is a flame in the region of the electrode means as a result of flame rectification, an f.e.t.

switch device controlled by the capacitor and arranged to prevent operation of the spark generating circuit when a flame is detected and a surge arresting device connected across the secondary winding and the electrode means to protect the f.e.t. from voltage spikes created in the transformer secondary winding during the production of a spark.

2. An apparatus according to claim 1 in which the detector capacitor is connected to the gate of the f.e.t. through a high value resistor.

3. An apparatus according to claim 1 or claim 2 in which a leakage resistor is connected across the electrode means.

4. An apparatus according to Claim 1 or Claim 2 in which negative feedback is provided from the output of the f.e.t. to the gate thereof.

5. An apparatus according to any one of the preceding claims in which the spark generating circuit further includes means for slowly charging an ignition capacitor and rapidly discharging its through the primary winding of the transformer.

6. An apparatus according to claim 5 including a mains transformer, the secondary of which charges the ignition capacitor through a resistor and diode connected in series.

7. An apparatus according to claim 6 in which the secondary of the main transformer is connected to the detector capacitor through a pair of series connected resistors, the junction of the pair of resistors being connected to the electrode means to provide the a.c. supply.

8. An apparatus according to claim 7 in which a second resistor and second diode are connected in series to the secondary of the main transformer to balance the load on the transformer.

9. An apparatus according to any one of claims 5 to 8 including a second transistor controlled by the f.e.t. and connected so as to prevent charging of the ignition capacitor when a flame is detected.

10. An apparatus according to any one of the preceding claims including a solenoid operated gas control valve, the solenoid being energised when a flame is detected.

11. A combined spark ignition and flame detection apparatus for a gas burner

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. high/low flame control. The solenoid 2() is connected across a bridge rectifier 22 encrgised by the live and neutral mains conductor. The solenoid 21 is connected between the cathode of a diode D5, which has its anode connected to the live rail. and the anode of a thyristor T1 which has its cathode connected to the neutral rail, a resistor Rl2 being interposed between the solenoid 21 and the anode of the thyristor T,. A capacitor C5 is connected across the solenoid 21. The gate of the thyristor T1 is connected by a resistor R13 to the neutral rail and by a resistor R14 to the anode of a zener diode D6 which has its cathode connected by a capacitor C6 to the neutral rail. A resistor Rl5 connects the cathode of zener diode D6 to the anode of a thyristor T2 which has its cathode connected to the neutral rail and which also has the additional secondary winding of the transformer 111 connected across its gate-cathode. A resistor R,6 connects the anode of the thyristor T2 to the cathode of the diode D5. The solenoid 20 is always energised when the circuit is connected to the mains. The solenoid 21, however, is only energised if there is a sufficient voltage on the capacitor C6 to breakdown the zener diode D6 and fire the thyristor T1. The resistor R16 is chosen so that it takes several seconds after power has been applied to the circuit for the capacitor C6 to charge such a voltage and the thyristor T2 conducts to discharge the capacitor several times each second during sparking (i.e. when no flame is present). Thus during sparking the solenoid 21 is not energised but becomes energised to permit the full gas flow to the burner a few seconds after sparking ceases. In the event of the flame becoming extinguished by a cause other than a protracted removal of the gas supply, e.g. by a draught or by a very brief interruption of the gas supply, sparking will commence as explained above, after a delay set by resistor R,(,. During this delay full gas flow continues so that R", should be set to give a delay short enough to ensure that the amount of gas released into the space into which the burner fires is not sufficient to create an explosive mixture. The solenoid 21 will be re-energised when the flame is re-established. If the gas supply to the burner is cut off for a period, the sparking will take place and the solenoid 21 will be de-energised. When the gas supply is restored, the burner will go through its normal starting sequence on low flame and then switch to high flame. In a modification of the circuit shown in Figure 3, the variable delay circuit is replaced with the resistor R9 of Figure 1. WHAT WE CLAIM IS:

1. A combined spark ignition and flame detection apparatus for a gas burner comprising the combination of electrode means, a spark generating circuit including a stepup transformer having the electrode means connected to its secondary winding, a.c.

supply for supplying an a.c. signal across the electrode means, a detection capacitor connected in circuit with said a.c. supply and the electrode means whereby the detection capacitor acquires a charge when there is a flame in the region of the electrode means as a result of flame rectification, an f.e.t.

switch device controlled by the capacitor and arranged to prevent operation of the spark generating circuit when a flame is detected and a surge arresting device connected across the secondary winding and the electrode means to protect the f.e.t. from voltage spikes created in the transformer secondary winding during the production of a spark.

2. An apparatus according to claim 1 in which the detector capacitor is connected to the gate of the f.e.t. through a high value resistor.

3. An apparatus according to claim 1 or claim 2 in which a leakage resistor is connected across the electrode means.

4. An apparatus according to Claim 1 or Claim 2 in which negative feedback is provided from the output of the f.e.t. to the gate thereof.

5. An apparatus according to any one of the preceding claims in which the spark generating circuit further includes means for slowly charging an ignition capacitor and rapidly discharging its through the primary winding of the transformer.

6. An apparatus according to claim 5 including a mains transformer, the secondary of which charges the ignition capacitor through a resistor and diode connected in series.

7. An apparatus according to claim 6 in which the secondary of the main transformer is connected to the detector capacitor through a pair of series connected resistors, the junction of the pair of resistors being connected to the electrode means to provide the a.c. supply.

8. An apparatus according to claim 7 in which a second resistor and second diode are connected in series to the secondary of the main transformer to balance the load on the transformer.

9. An apparatus according to any one of claims 5 to 8 including a second transistor controlled by the f.e.t. and connected so as to prevent charging of the ignition capacitor when a flame is detected.

10. An apparatus according to any one of the preceding claims including a solenoid operated gas control valve, the solenoid being energised when a flame is detected.

11. A combined spark ignition and flame detection apparatus for a gas burner

substantially as hereinbefore described with reference to and as shown in Figure 1, or Figure 2, or Figure 3 of the accompanying drawings.

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