GB2339945A

GB2339945A - An ignition and flame detection circuit

Info

Publication number: GB2339945A
Application number: GB9916130A
Authority: GB
Inventors: Richard Arthur George Kinge
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1998-07-25
Filing date: 1999-07-12
Publication date: 2000-02-09
Anticipated expiration: 2019-07-12
Also published as: GB9816172D0; GB2339945B; GB9916130D0

Description

2339945 IGNMON SYSTEMS This invention relates to ignition systems.

The invention is more particularly concerned with systems for both causing and detecting ignition.

Conventional ignition systems often include some means for detecting the presence or absence of a flame caused by the ignition. One arrangement for detecting the presence of a flame employs the flame-rectification principle where the ionized flame enables more current to flow from a small area electrode to a large area electrode than in the opposite direction. Most commonly, the small area electrode is provided by the spark electrode and the large area electrode is the grounded housing of the burner itself If an alternating current is applied to the small area electrode, the presence of a flame is indicated by a negative dc bias component, which can be readily detected in conventional ignition systems.

In some ignition systems, however, this form of flame detection cannot be used for various reasons. If the ignition circuit includes a path to ground via, for example, a rectifier circuit, this may distort the applied alternating flame detection signal. Also, if the circuit produces sparking pulses continuously or at irregular intervals, it may not be possible to isolate the flame detection signals from the much larger sparking voltages.

It is an object of the present invention to provide an improved ignition system.

2 According to one aspect of the present invention there is provided an ignition system including an ignition circuit for supplying ignition pulses to a spark electrode, the spark electrode being closely spaced from a ground electrode to define a spark gap across which a spark is produced, the ignition circuit including rectifier means and capacitor storage means in which energy is stored for producing a spark across the gap, and the system including a flame detection circuit for supplying an alternating signal to the spark electrode, and means for preventing passage of the alternating signal to ground except via the spark gap and when a flame is present at the gap.

The means for preventing passage of the alternating current preferably includes voltage clamping means such as a voltage-dependent resistor. The rectifier means may have two outputs, one output being connected to ground and the other output being connected to the spark electrode, the means for preventing passage of an alternating signal to ground being connected in the other output between the rectifier means and the flame detection circuit. The storage capacitor is preferably connected in series between the spark electrode and a source of the alternating current. The storage capacitor and source of alternating current are preferably connected to ground via voltage clamping means arranged to prevent any substantial current flow to ground at the voltages of the alternating current.

According to another aspect of the present invention there is provided an ignition system including a spark electrode closely spaced from a return electrode to form a spark gap, pulse generator means, a step-up transformer arranged to step up the voltage of pulses from said step-up transformer, the step-up transformer having two secondary output terminations connected via a rectifier circuit respectively to a return path and to the spark electrode, first 3 voltage clamping means connected between the spark electrode and the rectifier circuit, a storage capacitor having one electrode connected between the spark electrode and the first voltage clamping means and another electrode connected to the return path via second voltage clamping means, a source of alternating voltage arranged to supply an alternating signal to the spark electrode via the storage capacitor, and means for detecting flow of part of the cycle of the alternating current from the spark electrode to the return electrode during the presence of a flame at the gap.

A conventional ignition system including a flame detection circuit, and an ignition system according to the present invention including a flame detection, will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure I is a circuit diagram of a conventional system; and Figure 2 is a circuit diagram of a system according to the present invention.

With reference to Figure 1, the conventional system includes a pulse generator circuit I connected with the primary winding 2 of a step-up transformer 3. The secondary winding 4 of the transformer is connected at one end or termination to a spark electrode 5 positioned in the vicinity of the outlet of a gas burner 6. The gas burner 6 has a metal housing 7 connected to a ground return path. Reference in this specification to ground is intended to include other potentials. The other end or termination of the secondary winding 4 is connected via line 8 to a ground return path via a voltage-dependent resistor VDR 9, which clamps the voltage across it to about 270 volts.

4 The system includes a flame detection circuit indicated generally by the numeral 10. The circuit 10 has a medium voltage ac supply I I one side of which is connected to the ground return path. The other side of the ac supply 11 is connected via a protective resistor 13 and a capacitor 14 to line 8 at a junction 12 between the secondary winding 4 and the VDR 9. The flame detection circuit 10 also includes a comparator 15 having one input 16 connected to a reference source of negative dc voltage 17. The other input 18 of the comparator 15 is connected to the line 8 at a junction 19 between the junction 12 and the VDR 9. An integrator 20 is connected between the junction 19 and the second input 18 of the comparator 15 to sample signalson line 8.

In operation, the pulse generator circuit I supplies pulses to the primary winding 2 of the transformer 3, which steps these up to short- duration high voltage pulses of about l5kV. These pulses are applied to the electrode 5 causing a discharge across the gap between electrode and the burner housing 7. This causes ignition of the gas-air mixture flowing out of the burner 6. The VDR 9 ensures that the voltage at the junctions 12 or 19 does not rise above about 270 volts during the production of the ignition pulses. The flame detection circuit 10 applies a much lower alternating voltage, via junction 12, to the spark electrode 5. When no flame is present, this alternating signal will be integrated by the integrator 20 producing a zero signal to the input 18 of the comparator 15. This causes the comparator 15 to produce a "no-flame" output to a display or other utilisation means 21. When a flame is present, this allows preferential conduction of the positive half-cycles of the alternating signal at the electrode 5 to the ground return path via the ionized flame and the burner housing 7. As a result of this, the integrated alternating signal produced by the integrator 20 becomes negative, thereby causing the comparator 15 to produce a "flame present" output signal.

Turning now to the circuit shown in Figure 2, this has a pulse generator circuit 40 connected across the primary winding 41 of a step-up transformer 42. Opposite ends of the secondary winding 43 of the transformer are connected to a rectifier circuit 44 to provide a rectified output. The negative side of the output is connected to a ground return path. The positive side of the output is connected both to a spark electrode 45 and to one electrode of a storage capacitor 46, via a resistor 47 and a VDR 48, which clamps the voltage across it to about 270 volts. The other electrode of the storage capacitor 46 is connected to ground via a second VDR 49, which clamps voltage across it to about 270 volts. The spark electrode 45 is positioned in the vicinity of the outlet of a gas bumer 50. The gas bumer 50 has a metal housing 51 connected to the ground return path. An oscillator 140 is connected to the pulse generator 40 to switch it on and off periodically, typically enabling and interrupting supply of ignition pulses for equal intervals of one second duration. A similar ignition circuit employing a rectifier circuit to charge a capacitor is described in GB2327984.

The flame detection circuit is indicated generally by the numeral 52. The circuit 52 includes an ac supply 53 of medium voltage, such as a conventional mains supply voltage of about 200 volts peak-to-peak. One side of the supply is held at ground potential and the other side is connected via a protective resistor 54 and the storage capacitor 46 to the spark electrode 45. The flame detection circuit 52 includes a comparator 55 having one input 56 connected to a reference source of negative voltage 57. The other input 58 of the comparator 55 is connected to the spark electrode 45 via an integrator 59. The storage capacitor 46 6 prevents the passage of any dc component of the ac signal produced by the supply 53 to the input of the integrator 59 and, therefore, serves a dual function as an energy store for the spark voltage and a high impedance feed of the flame detection current to the spark electrode 45.

In operation, pulses from the generator 40 are stepped up by the transformer 42, are rectified and are supplied to one electrode of the storage capacitor 46, the other electrode of which is held close to ground voltage by the VDR 49. When, after several pulses, the charge on the capacitor 46 has built up sufficiently, breakdown occurs between the spark electrode 45 and the burner housing 51 causing ignition of the gas- air mixture flowing out of the burner 50. The alternating flame detection signal is supplied continuously to the spark electrode 45 but is masked by the much higher positive dc ignition voltage while the pulse generator 40 is on. Because the ignition voltage is always positive during spark production, it prevents the production of a false flame present signal- at the integrator 59.

When the oscillator 140 interrupts supply of ignition pulses by the generator 40, the flame detection circuit 52 is able to respond. When no flame is present, the alternating signal applied to the spark electrode 45 is unaffected and the output of the integrator 59 is, therefore, zero. The comparator 56 provides a "no-flame" output signal to a display or other utilization means 60. When a flame is present at the burner 50, the positive half-cycle is preferentially conducted to the ground return path by the ionized flame, thereby giving a negative bias to the input of the integrator 59. This causes the integrator 59 to apply a negative signal to the input 58 of the comparator 55, which produces a "flame present" output signal to the 7 utilization means 60. A flame present signal may be used to halt the production of further sparks while the flame is present The VDR 48 prevents passage of the alternating flame detection signal, because of its lower voltage, to the ground return path via rectifier circuit 44 but will conduct the high voltage pulses from the rectifier 44 during spark production. Similarly, the second VDR 49 prevents passage of the alternating flame detection signal to the ground return path via the connection between the storage capacitor 46 and ground but will conduct during spark production to provide the return path for the spark voltage. Although some of the available voltage to charge the storage capacitor 46 is lost because of the voltage drop across both the VDR 48 and the VDR 49, the amount is insignificant compared with the voltage available from the transformer 42.

Various modifications are possible. For example, instead of VDRs, other voltage clamping devices could be used, such as, air gaps, gas discharge tubes, zener or avalanche diodes or diacs.

8

Claims

1. An ignition system including an ignition circuit for supplying ignition pulses to a spark electrode, the spark electrode being closely spaced from a ground electrode to define a spark gap across which a spark is produced, wherein the ignition circuit includes rectifier means and capacitor storage means in which energy is stored for producing a spark across the gap, and wherein the system includes a flame detection circuit for supplying an alternating signal to the spark electrode, and means for preventing passage of the alternating signal to ground except via the spark gap and when a flame is present at the gap.

2. An ignition system according to Claim 1, wherein the means for preventing passage of the alternating signal to ground includes voltage clamping means.

3. An ignition system according to Claim 2, wherein the voltage clamping means is a voltage-dependent resistor.

4. An ignition system according to any one of the preceding claims, wherein the rectifier means has two outputs, wherein one output is connected to ground and the other output is connected to the spark electrode, and wherein the means for preventing passage of an alternating signal to ground is connected in said other output between the rectifier means and the flame detection circuit.

9 5. An ignition system according to any one of the preceding claims, wherein the storage capacitor is connected in series between the spark electrode and a source of the alternating current.

6. An ignition system according to Claim 5, wherein the storage capacitor and the source of alternating current are connected to ground via voltage clamping means arranged to prevent any substantial current flow to ground at the voltages of the alternating current.

7. An ignition system including a spark electrode closely spaced from a return electrode to form a spark gap, pulse generator means, a step-up transformer arranged to step up the voltage of pulses from said step-up transformer, the step-up transformer having two secondary output terminations connected via a rectifier circuit respectively to d return path and to the spark electrode, first voltage clamping means connected between the spark electrode and the rectifier circuit, a storage capacitor having one electrode connected between the spark electrode and the first voltage clamping means and another electrode connected to the return path via second voltage clamping means, a source of alternating voltage arranged to supply an alternating signal to the spark electrode via the storage capacitor, and means for detecting flow of part of the cycle of the alternating current from the spark electrode to the return electrode during the presence of a flame at the gap.

8. An ignition circuit substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

9. Any novel and inventive feature or combination of features as hereinbefore described.