ELECTRONIC IGNITION CIRCUIT FOR OIL BURNERS

Abstract: The present invention relates to an electronic ignition circuit for oil burners, in particular a high voltage high frequency ignition circuit, said circuit comprising an oscillator and a high frequency high voltage transformer wound on a core (1). The core (1) on which the high frequency transformer is wound forms a substantially closed magnetic path for the magnetic flux generated by the winding of the transformer (T1'), when current flows through the windings (4a-4d).

Description: Electronic ignition circuit for oil burners. The present invention relates to an electronic ignition circuit for oil burners, in particular a high voltage high frequency ignition circuit, where the circuit comprises an oscillator and a high frequency transformer wound on a core, and a transformer for such a circuit . In the oil burners typically for heating in single-family houses, electric ignition is employed. In such burners a blower creates an airflow into which the oil is sprayed through a spray nozzle to form an oil mist in the airflow. In order to ignite the oil an ignition spark gap is located in the vicinity of, but not too close to the spray nozzle. Typically- the spark gap is located downstream from the blower and the spray nozzle with respect to the airflow. The distance from the spray nozzle to the spark gap in the direction of the airflow is, how- ever, quite small, e.g. approximately 1-2 mm. The spray nozzle and the spark gap are usually off-set slightly in the direction across the airflow, so as to prevent the oil mist from reaching the electrodes of the spark gap. Other arrangements are of course possible. In particular, the spark gap could be located upstream from the spray nozzle. The important thing, however, is that the ignition spark gap is located sufficiently far away from the spray nozzles, or rather the oil mist, to prevent soot forming on the electrodes from unburned oil from the oil mist. On the other hand the spark gap must be located close enough to the spray nozzle to allow the arc formed in the spark gap to actually reach the oil mist and ignite it. When the arc is formed between the electrodes of the spark gap it will be deformed inter alia by the airflow so as to extend downstream into the oil mist. This, however, cools the arc, and under certain conditions, the part of the arc, which extends into the oil mist, may only be able to ignite the oil mist with a delay, or even not be able to ignite the oil mist at all . In both cases unburned oil products are blown away into the environment causing pollution thereof. As already stated, the present invention in particular relates to high voltage high frequency ignition. In fig. 1 a known electronic circuit for producing the arc is shown. The circuit comprises a spark gap G connected to the secondary of a high voltage high frequency transformer TI . The high fre- quency transformer TI is wound on a ferrite rod. An example of such a high frequency high voltage transformer is found in US-A-4683518. The electronic circuit incorporates an oscillator circuit Rl, R2 , R3 , R4, C3, C4, C5, C6, DZ1, DZ2 , TR1 and TI . It should be noticed that the transformer TI is coupled with the basis of the transistor TR1, so as to provide the feedback needed for the oscillator. The electronic circuit further comprises a half-wave rectifier circuit Dl, C2 as well as noise suppression circuitry LI, Cl, R5 , R6 , the details of which are not considered relevant for the present invention and will not be described in further detail . The oscillator is fed with the half-wave rectified current from the half-wave rectifier, and thus produces high frequency bursts to the high frequency transformer TI. Though a prior art ignition unit with the above circuit has worked well over a number of years, the size of the components in the circuit places limita- tions on the size of the ignition unit. The size of the unit also limits the freedom in locating it. Both the size of the unit and the limitation in locating it puts constraints on the size of the burner, which makes it difficult to build compact burners. It is a first object of the invention to build more compact oil burners . It is a second object of the invention to improve the ignition in oil burners. According to a first aspect of the present in- vention, these objects are met by an electronic ignition circuit for oil burners according to the opening paragraph, characterized in that the core on which the high frequency transformer is wound forms a substantially closed magnetic path for the magnetic flux generated by the windings of the transformer, when current flows through the windings . By the use of a core forming a substantially closed magnetic path for the magnetic flux a substantial reduction of the size transformer, because not only the core itself but also the windings may be reduced in size. According to a preferred embodiment the core comprises two parts between which a gap is provided. This allows for easy assembly of the trans- former. In a first preferred embodiment the core comprises at least one essentially E shaped core and a yoke. In a second preferred embodiment the yoke also comprises a second essentially E shaped core. In a third preferred embodiment the gap is formed between the central leg of the E shaped core and the yoke . All of the above embodiments improve the advantage of using a MOS-FET transistor in the oscillator circuit . Thus, in a further preferred embodiment of the electronic circuit the oscillator comprises a MOS-FET transistor. The use of a MOS-FET is advantageous as it is more power efficient as compared to traditional bipolar transistors, and thus contributes to the over- all efficiency of the electronic circuit. Moreover it allows a greater deflection of the arc. According to a second aspect the present invention provides a burner comprising the above electronic circuit is provided. According to a third aspect the invention involves the use, in an electronic ignition circuit for oil burners, of a high voltage high frequency transformer comprising a core forming a substantially closed magnetic path for the magnetic flux generated by the windings of the transformer, when current flows through the windings. According to a fourth aspect the invention relates to a high voltage high frequency ignition transformer for oil burners, where the transformer comprises a core forming a substantially closed magnetic path for the magnetic flux generated by the windings of the transformer, when current flows through the windings . The present invention will now be described in greater detail by means of a non-limiting example and with reference to the drawings on which, Fig. 1 is a diagram of an electronic ignition circuit for oil burners, Fig. 2 is a perspective view in partial section of a substantially closed core employed in a pre- ferred embodiment of the present invention to form a substantially closed magnetic path for the magnetic flux generated by the windings of the transformer, Fig. 3 is a schematic drawing of an ignition spark gap an oil burner, and Figs. 4a-4c illustrate the ignition arc under different experimental conditions. Fig. 1 is a diagram of an electronic ignition circuit for oil burners. Except for the high voltage high frequency transformer TI, which is in the present invention substituted by a high voltage high frequency transformer TI', this circuit corresponds to a prior art circuit. For the sake of convenience, i.e. to avoid having two substantially identical fig- ures, reference will be made to fig. 1 also in the context of the present invention, only with TI' replacing TI . As already mentioned, the circuit comprises a spark gap G connected to the secondary of a high voltage high frequency transformer TI ' . The spark gap of the burner is located remote from the ignition unit containing the electronic circuit including the high voltage, high frequency transformer TI' . The electronic ignition circuit incorporates an oscilla- tor circuit Rl, R2 , R3 , R4 , C3 , C4 , C5 , C6, DZ1, DZ2 , TR1 and TI' . It should be noticed that the transformer TI' is coupled with the basis of the transistor TR1, so as to provide the feedback needed for the oscillator. The electronic circuit further comprises a half-wave rectifier circuit Dl, C2 as well as noise suppression circuitry LI, Cl, R5 , R6, the details of which are not considered relevant for the present invention and will not be described further. The transformer TI' is shown in perspective view and partial section in fig. 2. The transformer TI' has a substantially closed core 1 comprising two parts la and lb, which are in the preferred embodiment identical, both being substantially E shaped and both preferably of a ferrite material . The two parts are arranged in an opposing mirror image configuration. For convenience, only one of the E shapes will be described below. The middle leg 2 of the E is preferably cylindrical, so as to match a likewise cy- lindrical bobbin 3 on which the windings 4a, 4b, 4c are wound. The two outer legs 5 of the E are essentially rectangular in cross section. It is, however preferred that the inner side is curved, cf. fig. 2, so as to match the outer shape of the bobbin 3. Evi- dently also the outer side may be curved, e.g. so that the cross sections are circular segments matching the outer shape of the bobbin 3. The middle leg 2 is slightly shorter than the two outer legs 5. Thus, when the outer legs 5 of the two respective parts la, lb abut, a gap 6 is formed between the middle legs 2. The interconnections 7 between the outer legs 5 and the central leg 2 are slightly tapered, so as to present a trapeze shaped cross section. On the bobbin 3 four windings are located in appropriate grooves. There are two secondary high voltages windings 4b and 4c each distributed between several grooves, three for each respective secondary winding in fig. 2, so as to reduce the voltage between adjacent turns. The two secondary windings are connected in series, so as to have a neutral point between them. In the burner this neutral point may be grounded using the metal parts of the burner, thereby reducing electrocution hazards, and the risk of unde- sired sparks to conducting parts of the burner. In the bobbin 3 there is a further groove in which the primary low voltage winding 4a is located. Together with the primary winding 4a the coupling winding 4d (not shown in fig. 2) , which is connected to the basis of the transistor TRl, is located. In the preferred embodiment the core used is a pair of ETD 45G 19 14 07 - 050 core parts, supplied by Iskra Feriti, Ljubljana, Slovenia. The use of these two core parts gives a gap of approximately 1 mm. The overall dimensions for each core part is ap- proximately 19,6 mm x 7,4 mm, giving the core an overall weight of approximately 13,4 g, which is a substantial reduction over the rod core presently employed by the applicant, weighing approximately 16 g. Moreover, the use of this core, which forms a substantially closed magnetic path for the magnetic flux generated by the windings of the transformer, when current flows through the windings, reduces the amount of copper needed for the transformer windings with approximately 20 percent. With a view to the above, preferred embodiment, it is appropriate to mention that substantially closed in the present context is not to be understood as a including a fully closed magnetic circuit . Such fully closed magnetic circuits, do not give rise to the advantages of the invention, as will be apparent from description of experiments given below. Moreover, the use of the core according to the present invention, which forms a substantially closed magnetic path for the magnetic flux, makes the use of a MOS-FET type transistor as the transistor TRl in the oscillator circuit. The MOS-FET generates sharper flanks in the high frequency oscillations produced, and gives rise to a larger deflection of the arc, as will be described below in connection with Figs. 4a- 4c. Further, the use of a MOS-FET transistor is more energy efficient . It should also be mentioned that though, in the preferred embodiment, a pair of identical core parts are used, other core part configurations may be used. I.e. the configuration of the two parts of the core may be any conventional e.g. such as C-I, C-C, E-I, E-C, E-E, or C-T. Experiments have shown that the gap is essential for the behaviour of the arc in the airflow. Fig. 3 illustrates a spark gap G with two electrodes 12, 13 and two terminals 14 and 15 for connection to secondary of the ignition transformer TI' via a cable (not shown) . When high voltage is applied to the spark gap a plasma arc is formed between the electrodes 12, 13. Partly because of the airflow, as mentioned initially, the arc is blown away from the electrodes. At the same time the airflow cools the arc. It has been found that in a given air flow the distance that the arc is blown away from the spark gap depends on the high voltage high frequency transformer TI' in the electronic ignition circuit. Figs. 4a-4c illustrate the ignition arc under experimental conditions, during tests performed with different high voltage high frequency transformers inserted in the electronic circuit of fig 1. The darker areas illustrate the central channels of hot, highly luminous plasma emitting white light, whereas the grey areas illustrate the outer cloud of colder blue/white luminous plasma. The scale coves 0-2 cm. Fig. 4a illustrates the arc, which is achieved if a closed core is used for the high voltage high frequency transformer. Fig. 4b illustrates the arc, which is achieved with the prior art high voltage high frequency transformer. Fig. 4c illustrates the arc achieved using the high voltage high frequency transformer of the preferred embodiment using ETD 45G 19 14 07 - 050 core parts and a gap of 1 mm. As can be seen, the luminous cloud of plasma extends about 13 mm from the tips of the electrodes in Fig. 4b where the prior art transformer TI is used in the ignition circuit. If instead, the transformer according to the present invention is used as illustrated in fig. 4c the luminous cloud of plasma ex- tends more than 15 mm from the tips of the electrodes. On the other hand, if a transformer with a closed core is used, the arc is only deflected slightly and the luminous cloud extends only about 5 mm from the electrodes . Thus, not only does the present invention provide a longer arc of blue/white plasma- reaching deeper into the oil mist for ignition, but also the white-hot central arc channel reaches as far using the present invention as does the colder blue/white plasma when using the prior art. Accordingly a much better ignition of the oil mist is achieved.

Claims: P A T E N T C L A I M S 1. Electronic ignition circuit for oil burners, in particular a high voltage high frequency ignition circuit, said circuit comprising an oscillator and a high frequency transformer (TI') wound on a core (1), c h a r a c t e r i z e d in that the core (1) on which the high frequency transformer is wound forms a substantially closed magnetic path for the magnetic flux generated by the windings of the transformer (TI'), when current flows through the windings (4a- 4d) . 2. Electronic ignition circuit according to claim 1, c h a r a c t e r i z e d in that the core comprises two parts (la, lb) between which a gap (6) is provided. 3. Electronic ignition circuit according to any preceding claim, c h a r a c t e r i z e d in that the core comprises at least one essentially E shaped core (la) and a yoke (lb) . 4. Electronic ignition circuit according to claim 3, c h a r a c t e r i z e d in that the yoke (lb) comprises a second essentially E shaped core. 5. Electronic ignition circuit according to any one of claims 3 to 4, c h a r a c t e r i z e d in that the gap (6) is formed between the central leg of the E shaped core (la) and the yoke (lb) . 6. Electronic ignition circuit according to claim any one of the preceding claims, c h a r a c - t e r i z e d in that the core (1) is a ferrite core . 7. Electronic ignition circuit according to any one of the preceding claims, c h a r a c t e r i z e d in that the oscillator comprises a MOS-FET transistor (TRl) . 8. Electronic ignition circuit according to any one of the preceding claims, c h a r a c t e r i z e d in that the core (1) weighs less than 14 g. 9. Oil burner comprising an electronic ignition circuit according to any one of the preceding claims. 10. The use, in an electronic ignition circuit for oil burners, of a high frequency transformer (TI') comprising a core (1) forming a substantially closed magnetic path for the magnetic flux, generated by the windings (4a-4d) of the transformer (TI'), when current flows through the windings (4a-4d) . 11. High frequency ignition transformer for oil burners, c h a r a c t e r i z e d in that the transformer (TI') comprises a core (1) forming a sub- stantially closed magnetic path for the magnetic flux, generated by the windings (4a-4d) of the transformer (TI'), when current flows through the windings (4a-4d) . 12. High frequency ignition transformer accord- ing to claim 11, c h a r a c t e r i z e d in that the core (1) comprises two parts (la, lb) between which a gap (6) is provided. 13. High frequency ignition transformer according to any one of claims 11 or 12, c h a r a c - t e r i z e d in that the core (1) is a ferrite core. 14. High frequency transformer according to any one of claims 11 to 13, c h a r a c t e r i z e d in that the core (1) weighs less than 14 g.

Inventors: PETERSEN KEN, PETERSEN, KEN
Applicants: DANFOSS AS, PETERSEN KEN, DANFOSS A/S, PETERSEN, KEN
Application year: 2001
Main classification: F23Q3
IPC codes: F23Q3, H01F3, H01F38