The present invention refers to improvements in ignition devices for liquid fuel burners and to control apparatus for said burners which incorporates said improvements.
The invention particularly refers to a burner control apparatus as described in Italian patent application No. 30885 A/75 of the applicant. Essentially, said apparatus comprises a motor actuated by a means which generates a heat request signal, a fan for feeding air to the burner, a pump for feeding liquid fuel to the burner, a valve means which in a "closed" position so as to inhibit the flow of liquid fuel from the pump to the burner and which is in an "open" position so as to permit such a flow of fuel, a means for producing a spark for igniting the liquid fuel which flows out of the burner, which ignition means comprises ignition electrodes fed by the secondary winding of a high frequency ignition transformer, and means for feeding the primary winding of said transformer with a low voltage of a high frequency. Said means for feeding the primary winding of the transformer comprises an oscillating circuit which in turn is fed a unidirectional current. Said unidirectional current is in turn obtained by rectifying an alternating current, generally having a low frequency, usually of the line frequency, and preferably derived from the motor winding.
Summing up, therefore, a low voltage of a low frequency, generally of the line frequency, is rectified so as to obtain a unidirectional low voltage, and said recrified low voltage is fed to an oscillating circuit which produces a low alternating voltage of a high frequency with which the primary winding of a transformer is fed, the secondary winding of which furnishes a high voltage of a high frequency current to the electrodes for the production of the spark used for igniting the fuel.
Furthermore, the invention is also applicable to ignition devices which are different from that described in said patent application, as will appear hereinafter and within the limits of which will be specified.
In the control boxes for burners in general, and more particularly in the one described hereinbefore, a so called lock out relay is included, the function of which is to stop the operation of the burner by stopping the feed thereto if the flame is not formed after a certain predetermined time, or if, once the flame has disappeared during the operation and the ignition cycle has been repeated, the flame is not formed anew. Said lock out relay comprises in general, and in the case which is relevant here, a bimetallic lamina (or analogous element) which, when heated by a winding through which current flows, becomes gradually deformed with the passage of time until, when a predetermined period of time has passed and if the flame has not formed and therefore the current has not ceased to pass through the heating winding, it interrupts in a suitable way the electrical feed circuit of the burner (i.e.-locks out the burner).
Obviously, the time required by the bimetallic lamina to become deformed to the extent necessary to cause the lock out depends, non-linearly, on the intensity of the current which flows through the heating winding. In the type of device to which the present invention applies, as previously recalled, said current--as that will better be seen hereinafter--is practically the same as absorbed by the aforesaid oscillating circuit--except that the rectifier intervenes--and is anyway closely related to the length of the spark, viz. to the distance between the ignition electrodes. Substantially the same current is therefore applied both to said winding and to said oscillating circuit and therefore to the ignition transformer. The current which flows through the winding is therefore at a maximum when the electrodes are correctly positioned, which occurs in practice when they are at a distance of about 4 mm apart from one another and the current decreases if they are displaced from the optimal position. This causes insecurity in the operation of the lock out device, because the blocking time may vary according to the position of the electrodes. Said variations of the blocking time may render the apparatus insufficiently safe.