Abstract:
The disclosure includes an ignition control circuit comprising a transformer having at its secondary side an operating winding, and at least one filament winding. In a pre-heating phase, only the filament winding, which is normally connected to the filaments of a lamp, is active. In this way, the filaments of the lamp are heated first. After a predetermined time, the secondary operating winding is switched on, by a triac. This triac is a compact component producing no or little heat. The invention provides a simple and economic ignition circuit for gas discharge lamps.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an electronic circuit for controlling the ignition of a gas discharge lamp. 
   PRIOR ART 
   In such circuits before ignition of the lamp, filaments of the lamp are heated by a low voltage AC current. Then, after a delay, a high voltage AC operating current is applied across opposite filaments or electrodes of the lamp. By pre-heating the filaments the lifetime of fluorescent lamps is prolonged. The duration of the delay times between the pre-heating of the filaments and the ignition of the lamp is crucial. Known circuits consist of an oscillator and a transformer including an operating winding and a filament heating winding. Initially, current through the operating winding is switched off by way of a serial switch. Known circuits for example use a relay as the switching device. These relays take up a considerable amount of space and are not very reliable. As an alternative, circuits are designed using non-moving elements. One example is described in U.S. Pat. No. 4,214,186. There, a rectifier is provided having AC input terminals, one connected to an end of an operating winding and the other connected to an end of an auxiliary ignition winding by a resistor. A resistive ballast is connected across a pair of DC output terminals of the rectifier. This resistive ballast comprises an NTC. The normally high resistance of the NTC decreases after heating. This results in a current through the ignition winding, which results in a delayed ignition of a lamp. 
   Once the lamp is burning, a DC current is sill flowing through the NTC. This results in a continuous heating of the NTC component, which leads to energy loss and unwanted heating of the total circuit. 
   It is an object of the present invention to provide an ignition control circuit for the ignition of gas discharge lamps, which is compact and wherein heat production in the circuit is minimised. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is achieved by an electronic circuit for controlling the ignition of a pre-heat type gas discharge lamp comprising:
         an operating winding for supplying a high voltage AC to opposite electrodes of the gas discharge lamp;   filament power supply means for heating filaments of the gas discharge lamp;   a delay switch for switching on of the operating winding only after a predetermined delay time after a switching on of the filament power supply means,
 
wherein said delay switch comprises a solid-state switch.
 
By using a single solid state switch, a switching circuit is provided with very few components, and with no or little heat dissipation.
       

   In one embodiment the electronic circuit comprises;
         a transformer comprising primary windings and secondary windings, arranged to transform a low voltage AC current in a primary part of the circuit into a high voltage AC current in a secondary part of the circuit;   an oscillator connected to the primary windings of the transformer, and wherein the secondary windings comprise the operating winding.       

   Advantageously, a simple circuit is realized. 
   Moreover, in a further embodiment the present invention relates to a circuit as described above, wherein the filament power supply means comprise filament heating windings which are being part of the secondary windings. 
   By using filament heating windings as filament power supply means and by using only one transformer in which the secondary windings comprise both the operating winding and the filament windings, an even simpler circuit is realized. 
   In a preferred embodiment the solid-state switch comprises a triac. The triac is very suitable for switching an AC current. The triac is a solid state switch which produces very little heat. This saves energy which may be very important in application wherein a lamp is powered by a low power a battery. 
   Preferably a capacitor is arranged in the secondary part of the circuit which contains the operating winding and the triac, wherein the capacitor and the operating winding have been dimensioned in such a way that the overall frequency of the circuit decreases as soon as the triac is switched on and the discharge process in the fluorescent lamp has started. 
   The present invention also relates to a lighting fixture comprising an electronic circuit as described above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Below, the invention will be explained with reference to some drawings, which are intended for illustration purposes only and not to limit the scope of protection as defined in the accompanying claims. 
       FIG. 1  shows an ignition control circuit according to the invention. 
       FIG. 2  is a schematic diagram of a secondary pan of the circuit according to an embodiment of the invention, connected to a gas discharge lamp. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows an ignition control circuit according to the invention, connected to a low voltage DC voltage source  1  via a positive  2  and a negative  4  terminal. The voltage source  1  may for example be a 12 or 24 DC voltage source. At the positive terminal  2  a first side of a fuse  3  is connected, the other side of which is connected to a junction point  5 . To the junction point  5 , a diode  6  and a capacitor  7  are connected at their first side, both of which are at their other side, connected to the negative terminal  4 . In addition, the junction point  5  is connected to a capacitor  9 , which is serially connected with a resistor  11  and another resistor  13 . The junction point  5  is also connected to a resistor  15 , which is serially connected to the collector of a transistor  17 . The base of transistor  17  is connected to a junction point between resistor  11  and resistor  13 . The emitter of transistor  17  is connected to the negative terminal  4 . The junction point  5  is also connected to a first side of two resistors  19 ,  21 , which are connected with each other at their second side via a primary winding  23  of a transformer. The second side of the resistor  19  is also connected to the base of a transistor  25 . The second side of the resistor  21  is connected to the base of another transistor  27 . Another primary winding  28  of the transformer is connected with one side to junction point  5  and with another side to a junction point  8 . The collector of the transistor  25  is connected to one side of yet another primary winding  29 , the other side of which is connected to the junction point  8 . The collector of the transistor  27  is connected to one side of another primary winding  31 , the other side of which is connected to the junction point  8 . The emitters of both transistors  25  and  27  are connected to negative terminal  4 . Between the collectors of the transistors  25  and  27 , a capacitor  30  is situated 
   A second part of the ignition circuit comprises three secondary windings of the transformer mentioned above, i.e. an operating winding  33  and two filament windings  39 ,  41 . The operating winding  33  connects an output terminal  47  with an output terminal  53  via a series connection of a capacitor  35  and a triac  37 . The filament winding  39  connects output terminal  47  to an output terminal  49  via a capacitor  43 , and the filament winding  41  connects output terminal  53  to an output terminal  51  via a capacitor  45 . Triac  37  has a control gate that is controlled by a low voltage junction in the primary part of the circuit. In the embodiment shown, this low voltage junction is the collector of transistor  17 . 
   In this circuit the part comprising the primary windings  23 ,  29 ,  31  can be regarded as a primary part of the circuit, the part of the circuit comprising the secondary windings  33 ,  39 ,  41  is the secondary part of the circuit. 
   When the voltage source  1  is switched on, the primary part of the circuit, i.e. the resistors  19  and  21 , the primary windings  23 ,  29  and  31  and the transistors  25  and  27  will behave as an oscillator. The oscillator will have a startup-resonance frequency in accordance with the impedance of the circuit as determined while the triac still in its off-state. This oscillator contains the primary windings  23 ,  29 ,  31  of the transformer. Via a magnetic core of the transformer, a magnetic field produced by the oscillating primary windings  23 ,  29 ,  31  may produce an oscillating current in all the secondary windings  33 ,  39 ,  41 . 
   As will be obvious to the skilled person, other embodiments are possible. In stead of using only one transformer, two or more transformers can be used. For example, the operating winding may be part of one transformer, while the filaments maybe part of another transformer. Further, it is noted that alternatively the filaments may be heated by a different type of power supply such as a DC source, in stead of filament heating windings. 
     FIG. 2  is a schematic diagram of the secondary part of the ignition circuit according to an embodiment of the invention. In  FIG. 2 , the circuit is connected to a gas discharge lamp  61 . The gas discharge lamp  61  comprises two filaments  63 ,  65 . In the gas discharge lamp  61  a gas is present which will get discharged if a sufficient voltage is applied between the filaments  63 ,  65 .  FIG. 2  shows the four circuit output terminals  47 ,  49 ,  51 ,  53  as already described in FIG.  1 . If the circuit is connected to the gas discharge lamp  61 , the terminals  47  and  49  are connected to filament  63 , and the terminals  51  and  53  are connected to filament  65 . When a magnetic field is present in the transformer, an AC current will flow through the filament windings  39 ,  41 . This results in heating of the filaments  63 ,  65  of the lamp  61 . 
   The operating winding  33  is arranged to produce a high voltage. This voltage will ignite gas present in the gas discharge lamp  61 , as soon as the triac  37  switches to the on state. The switching of the triac  37  is controlled by a voltage divider formed by resistor  15  and transistor  17 . This transistor  17  is controlled by another voltage divider formed by capacitor  9 , resistor  11  and resistor  13 . If the voltage source  1 , connected between terminals  2  and  4 , is switched on, the capacitor  9  first functions as a ‘short circuit’ and the base of transistor  17  receives a voltage to open transistor  17 , i.e. to bring the collector of transistor  17  to a voltage close to the emitter voltage. 
   Consequently, triac  37  is closed. However, capacitor  9  will gradually be charged, resulting in a decreasing potential at the base of the transistor  17 . The transistor  17  functions as a switch to control the gate of the triac  37 . Once the transistor  17  closes, the potential at the gate of triac  37  is triggered. This will result in a conducting triac  37 , which in turn facilitates an operating AC current through the operating winding  33 . Because the capacitor  7  needs some time to be charged, the triac  37  will only be switched on after a predefined delay period which is defined by the value of the capacitor  9 , the resistor  11  and the resistor  13 . 
   In the secondary part of the circuit, the operating winding  33  and the capacitor  35  form a resonator, which has a resonance frequency determined by the capacitance and inductance of the elements  33  and  35 , respectively. Preferably, the operating winding  33  and the capacitor  35  are dimensioned in such a way that the resonance frequency of the resonator is lower than the startup-resonance frequency of the oscillator in the primary part of the circuit. Thus, since the overall impedance of the circuit changes when the triac enters its on-state (to switch on the operating winding  33 ) and a discharge in the gas discharge lamp  61  is started, the frequency of the oscillator in the primary circuit during the pre-heat period, at startup resonance frequency F fil , will drop considerably at that moment, resulting in an operational frequency F op , which can be approximately two-thirds of F fil  or less. The higher frequency F fil  causes higher heat production in the filaments, which is favourable for a quick start in cold circumstances. Lower operational frequency results in lower power consumption by the filaments and lower heat production, which are both advantageous in battery powered and/or cooled environments. This effect may also extend the lifetime of the filaments. 
   It is further noted that although in  FIG. 1  the capacitor  35  is located in between the operating winding  33  and the triac element  37 , persons skilled in the art will recognise that the capacitor  35  may be located anywhere in the secondary part of the circuit. 
   The circuit according to the present invention, does not use an NTC or a PTC. Heat dissipation is minimised in this way. The invention provides a very compact ignition circuit, which can be used in a lighting fixture for a gas discharge lamp, like for example a fluorescent lamp. The circuit according to the invention can be powered by a low voltage DC source, which is favourable in many outdoor applications. 
   The diode  6  and the capacitor  7  are not relevant for the invention. They are used as a stabilising circuit. The fuse  3  is optional too. Furthermore, the resistors may comprise several resistors in parallel, as is know to the person skilled in the art. 
   It will be clear that other embodiments are possible. For example the triac  37  may be replaced by a rectifier and a transistor, or any other solid state switch.