Pulse generating circuit
The present invention relates to a pulse generator circuit and more particularly to a circuit which can generate a high voltage pulse, suitable for lighting the gas from a gas cigarette lighter, from a low voltage d.c. source. 
     According to the present invention there is provided a pulse generating circuit including a first transformer the primary winding of which is connected in an oscillator circuit and the secondary winding of which is connected with a capacitor and diode in a charging path. The capacitor also is connected with the primary winding of a second transformer in a discharge path upon the operation of a switch, resulting in a pulse being generated through a secondary winding of the second transformer upon discharge of the capacitor. The invention is characterized in that the capacitor discharge path includes a part or the whole of the secondary winding of the first transformer. 
     An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing which shows a circuit diagram of a pulse generating circuit. 
     An oscillator circuit is formed by a transistor TR1, windings L 1 , L 2  of a transformer T 1 , diode D 1 , Zener diode D 2  and a resistor R 1 . Battery B 1  provides a low voltage d.c. supply which is connectable to the oscillator by means of a terminal 1 of a switch S. 
     A further winding L 3 of transformer T 1  is connected in series with a diode D 3  and a primary winding L 4  of a second transformer T 2  to form a charging path for a capacitor C. 
     A secndary winding L 5 of transformer T 2  is connected across a spark gap S.G. which is shown symbolically. A discharge path for the capacitor C is formed by the winding L 3 of the first transformer T 1 , the switch S when placed in position 2, and the primary winding L 4  of transformer T 2 . 
     The operation of the circuit is as follows. The rest position for the switch S is in position 2 and the switch in a preferred embodiment is so arranged that it is spring biassed towards that position. The switch S is operated to position 1 which connects the battery B across the series combination of resistor R 1  and diode D 1 . The diode D 1  is a bias stabiliser since it has a voltage drop similar to the voltage Vbe of transistor TR1 and therefore allows the oscillator to operate with a large variation in supply voltage. This is particularly valuable with low voltage supplies of the order of 1.5 volts from battery B. 
     Transistor TR1 turns ON and due to the coupling between coils L 1  and L 2  of transformer T 1   is turned OFF again by means of the known ringing choke action. 
     When transistor TR1 is ON the coils L 1  and L 2  store energy and when TR1 is turned OFF this energy is transferred via winding L 3  to capacitor C, the diode D 3  preventing loss of energy from capacitor C via winding L 4 . 
     As the ringing choke action continues the capacitor C is charged up so that the voltage across the capacitor C builds up. If this were allowed to continue the voltage rating of the capacitor C might well be exceeded and therefore Zener diode D 2  is included in the oscillator circuit to limit the collector swing of transistor TR1 and therefore to limit the transformed voltage which charges capacitor C. 
     When the capacitor C is charged the switch S is returned to its rest position 2. A discharge path for the capacitor is now formed via winding L 3 , switch S and primary winding L 4  of transformer T 2 . A spark is produced across the spark gap S.G. since the transformer T 2  is arranged to have a high step up ratio (approxiamately 1 : 30). It is also, in a further embodiment, possible to have the blocking oscillator continuously running and to discharge the capacitor by using a single make contact switch. 
     An advantage of using the winding L 3  in the discharge path is found in the use of the spark produced across the spark gap S.G. in igniting a gas/air mixture such as is found in gas cigarette lighters or in gas appliances. This advantage is that the winding L 3  provides a leakage inductance which controls the initial surge of current in the primary winding L 4  and as a result the spark current is controlled to provide a longer lasting spark at a lower energy level. Such a spark is found to be better for gas ignition than the otherwise obtained short duration high energy spark. 
     The winding L 3  also results in a substantial reduction in peak current through contact 2, which makes it possible to use a smaller contact. In a practical system the current reduction is from 40 amps down to 6 amps. Since the contact may normally be expected to bounce, the reduction in current improves the reliability of such a contact. 
     In a further embodiment, contact 2 of the switch S may be connected to a tapping on the secondary of transformer T 1  as shown in dotted lines. This allows a compromise to be obtained between the discharge part of the inductance of L 3  and the inductance of the primary winding L 4  so as to provide a better compromise between the initial spark current and the spark duration.