GB1569869A

GB1569869A - E h t voltage generators

Info

Publication number: GB1569869A
Application number: GB4974575A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1976-12-03
Filing date: 1976-12-03
Publication date: 1980-06-25

Description

(54) IMPROVEMENTS IN OR RELATING TO E.H.T. VOLTAGE GENERATORS (71) We, THE PLESSEY COMPANY LIMITED, a British Company of 2/60 Vicarage Lane, Ilford, Essex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to E.H.T. voltage generators and to the application of such voltage generators in fly killers.

It is an object of the present invention to provide a d.c. E.H.T. voltage generator which operates with a relatively small quiescent current from a relatively low d.c.

voltage source to produce a d.c. E.H.T.

voltage which is suitable for fly killing purposes.

According to the present invention an E.H.T. voltage generator comprises a transistor and a single transformer connected to define a blocking oscillator, the transistor being operatively associated with switch means responsive to a feedback pulse produced in one secondary winding of the transformer consequent upon the generation of each output pulse in another main secondary winding of the transformer, for changing the switch means from one switch state to another for a period determined in dependence upon delay means so as to inhibit the generation of the next successive output pulse from the said main secondary winding.

The E.H.T. voltage generator may be arranged to feed a d.c. storage capacitor via a rectifier whereby a d.c. output signal is provided across the storage capacitor suitable for fly killing purposes.

The feedback pulse may be derived from the said one secondary winding of the transformer and applied to a transistor switch arranged to control the transistor of the blocking oscillator.

The delay means may comprise a capacitor the charged state of which is arranged to be changed by the feedback pulse, the transistor switch being rendered conductive or non-conductive in dependence upon the charged state of the capacitor, the arrangement being such that the charge level of the capacitor during the said period is effective to inhibit operation of the transistor switch.

An exemplary embodiment of the invention will now be described with reference to the drawing accompanying the Provisional Specification in which: Figure 1 is a circuit diagram of a d.c.

E.H.T. voltage generator, and in which Figure 2 shows three waveform diagrams appertaining to operation of the circuit shown in Figure 1.

Referring now to the drawing the circuit comprises a transistor T1 which is connected to drive a primary winding 1 of a transformer 2 so as to produce across a secondary winding 3 of an E.H.T. voltage pulse which is rectified by the diode 4 and smoothed by the resistor 5 and capacitor 6 so as to produce a substantially constant d.c. E.H.T. voltage across the terminals 7. The transistor T1 is connected in operative association with the transformer 2 in blocking oscillator configuration and in accordance with blocking oscillator design a feedback winding 8 is provided on the transformer 2 providing a feedback signal which is fed via resistor 9 and diode 10 to the base of the transistor T1. The end of the feedback winding 8 remote from the diode 10 is connected to a positive voltage supply rail 11 via a switch arrangement shown within a broken line 12. The switch arrangement comprises a transistor T2 the collector of which is connected to the feedback winding 8, the emitter being connected to the positive supply rail 11.

The transistor T2 is biassed by means of resistors 13 and 14 the base of transistor T2 being fed from the collector of a further transistor T3 which forms part of the switch, the emitter of transistor T3 being connected to the negative supply rail 15.

The base of this further transistor T3 is connected to the positive supply rail 11 via a resistor 16 and to the negative supply rail via a capacitor 17. The capacitor 17 is fed also from a feedback control winding 18 of the transformer 2 via a resistor 19 and a diode 20.

In operation of the circuit, as may be seen from the waveform of Figure 2, on 'switch-on' transistor T1 bottoms and the collector current then begins to rise at a rate determined in dependence upon the primary winding inductance of the transformer T2 until a condition is reached whereat it comes out of the bottomed condition and begins to turn off. The switchoff, due to regenerative action then occurs very quickly and the primary circuit rings to produce a very large E.H.T. voltage pulse of the order of 5 kV or 6 kV, determined by the particular primary/sec turns ratio across the secondary winding 3 of the transformer 2. At the same time a negative voltage pulse is produced in the feedback control winding 18 and fed via the diode 20 and resistor 19 effectively to switch off the transistor T3 and therefore the transistor T2. The capacitor 17 therefore discharges through the diode 20. The transistors T and Ts remain non-conductive until the capacitor 17 has re-charged through the resistor 16 to produce a voltage at the base of the transistor T3 to cause it to conduct. The resistor 16 and the capacitor 17 therefore constitute a delay network and the value of these two components determine the period for which the switch defined by transistors T2 and Ts is non-conductive.

With a conventional blocking oscillator pulse as shown in Figure 2 at the collector are produced repetitively with little delay between negative pulses and therefore considerable quiescent current is required to maintain the circuit in operation whereas with the present arrangement the production of each pulse is delayed for 20 milIiseconds and in this way the quiescent current of the circuit is considerably reduced.

The diode 10 is provided for protection purposes since large voltage spikes may obtain in the winding 8 which may damage the base emitter junction of the transistor T1 and diode 20 is provided also for protection purposes to prevent the voltage spikes present in the winding 8 from damaging the transistor T2. A capacitor 21 is provided shunting a battery 22 which is connected to the supply lines 11 and 12 the capacitor 21 serving to reduce the effective impedance of the battery as necessary to provide for efficient operation.

An E.H.T. voltage generator as just before described with reference to the accompanying drawings is particularly suitable for use in a fly killer in which the d.c. E.H.T. output voltage across the terminals 7 is applied to a bar or grill to which the flies or other insects are attracted by means of an ultra violet light source to be electrocuted or burnt by an electrical discharge as they pass in the region of the grill. With this kind of application large energy pulses can be provided by the capacitor 6 intermittently as necessary in order to destroy flies or other insects but a small quiescent current obtains as desirable for battery operation.

Various modifications may be made to the arrangements shown in Figure 1 without departing from the scope of the invention and for example any suitable kind of controllable switch may be utilised to switches the transistor T1 on or off.

WHAT WE CLAIM IS: - 1. An E.H.T. Voltage Generator comprising a transistor and a single transformer connected to define a blocking oscillator, the transistor being operatively associated with switch means responsive to a feed-back pulse produced in one secondary winding of the transformer consequent upon the generation of each output pulse in another main secondary winding of the transformer, for changing the switch means from one switch state to another for a period determined in dependence upon delay means so as to inhibit the generation of the next successive output pulse from the main secondary winding.

2. An E.H.T. Voltage Generator as claimed in claim 1 arranged to feed a D.C.

Storage capacitor via a rectifier whereby a D.C. output signal is provided across the storage capacitor.

3. An E.H.T. Voltage Generator as claimed in claim 1 or claim 2 wherein the feed-back pulses derived from the said one secondary winding of the transformr are applied to a transistor switch arranged to control the transistor of the blocking oscillator.

4. An E.H.T. Voltage Generator as claimed in any preceeding claim wherein the delay means comprises a capacitor the charge state of which is arranged to be

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. T2 being fed from the collector of a further transistor T3 which forms part of the switch, the emitter of transistor T3 being connected to the negative supply rail 15. The base of this further transistor T3 is connected to the positive supply rail 11 via a resistor 16 and to the negative supply rail via a capacitor 17. The capacitor 17 is fed also from a feedback control winding 18 of the transformer 2 via a resistor 19 and a diode 20. In operation of the circuit, as may be seen from the waveform of Figure 2, on 'switch-on' transistor T1 bottoms and the collector current then begins to rise at a rate determined in dependence upon the primary winding inductance of the transformer T2 until a condition is reached whereat it comes out of the bottomed condition and begins to turn off. The switchoff, due to regenerative action then occurs very quickly and the primary circuit rings to produce a very large E.H.T. voltage pulse of the order of 5 kV or 6 kV, determined by the particular primary/sec turns ratio across the secondary winding 3 of the transformer 2. At the same time a negative voltage pulse is produced in the feedback control winding 18 and fed via the diode 20 and resistor 19 effectively to switch off the transistor T3 and therefore the transistor T2. The capacitor 17 therefore discharges through the diode 20. The transistors T and Ts remain non-conductive until the capacitor 17 has re-charged through the resistor 16 to produce a voltage at the base of the transistor T3 to cause it to conduct. The resistor 16 and the capacitor 17 therefore constitute a delay network and the value of these two components determine the period for which the switch defined by transistors T2 and Ts is non-conductive. With a conventional blocking oscillator pulse as shown in Figure 2 at the collector are produced repetitively with little delay between negative pulses and therefore considerable quiescent current is required to maintain the circuit in operation whereas with the present arrangement the production of each pulse is delayed for 20 milIiseconds and in this way the quiescent current of the circuit is considerably reduced. The diode 10 is provided for protection purposes since large voltage spikes may obtain in the winding 8 which may damage the base emitter junction of the transistor T1 and diode 20 is provided also for protection purposes to prevent the voltage spikes present in the winding 8 from damaging the transistor T2. A capacitor 21 is provided shunting a battery 22 which is connected to the supply lines 11 and 12 the capacitor 21 serving to reduce the effective impedance of the battery as necessary to provide for efficient operation. An E.H.T. voltage generator as just before described with reference to the accompanying drawings is particularly suitable for use in a fly killer in which the d.c. E.H.T. output voltage across the terminals 7 is applied to a bar or grill to which the flies or other insects are attracted by means of an ultra violet light source to be electrocuted or burnt by an electrical discharge as they pass in the region of the grill. With this kind of application large energy pulses can be provided by the capacitor 6 intermittently as necessary in order to destroy flies or other insects but a small quiescent current obtains as desirable for battery operation. Various modifications may be made to the arrangements shown in Figure 1 without departing from the scope of the invention and for example any suitable kind of controllable switch may be utilised to switches the transistor T1 on or off. WHAT WE CLAIM IS: -

1. An E.H.T. Voltage Generator comprising a transistor and a single transformer connected to define a blocking oscillator, the transistor being operatively associated with switch means responsive to a feed-back pulse produced in one secondary winding of the transformer consequent upon the generation of each output pulse in another main secondary winding of the transformer, for changing the switch means from one switch state to another for a period determined in dependence upon delay means so as to inhibit the generation of the next successive output pulse from the main secondary winding.

2. An E.H.T. Voltage Generator as claimed in claim 1 arranged to feed a D.C.

changed in dependence upon the feed-back pulse, the transistor switch being rendered conductive or non-conductive in dependence upon the charge state of the capacitor, the arrangement being such that the charge level of the capacitor during the said period is effective to inhibit operation of the transistor switch.

5. An E.H.T. Voltage Generator substantially as hereinbefore described with reference to the drawings accompanying the provisional specification.

6. Fly killing apparatus including an E.H.T. Voltage Generator as claimed in any preceeding claim.