GB2258988A - Insect killers - Google Patents

Abstract

An insect killer comprises a discharge lamp 10 and a grid 13 towards which insects are attracted by the lamp. The grid 13 is connected to the secondary winding of a pulse transformer 14, through the primary winding which current pulses are periodically passed by discharge of a capacitor 12 therethrough. Charging of the capacitor 12 is effected by means of a voltage doubler comprising diodes 20 to 23 and capacitors 18 and 19 which are charged alternately through a common resistor 17. <IMAGE>

Description

INSECT KILLERS This invention relates to insect killers of the general kind which include a discharge lamp and a high voltage grid for electrocuting insects attracted towards said lamp.

Conventionally a step up transformer is used for creating the high voltage on the grid. This is, however, heavy, bulky and expensive and alternative arrangements making use of a capacitor which periodically discharges through the primary winding of a pulse transformer have therefore already been proposed.

These capacitor discharge systems have a power dissipation problem, however, resulting from the use of current limiting resistors in the charging path of the capacitor.

The applicants have themselves considered using the discharge tube and its ballast choke to limit charging current to the capacitor and this is extremely effective when the mains supply voltage is considerably in excess of the breakdown voltage of the discharge tube.

When a mains supply voltage of only llOV is used, however, some dimming of the lamp may occur.

It is an object of the present invention to provide an insect killer which overcomes all of the above mentioned deficiencies.

In accordance with the invention, an insect killer comprises a discharge lamp, a grid towards which flying insects are attracted by said lamp and an energising circuit for said grid, said energising circuit comprising a step-up pulse transformer having a secondary winding connected to the grid and a primary winding, a discharge capacitor, a trigger device connected in series with said primary winding across said discharge capacitor and a charging circuit for said discharge capacitor, said charging circuit comprising a full wave voltage-doubler circuit.

Said full-wave voltage-doubler circuit may comprise a common input resistor, a pair of input capacitors and two pairs of diodes, said input capacitors being connected via said common input resistor to one supply terminal and via respective ones of one of the diode pairs to the other supply terminal, said diodes of said one diode pair being oppositely connected so that the capacitors can charge in alternate half-cycles of the supply, the diodes of the other diode pair connecting the input capacitors to opposite sides of the discharge capacitor.

With this arrangement the discharge capacitor is charged in the same sense on each supply half cycle. The trigger device can therefore be a uni-directional device such as a thyristor instead of a triac which would be required if bi-directional current flow were required.

An example of the invention is shown in the sole figure of the accompanying drawings.

In the example shown, terminals L and N are intended for connection to a.c. mains for providing power for the insect killer. A discharge lamp 10 is connected in series with a ballast choke 11 between these terminals. The terminals L and N are also connected to a full-wave voltage-doubler for charging a discharge capacitor 12. This capacitor 12 is used for energising a grid which is arranged in well-known manner to surround the lamp 10 so that insects flying towards the lamp 10 touch the grid and are electrocuted. The grid 13 is connected across the secondary winding of a pulse transformer 14, the primary winding of which is connected in series with a triggering device 15 (in the form of a thyristor) across the discharge capacitor 12.A zener diode 16 connects the anode and gate of the thyristor together so that whenever, the anode/gate voltage exceeds the zener breakdown voltage the thyristor is fired.

The full-wave voltage-doubler circuit employed makes use of two conventional voltage-doublers sharing a common input resistor 17 and having the discharge capacitor 12 as a common output capacitor. Two input capacitors 18 and 19 and four diodes 20 to 23 make up the circuit. Capacitors 18 and 19 each have one terminal connected to one end of the input resistor, the other end of which is connected to the terminal L. The other terminals of capacitors 18, 19 are connected by respective diodes 20 and 21 to the terminal N, the diodes 20 and 21 being oppositely connected so that charging current flows into the capacitors 18 and 19 in alternate half-cycles of the a.c. supply. The remaining two diodes 22 and 23 connect the other terminals of the capacitors 18, 19 to opposite sides of the capacitor 12.Thus, as will be seen from the drawing, one side of capacitor 12 is connected to terminal N by the two diodes 20, 22 in series and the other side thereof is connected to terminal N by the other two diodes 21, 23 in series, the four diodes forming a single series circuit connected across the capacitor 12 with all the diodes in this series circuit being connected in the same sense.

In operation, a situation where all four diodes are forwardly biassed simultaneously cannot occur, as in every half-cycle one or other of the diodes 20, 21 will be reversed biassed. During negative half-cycles capacitor 18 charges so that a negative voltage appears on its input side terminal. When the next half cycle is commenced, this input side terminal is forced to a positive voltage so that output side terminal of capacitor 18 is forced to an increasingly positive voltage and voltage is transferred to the "upper" side of capacitor 12. During each positive half-cycle capacitor 19 is charged in the opposite sense so that when the succeeding negative half cycle is commenced the voltage on the "lower" side of capacitor 12 is forced in a negative direction.

In each half cycle, when the voltage on capacitor 12 becomes high enough, thyristor 15 fires and the capacitor 12 discharges rapidly through the transformer primary winding.

The arrangement described above enables a high voltage to be maintained efficiently on the grid without reducing the voltage across the lamp. Power consumption is minimised and no heavy, bulky mains frequency transformer is required.

Claims (5)

1. An insect killer comprising a discharge lamp, a grid towards which flying insects are attracted by said lamp and an energising circuit for said grid, said energising circuit comprising a step-up pulse transformer having a secondary winding connected to the grid and a primary winding, a discharge capacitor, a trigger device connected in series with said primary winding across said discharge capacitor and a charging circuit for said discharge capacitor, said charging circuit comprising a full wave voltage-doubler circuit.

2. An insect killer as claimed in Claim 1, in which said voltage-doubler circuit comprises a common input resistor, a pair of input capacitors and two pairs of diodes, said input capacitors being connected via said common input resistor to one supply terminal and via respective ones of one of the diode pairs to the other supply terminal, said diodes of said one diode pair being oppositely connected so that the capacitors can charge in alternate half-cycles of the supply, the diodes of the other diode pair connecting the input capacitors to opposite sides of the discharge capacitor.

3. An insect killer as claimed in Claim 2, in which said trigger device is a uni-directional current flow device.

4. An insect killer as claimed in Claim 2, in which said trigger device is a thyristor.

5. An insect killer substantially as hereinbefore described with reference to and as shown in the accompanying drawings.