GB2411358A

GB2411358A - Current and voltage control for electronic microbicidal devices

Info

Publication number: GB2411358A
Application number: GB0404136A
Authority: GB
Inventors: Jonathan Hugh Lambert Copus
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-02-25
Filing date: 2004-02-25
Publication date: 2005-08-31
Anticipated expiration: 2024-02-25
Also published as: GB2411358B; GB0404136D0

Abstract

The output current of an electronic microbicidal device is used to control its voltage-generating circuitry in such a way as to maintain a constant output current despite a varying load resistance. In one manifestation of the circuitry the magnitude of the current flowing in the output circuit of the said device is used to control a transconductance amplifier 13 which determines the operation of a pulse-width control circuit 8 to modify the signal from an oscillator 7 before it is fed to a drive element 9 which controls a switching element 10 feeding a voltage multiplier 11 connected to an ion-emitting element 1. This ion-emitting element is approximated to a conductive surface 5 which is connected to the opposite pole of the voltage generator from that to which the ion-emitting element is connected and the current flowing in the output circuit thus formed is sensed by a current-sensing element 12 whose output controls the transconductance amplifier 13 in such a way as to maintain as far as possible a constant current n the output circuit. A voltage limiter 14 modifies the behaviour of the transconductance amplifier 13 in such a way as to limit the width of the pulses emerging from the pulse-width control circuit 8 and thus the output voltage of the voltage generator.

Description

241 1 358

CURRENT AND VOLTAGE CONTROL FOR ELECTRONIC MICROBICIDAL

DEVICES

This invention relates to a circuit for controlling the output current of an electronic mcrobcidal device at a constant and predetermined level despite changes in the electrical characteristics of the circuit which constitutes the load resistance of the device It also Incorporates means of preventing the output voltage of the device rising to undesirable levels when the output element is powered but not in use.

Patent number GB2246955 discloses a method of killing micro-organisms by electrical means. Several embodiments of this invention Include means whereby an ion emitter consisting of a sharp point charged to a relatively high voltage with respect to earth is mounted in a handpiece and brought close to an electrically-conductive surface for the purpose of killing the micro-organisms on the surface by means of the stream of gaseous ions formed at the point and attracted towards the surface, the electrical potential of the surface being maintained closer to earth potential than that of the ion- emitting element or at some potential with the opposite polarity to that of the gaseous ions. In these circumstances a current flows from the ion emitter in the form of a stream of gaseous ions which give up their charge on impinging on the surface and the current returns via an external connection to the base unit powering the ion emitter.

This current Is directly related to the number of ionisations occurring at the point of the ion-emtting element per unit time, and it is usually desirable to control the current and therefore the rate of ionsaton Without some means of automatic control the current flowing In the output circuit which includes the ion emitter and the conductive surface varies In inverse proportion to the distance between the point of the ion emitter and the conductive surface, and in order to maintain this current at an optimum value the operator Is obliged to maintain a constant watch on a meter included in the circuit. i

Figure 1 Illustrates this situation, where an ion-emitting element 1 is mounted in a handpiece 2 connected to a base unit containing a power supply 3. The On-emitting element Is held close to an electricallyconductve surface 5 which Is connected via a current meter 4 to the opposite pole of the power supply 3 to that which is connected to the ionemitting element. As the distance 6 between the ion-emitting element and the conductive surface varies, so does the current flowing through the meter, becoming greater as the distance becomes less.

The present invention removes the need for vigilance on the part of the operator by automatically controlling this current at a predetermined level regardless of variations in the distance between the ion-emtting element and the surface being treated. It also overcomes another drawback found in previous electronic microbcidal devices by limiting the tendency of the electrical potential on the On-emitting element to rise to undesirable levels when the distance between the ion-emittng element and the surface being treated becomes larger than that at which a microbicidally-effective current can be generated by the apparatus, for example while the operator is moving the On-emitting element to a different treatment site.

Although the examples given so far have all been concerned with Instances where the ion-emtting element is mounted in a handpiece, the same drawbacks and the same solution apply to other embodiments of electronic microbcidal technology, for instance where food Is passed close to one or more fixed ion-emitting elements for the purpose of killing the microorganisms on the surface of the food. In these circumstances, too, it is desirable for the rate of ionsaton to be controlled automatically within preset limits, regardless of any instantaneous variations In the distance between the ion-emitting element or elements and the surface of the food.

Electronic mcrobicdal systems are generally powered by circuits such as that shown in outline in Figure 2, where the width of pulses generated by an oscillator 7 is controlled by a pulse-width control circuit 8, the pulses then triggering a switching transistor 9 which controls the current flowing in the primary of a step-up transformer 10, the output of which is fed to a voltage multiplier 11 whose output Is connected to the ion-emittng element 1. The width of the pulses fed from the pulse-width control circuit to the switching transistor determines the proportion of the cycle represented by the oscillator frequency for which current flows through the primary of the step-up transformer, and by this means the output voltage of the voltage multiplier can be set for a range of given load conditions.

In previous embodiments of electronic microbicidal apparatus the pulse width has generally been set by hand at a fixed value, this practice giving rise to the undesirable vanatons in output current mentioned above when the distance between the ion emitting element and the surface under treatment is varied.

The present application proposes to control the operation of the pulsewidth control circuit by sensing the current flowing in the output circuit and using this current to generate a voltage fed to the inverting Input of an operational transconductance amplifier, which vanes the current supplied to the pulse-width control circuit in accordance with variations In its input voltage, thus effectively controlling the output voltage of the voltage multiplier according to the principles already described.

The proposed circuit Is outlined In Figure 3, in which the output of the oscillator 7 is fed as before into a pulse-width control circuit 8 which controls the switching transistor 9 dnvng the step-up transformer 10 the output of which Is multiplied by the voltage multiplier 11 and applied to the ion-emtting element 1. This Is approximated to the conductive surface under treatment 5. The output of current-sensing element 12 In the form of a voltage is fed to transconductance amplifier 13 which controls the current supplied to the pulse-width control circuit 8.

However, the effect of the circuit as shown Is to Increase the voltage at the tip of the ion-emittng element as the distance between it and the surface under treatment increases, In order to maintain a constant current. The result Is that when this distance Increases beyond the limit at which a useful current can be maintained, the voltage at the tip of the ion-emitting element rises undesirably to the maximum which the power supply can sustain, leading In practice to unwanted effects such as the build-up of excessive voltages induced on the outside of the insulation of the lead connecting the power supply unit to the handpiece, when the effects of static electricity can be alarming to the operator The following modification to the circuit circumvents this problem and is Illustrated in outline by reference to figure 4, in which the non'nverting Input of the transconductance amplifier 13 is referenced to the potential at a point in a voltage- limiting circuit 14 limiting the peak voltage at the collector of the switching transistor, in such a way that when the voltage-lmiting circuit begins to operate it causes the transconductance amplifier to reduce the width of the pulses emerging from the pulse width control circuit, thus preventing an excessive voltage from developing at the output of the voltage multiplier The complete circuit of one embodiment of the invention is shown In detail in Figure 5, in which D11-30 and C41-60 constitute the voltage multiplier and R74-76 are current limiting resistors. The oscillator comprises the first two gates of U2 ('e pins 1- 6) together with tuning components V29, R89 and C5. The remaining two gates of U2 (ie pins 8-13) together with coupling capacitor C68 form the pulse-wdth control circuit, the output of which is fed via resistor R18 to the base of switching transistor Q8 whose collector circuit Includes the primary winding of step-up transformer T3, the secondary of which feeds the voltage multiplier. Current-sensng circuitry V28, R85 and R36 presents a voltage which varies in direct proportion to the current flowing from sockets J13, J17 (either of which is connected to the surface being treated) to the Inverting input of transconductance amplifier U4, whose gain and frequency response are set by feedback components C66 and R84. The transconductance amplifier therefore supplies a current to the pulse-width control circuit via R81, the value of which vanes the current supplied by R69 and modifies the width of pulses emerging from U2, thus controlling the time for which Q8 conducts and therefore the voltage fed to the voltage 1 0 multiplier.

The circuit limiting the peak voltage developed across the primary winding of T3 centres on zener diode Z12. When this diode conducts a voltage is developed across R12 which is applied to the non-'nverting input of transconductance amplifier U4, causing it to reduce the width of the pulses emerging from U2 C65 serves to tailor the response of this action.

It is to be understood that the expressions 'inverting' and 'non'nverting' may be reversed in any particular embodiment of the invention with appropriate modifications of or additions to the accompanying circuitry.

Claims

1. An electronic microbicide apparatus comprising a drect-voltage generator circuit of the kind comprising a relatively low-voltage alternating or pulsed current source and a voltage step-up and rectifying means arranged for transforming alternating or pulsed current from the alternating or pulsed current source Into a relative high direct voltage, in which the generator circuit comprises an output stabilizing means so arranged as substantially to stabilise the output current of the generator circuit over a range of load resistances, the output means comprising a current sensing means arranged to sense current in the generator circuit output, and feedback means for adjusting the Input to the voltage step-up and rectifying means In response to the output of the current sensing means.

2. An electronic microbcide apparatus as claimed in claim 1 whereof the output current is held relatively constant by the action of a transconductance amplifier governed by the said output current.

3. An electronic microbcide apparatus as claimed in claims 1 or 2 wherein the performance of the current-control circuitry is modified by feedback circuitry in such a way as to limit the maximum voltage appearing at the output of an electronic microbcidal device.

4. A generator and control circuitry substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

5. A generator and control circuitry substantially as herenbefore described with reference to figure 4 of the accompanying drawings.

6. A generator and control circuitry substantially as herenbefore described with reference to Figure 5 of the accompanying drawings