GB2360882A

GB2360882A - EHT generator

Info

Publication number: GB2360882A
Application number: GB0000849A
Authority: GB
Inventors: Michael Leonard Lucas
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-01-17
Filing date: 2000-01-17
Publication date: 2001-10-03
Also published as: GB0000849D0

Abstract

A transformer T1 generates a high tension voltage in secondary winding C from a supply to a primary winding A, the current supply to which is controlled using electronic switch Q1, Q2. An auxiliary sensor winding B is provided to monitor the magnetic flux in the transformer to detect an over current condition and turn off the electronic switch in response thereto by discharging capacitor C2. Operation of the electronic switch is restored after a delay in which the capacitor is recharged.

Description

1 2360882 EI-IT GENERATOR

TECHNICAL FIELD OF THE INVENTION

This invention relates to high voltage (EHT or extra high tension) generators which can be used for creating large quantities of charged particles.

BACKGROUND

It is known to generate very high voltages by means of a high ratio transformer driven by an oscillator, e.g. in the line timebase circuit of a television receiver. However, in such circuits the magnetic field around the transformer builds up and decays at its own natural rate depending on the inherent characteristics of the transformer core. As a result, such circuits rarely operate at maximum efficiency, especially over a range of output load conditions.

The present invention seeks to provide a new and inventive form of EHT generator.

SUMMARY OF THE INVENTION

2 The present invention proposes an EHT generator comprising: - a high ratio step-up transformer; - an electronic switch arranged to control the current supplied to the transformer; and - sensing means arranged to sense the magnetic flux in the transformer and turn off the switch in response to the magnetic field produced by a rising current in the transformer.

The sensing means preferably comprises a sensor coil.

The electronic switch is preferably associated with a delay arrangement which provides a delay before the switch is turned on again. A convenient and simple form of delay may be provided by charge storage means (e.g. a capacitor) arranged to charge at a controlled rate. To reduce the amount of charge which the switch draws from the storage means the switch preferably has a high input impedance and may, for example, comprise an IGFET or a pair of transistors connected in a darlington configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings:

Figure 1 is a circuit diagram of an EHT generator in accordance with the invention; and 1 3 Finure 2 is an alternative circuit for Sections 11 and Ill of the generator. diagram.

DETAILED DESCRIPTION OF THE DRAWINGS

The generator comprises a power supply section 1, an oscillator section 11 and an EHT section Ill.

Considering the power supply section 1, a d.c. supply of between 8v and 37v (e.g. as provided by a 12v car battery) is fed via fuse Fl and reverseprotection diode D1 to a voltage regulator REG 1. A potential divider formed by Rl and preset resistor R2 is connected across the output of the regulator allowing the regulated output voltage to be set to 9.8v d.c. D5 protects the regulator from voltage spikes on the output. Electrolytic capacitor Cl smooths the output voltage for supplying the oscillator section 11, while LED1, fed via current-limiting resistor R3, illuminates to indicate when the generator is on.

The oscillator section 11 comprises a driver transistor Q1 and an output transistor 02 connected in a darlington configuration to provide a high input impedance. A charge-storage capacitor C2 is charged from the supply voltage via series resistors R4 and R5. As the capacitor charges the potential on the base of driver transistor 01 rises, and when the potential reaches about two thirds of the supply voltage 01 and Q2 are turned hard on causing a high current (about 1 amp) to flow through the primary Winding A of output transformer T1, limited to a safe value by emitter resistor R6. The transformer may be wound on a ferdte core and has a high step-up 4 ratio. The primary winding A typically has about 5 turns and the secondary winding C between 1,500 and 1,700 turns.

The transformer Tl also has a sensor winding B, typically having a ratio of aboutlA with the primary winding. The current flowing in winding A induces a voltage in the sensor winding B which is rectified by series diodes D2 and D3 to drive the junction of resistors R4 and R5 negative. This reverse bias rapidly turns off the darlington pair, Q1 and 02 and discharges the capacitor C2. The current therefore ceases to flow through the winding A resulting in a very rapid collapse of the magnetic field around the transformer T1. The cycle then re-commences as C2 starts to charge again via R4 and R5, resulting in a fierce transition of the magnetic field around transformer T1. The time constant of R4/R5 and C2 primarily determines the frequency of oscillation. The sensor winding B effectively provides negative feedback in the oscillator stage which controls the saturation of transformer Tl and causes the circuit to automatically operate at maximum efficiency.

The secondary winding C produces an EHT voltage of typically between 12 and 20kv, which is rectified and smoothed in section Ill by high voltage capacitor C3 and diode D4. The resulting negative d.c. potential at cathode X produces a fierce electrostatic discharge resulting in the emission of electrons and causing streams of charged particles to be emitted from the cathode.

Fig. 2 shows an alternative circuit for Section 11.]Cl is an oscillator, the frequency of which can be adjusted by a variable resistor. The output is a square wave which is fed to inverting amplifier 1C2so that the two outputs are in antiphase. The peak-to-peak amplitude of both signals is equal to the 4 supply voltage. 1C1 receives a feedback signal from the feedback winding of the output transformer, thereby controlling the voltage applied to the primary of the transforner.

Claims

It will be appreciated that the features disclosed herein may be present

in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

R_y 6 CLAIMS 1. An EHT generator comprising: - a high ratio step-up transformer; - an electronic switch arranged to control the current supplied to the transformer; and - sensing means arranged to sense the magnetic flux in the transformer and turn off the switch in response to the magnetic field produced by a rising current in the transformer.
2. An EHT generator according to Claim 1, in which the sensing means comprises a sensor winding of the transformer.
3. An EHT generator according to Claim 1 or 2, in which the electronic switch is associated with a delay arrangement which provides a delay before the switch is turned on again.
4. An EHT generator according to Claim 3, in which the delay arrangement comprises charge storage means arranged to charge at a controlled rate.
5. An EHT generator according to any preceding claim, in which the electronic switch has a high input impedance.
6. An EHT generator according to Claim 2, in which the electronic 7 switch comprises an inverting amplifier which is fed by an oscillator, the inverting amplifier and the oscillator drive the transformer in antiphase, and the output of the oscillator is controlled by the sensor winding of the transformer.
7. An EHT generator substantially as described with reference to FiguresIl or 2 of the accompanying drawings.