IE45749B1 - Self-adjusting horizontal deflection circuit for cathode ray tubes - Google Patents

Abstract

In this circuit, a regulating circuit (2) is used for changing the duration of the drive pulses of the line output stage (6) in such a manner that the amplitude of the deflection current and the reference voltages for the television receiver, generated by the output stage, are stabilised. The regulating circuit (2) has a protective circuit (10) which short circuits the drive pulses in the case of a fault, for example with an overload of the line output stage. The protective circuit (10) in the form of a flip-flop is improved to the extent that the collector of the transistor (12), short circuiting the regulator, of the flip-flop is connected via a time constant section (21, 22) to the base of the transistor (11) of the flip-flop receiving the fault information.

Description

The invention relates to a self-adjusting horizontal-deflection circuit for cathode ray tubes, more especially for picture tubes in television receivers, for the development of a saw-tooth deflection current flowing through a deflection coil. Such a horizontal deflection circuit which stabilizes the amplitude of the deflection voltage is well known (see IEEE transactions on Broadcast and Television Receivers August 1972 pages 177-182) and has been described in detail in the Valvo Development Bulletin Nr. 61 of April 1975. This horizontaldeflection circuit operates directly from rectified mains voltage and also produces stabilized operating voltages of different value for the television receiver owing to the fact that during the first portion of the line sweep, the transistor of the line deflection circuit output stage also functions to provide the power supply. By means of a control circuit the transistor is controlled with pulses of varying length in such a manner that the deflection current remains constant.

Due to troubles that could overload the line output stage, in the known circuit there is built into the control circuit a flip flop which responds in case of trouble‘and short-circuits the drive pulses for the line output stage. Troubles may occur e.g. in the - 3 form of over-loading by a too high current drain from the controlled power(supply unit or in the form of overvoltages or by short power failure. In the latter case, it is particularly critical if the ’ television receiver is again turned on immediately after a short interruption of the operating voltage, because the charge loss caused by the interruption of the supply voltage must be promptly made good by the line output stage.

Therefore, it is the object of the present invention to protect the horizontal-deflection output stage even in such a case.

The present invention provides a self-adjusting horizontal deflection circuit for cathode ray tubes, comprising means for generating, in response to a 'train of drive pulses from a control circuit, both a supply voltage for the deflection circuit and also a saw-tooth deflection current, the said control circuit being arranged to vary the length of said pulses in accordance with the amplitude of said deflection current in order to stabilize the supply voltage, and there being provided a protection circuit for blocking said control pulses in the event of an overload in the deflection circuit, the said protection circuit comprising a mono-stable flip flop including a first transistor arranged to block said drive pulses and a second transistor connected to receive a signal upon the deflection voltage, the collector of the first transistor being connected to the base of the second transistor via a time constant circuit, whereby said monostable flip flop is caused to respond rapidly to failures in the power supply and upon reapplication of the power supply is restored after a predetermined delay.

By this means a rapid switching off of the 49 - 4 output stage is effected upon power failure, and a longer delay is enforced until the next possible switching on, during which the starting conditions for the deflection and power supply circuits can be reestablished. At the same time, a reduction of the sensitivity of the protection circuit during the tuming-on phase is achieved.

The invention is illustrated by way of example in the accompanying drawing, the single figure of which is a circuit diagram of a horizontal deflection circuit in accordance with the invention.

Referring to the drawing, in block 1, rectangular line-frequency pulses are developed and then applied to the control circuit 2. As a function of the emitter/cOllector current of the transistor 3, a control voltage appears at the emitter of the transistor 4 so that dependently on this control voltage, the rectangular voltage developed at the collector of the transistor 5 can be taken off with different pulse duty cycles and will be applied to the line output stage 6. The line output stage 6 is of known type as described for example in the Valvo Development Bulletin referred to above. Briefly, the line output stage 6 is arranged to generate a sawtooth deflection current for the horizontal deflection coil 31 of the cathode ray tube, the deflection coil forming part of a resonant circuit of the output stage. A diode 32 completes the resonant circuit and conducts the deflection current during a first part of the scanning time, whereas a switching transistor 33 controlled by pulses from the control circuit 2 is connected in parallel with the diode 32 in order to conduct the deflection current flowing in the resonant circuit during a second part of the scanning time in which the deflection current is of opposite polarity. By means of the pulses from the control circuit 2 the transistor 33 is rendered conductive during the second part of the scanning . time and is blocked during the fly-back time. A transformer 34 has its primary winding connected in series between the rectified mains voltage source Ug and the collector of the transistor 33 and its secondary winding connected to the resonant circuit including the deflection coil 31 by way of a rectifier diode 35. The voltage source is isolated from the resonant circuit by means of a further diode 36 connected between the resonant circuit and the collector of transistor 33. By means of this arrangement the transistor 33 serves, during the second part of the scanning time, to conduct both the deflection current passing through the deflection winding 31 and also current flowing from the power supply through the primary winding of transformer 34, so that during this time energy is stored in the primary winding of transformer 34, said energy, during the fly-back time, being transferred from the secondary winding of the transformer 34 via the diode 35 to the resonant circuit. In addition, the current from the secondary winding of the transformer 34 is supplied to the primary winding 37 of a further transformer the secondary winding 38 of which provides the operating voltage U2 for the deflection circuit. The output voltage from the secondary winding 38 is rectified by means of a rectifying diode 39.

The control circuit 2 receives its information dependently on a deviation e.g. of the deflection voltage through the input 7 to the control electrode of the transistor 3. Also deviations of the supply voltage U3 from the power supply 9 are fed in through the input 8. Furthermore, the well known circuit arrangement has a protection circuit in the form of a flip flop 10 with a transistor 11 and a transistor 12 of which the collector-emitter path will connect the - 6 emitter of the control transistor 4 to ground in case of trouble, whereby the driving of the line output stage 6 is interrupted. With normal operating condition of the television receiving circuit, the transistor 11 is conductive and the transistor 12 blocked. As a criterion for the overload, an a.c. voltage proportional to the charging current is taken off through the resistor 15 and the capacitor 16 at the series connection of the resistor 13 and the capacitor 14 in the horizontal-deflection output stage 6. The positive portions are clamped to earth potential by the diode 17. The a.c. voltage, unipolar in this way, is filtered by means of the filter formed by the resistor 18 and the capacitor 19. When the negative voltage exceeds about -100 mV, the switching operation of the stage 10 is triggered. It is also triggered if the stabilized voltage U2 from the deflection circuit becomes too large and exceeds the Zener voltage of the Zener diode 20, or if the voltage U2 is interrupted due to a failure in the mains power supply. However, the well known circuit arrangement has the disadvantage that with short-time power failures - as they may occur for instance with loose contacts in wall sockets - it is not turned off fast enough. By inserting a time constant by means of the resistor 21 and the capacitor 22 which connect the collector of the transistor 12 with the base of the transistor 11, it is ensured that even with short-time interruptions of the voltage U2, the switching operation immediately becomes triggered, as the negative voltage jump is transferred to the base of the transistor 11 through the low-resistance path via the resistors 23, 21 and the capacitor 22. When the trouble is eliminated and the receiver is turned on again, there is now a longer time delay caused by the time constant 24, 22, 21, after which the 43749 - 7 flip flop 10 returns into its initial position so that the line output stage 6 is re-driven and ready for operation. After any turning-off, even if short-time, the flip flop 10 is kept in its unstable condition for about 5 seconds. The insertion of the time-constant section has another advantage: During the turning-on, the capacitor 22 is charged through the base-emitter path of the transistor 11. During this charging time, the protection circuit 10 is purposely rendered insensitive; otherwise it would react against the required increased charging current as upon overload and turn out the set at once. After the forced interval, the protection circuit is again automatic15 ally sensitive.

Claims (5)

1. CLAIMS;1. A self-adjusting horizontal deflection circuit for cathode ray tubes, comprising means for generating, in response to a train of drive 5 pulses from a control circuit, both a supply voltage for the deflection circuit and also a sawtooth deflection current, the said control circuit being arranged to vary the length of said pulses in accordance with the amplitude of said deflection 10 current in order to stabilize the supply voltage, and there being provided a protection circuit, for blocking said control pulses in the event of an overload in the deflection circuit, the said protection circuit comprising a monostable flip flop 15 including a first transistor arranged to block said drive pulses and a second transistor connected to receive a signal dependent upon the deflection voltage, the collector of the first transistor being connected .to the base of the second transistor via 20 a time constant circuit, whereby said monostable flip flop is caused to respond rapidly to failures in the power supply and upon reapplication of the power supply is restored after a predetermined delay. 25

2. A self-adjusting horizontal deflection circuit for cathode ray tubes, for the generation of a saw-tooth deflection current, comprising a resonant circuit including the horizontal deflection coil of the cathode ray tube, a diode which 30 completes said resonant circuit and conducts the deflection current during a first part of the scanning time, a switch controlled by pulses from a control circuit and connected in parallel with said diode in order to conduct the deflection 35 current flowing in said resonant circuit during a - 9 second part of the scanning time in which the deflection current is of opposite polarity, said switch being conductive during said second part of the scanning time and being blocked during the flyback time, a transformer having a primary winding connected in series with a voltage source and said switch and a secondary winding connected to said resonant circuit via a rectifier diode, said voltage source being isolated from said resonant circuit by a third diode connected between said resonant circuit and said switch, whereby said switch serves both to conduct said deflection current during said second part of the scanning time and also to chop the current flowing through the transformer primary winding so that during the flyback time current flows from the secondary winding of said transformer to provide the power supply for the horizontal deflection circuit, the said control circuit being arranged to vary the duration of the control pulses supplied to said switch in accordance with the amplitude of the deflection voltage in such a manner that said power supply is stabilised, and said control circuit further including a monostable flip flop arranged to block the drive pulses to said switch in the event of overloading of the deflection circuit, said monostable flip flop comprising a first transistor arranged to block said drive pulses and a second transistor connected to receive a signal dependent upon the deflection voltage, the collector of the first transistor being connected to the base of the second transistor via a time constant circuit, whereby said monostable flip flop is caused to respond rapidly to such over-loading and to be restored after a predetermined delay.

3. A horizontal deflection circuit as - 10 claimed in Claim 1 or 2, being a line deflection circuit for a television set.

4. A hprizontal deflection circuit as claimed in any one of Claims 1-3, in which said 5. Switch is a transistor.

5. A horizontal deflection circuit substantially as described herein with reference to the accompanying drawing.