DE2165262A1 - Cancellation circuit for a display tube with a storage screen - Google Patents

Description

Applicant: fc4.Y. Philips' Gloeilampenfabrieken File No., PHIT-5365

Registration dated «December 27, 1971

Cancellation circuit for a display tube with a Storage screen.

The invention relates to an erase circuit for a display tube with a storage screen, the one DC voltage source connected between the storage screen and the cathode circuit for storing information and contains a first capacitor, one of which is connected to a square-wave voltage source, by the interaction of the capacitor with resistors, a quenching voltage is obtained, which is the differential of the square wave voltage is that of the voltage of the DC voltage source is overlaid and which clears the information on the screen.

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. It is known to use cathode ray tubes for displaying electrical information in visible form use. By deflecting an electron beam from a cathode, a display screen local charge can be stored or the screen can begin to emit light. In the oscilloscope and television technology, display tubes are mostly used with a display screen that only emits light when

w when the electron beam hits the screen; if necessary, a luminescent screen is also provided. If, however, the oscillogram or the information is to be stored over a longer period of time, so that the screen should continue to glow for a very long time, a storage screen of a certain type in connection with a different type of tube can be used.

A tube of such a type can be fitted with a writing gun, (i.e. the one producing the steerable writing beam System), with one or more spray guns that send an even stream of electrons to the entire screen can send, and be provided with a special screen, which is a thin conductive layer on a glass plate on which a dielectric layer, e.g. of phosphorus, is applied.

Depending on the energy that the electrons have when they hit the phosphor layer, the is Secondary emission coefficient of the layer smaller or larger as 1. This · energy is not just from the tension between

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the cathode and the conductive layer, but also, as a result the insulating properties of the dielectric layer, depends on the charge on the layer ". Venn the emission coefficient is less than 1, a negative charge accumulates on the screen, which is continuously generated by the spraying guns, while the layer assumes the cathode potential. The screen is dark; it no light is emitted, so that information can now be entered. The writing cannon delivers for this purpose a beam of high energy electrons; namely, the cathode has a high negative potential of some A thousand volts in relation to the screen. At the point of impact The emission coefficient of the screen is greater than 1, which means that a positive charge builds up.

At these points the phosphor begins to emit light, and the oscilloscope or information is visible"

Due to the positive charge, spray electrons are turned on attracted, which have such an energy that the emission coefficient is still greater than 1, so that after the disappearance of the write beam the information still remains visible · In this way is a memory 'received

To access this memory, it is necessary equalize the local differences in charge or discharge the charges.

It is known to use a cancellation circuit for this purpose

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to use. The well-known quenching circuit ensures, among other things, that the correct setting voltage between the spray cathode and the conductive layer of the screen can be adjusted and maintained. To this end, will a tube circuit applied to a DC voltage source with a low internal resistance and is known as "cathode follower". t The anode of the tube used is equipped with a

Voltage source of about 500 V connected; the grid is connected via a resistor to a voltage divider with a setting range of 150 V to 250 V, while the cathode of the tube on the one hand has a resistor with a common line to which the spray cathode is connected is connected, and on the other hand is connected to the conductive layer of the screen.

In order to erase the image on the screen, it is necessary to keep the screen on for a certain period of time

^v to bring a high positive potential, whereby the emission coefficient for the entire dielectric layer is greater than 1, so that the entire layer is positively charged and the entire screen emits light (equalization of the charge), after which this charge can be removed again and a negative charge accumulates on the layer by temporarily bringing the conductive layer to a low or zero potential with respect to the spray cathode *

The positive and the negative voltage jump,

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which are considered to be superimposed on the adjustment voltage must have sufficient length of time to allow for that equalize the entire screen with respect to charges. With the storage tubes now available, this period should be in of the order of magnitude of 50 - 150 msec.

The known circuit uses voltage jumps, which decrease exponentially and can be obtained by drawing a square-wave voltage across a capacitance-resistance network is differentiated. For this purpose, a capacitor is attached to the grid of the tube with one of its layers and with its other covering connected to a changeover switch, which has a rest position in which it connects to the common Establishes line, and has a working position in which a connection with a potential of e.g. +250 V will be produced. The various resistances in the grid circle in conjunction with the capacitor provide one RC time constant which is determined in such a way that for Clear enough time is available.

By putting the changeover switch in the working position, a voltage jump of +250 V is obtained on the grid and thus on the cathode and on the screen, which decreases exponentially to the original screen setting voltage.

The fact that the changeover switch is returned to the rest position results in a jump, but now in a negative one Direction, received.

As the Beatreben goes there, the apparatus becomes smaller

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and to make them lighter and reduce their power dissipation, one becomes the application of semiconductor devices skip wherever possible. Also should the cost price of the equipment can be reduced.

The well-known extinguishing circuit has the disadvantage that a tube is used that is extensive and together with its high-voltage and heating current supply parts is heavy and the power dissipation is high.

Replacing the tube with a transistor

does not eliminate all disadvantages: the high-voltage supply source remains necessary and the cost price of such High voltage transistor is very high.

A quenching circuit according to the invention overcomes all of these disadvantages. Only one is cheaper Low voltage transistor is required while the supply voltage is halved and e.g. 250 V instead of 500 V.

A cancellation circuit according to the invention is

"characterized in that the DC voltage source contains an emitter follower circuit, the emitter circuit of which is connected between the storage screen and the cathode circuit and which contains at least one transistor whose maximum permissible collector-emitter voltage is lower than the sum of the DC voltage and the peak value of the Erasing voltage is that means are provided by means of which the emitter follower circuit from su high voltages is protected, and that the connection point of the other Coating of the first capacitor. in the circuit like that

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is arranged that in combination with the means mentioned, the ¥ iderstands and the capacitor together the deliver correct time constants

In the different embodiments according to According to the invention, it was assumed that voltage jumps formed by superimposing on the DC setting voltage can be obtained by capacitive coupling, without an amplifier element over the entire peak-to-peak voltage range Needs to be controlled, the amplifier element is switched off, in the saturated State or still has a constant voltage and is therefore protected from excessive voltages is.

It should be taken into account that the positive and negative voltage jump have a certain RC time constant so that it is deleted in the correct way. With the help of the protective equipment, the setting resistors and the A fixed capacitor should be used to determine the time constant for the two polarities.

Some embodiments of the invention are shown in the drawing and are described in more detail below. It shows

Fig. 1 schematically shows a quenching circuit according to the invention, in which the collector-emitter voltage is almost remains constant;

Fig. 2 is a circuit diagram with a series arrangement,

3 shows a circuit diagram with blocking diodes and time constant adaptation, and

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4 shows a circuit diagram with collector-base protection.

In Fig. 1, a supply of 250 V is connected to terminals 1 and 2 of the extinguishing circuit, which converts this voltage into the setting voltage of 130-230 V by means of the voltage divider consisting of resistors 3 and h and a potentiometer 5.

To have an adjustable DC voltage source

To obtain low internal resistance, the screen labeled S is over a display tube, not shown a line 6 and is the cathode circuit of the display tube, which is denoted by K, via a line 7 to the emitter circuit connected to an emitter follower.

This emitter follower is constructed from a transistor 8 with an emitter resistor 9 and is set by the tap of the potentiometer 5 via a resistor 10 connected to the base. The collector is connected to the positive terminal 1 via a collector resistor 11 comparable ψ inhibited while an emitter resistor 9 is connected to the negative terminal 2 through the line. 7 In this case, the transistor 8 is of the npn type.

The positive and negative voltage jumps required for deletion are obtained via a changeover switch 12, which serves as a square wave voltage source. For this purpose, a break contact 15 is placed on the line 7 while a Working contact 14 connected to terminal 1 and a main contact 13 is capacitively coupled to the emitter follower. The voltage pulses of 250 V are transmitted via an between the base of the transistor 8 and the main contact 13

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The capacitor 16 is switched on and it decreases exponentially due to the presence of a base equivalent resistance, which base equivalent resistance consists of the resistances of the voltage divider 3, ^ »5t the base resistance 10 and the input resistance of the transistor circuit. The peak-to-peak value of the voltage jumps is almost equal to 500 V _f while the transistor 8 has a maximum permissible collector-emitter voltage of 180 V, for example. To protect the transistor, a capacitor is attached between the main contact 13 and the collector of the transistor 8, while the time constant of the base circuit, which is determined by the capacitor 16 and the base equivalent resistance, is selected to be equal to or slightly greater than the time constant of the collector circuit, which is determined by a capacitor 17 and the parallel connection of the collector resistor 11, the Emitterwider standee 9 and the screen load. In this case, the same voltage jump is obtained at the three terminal electrodes of the transistor, which decreases exponentially in an almost identical manner. The voltages between the electrodes in question remain constant and there is no risk of transistor limit voltages being exceeded.

In FIG. 2, corresponding parts in FIG. 1 with the same reference numerals · In this case, the DC voltage source is floating but formed by the EinitterfcIgerschaltung from siraer separate winding ein.cs §pei8etz> ai? .8form% torsg z _e B _s one ¥ ee! iseirieh ®tb or one © Heiätx * ft? seCor ^ ators _g tifeer © inesü Glsieteiehos · = issad BAD 0RIG1NAL2 0 9 8 31/0593

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optionally voltage stabilizer circuit 19 is fed.

The terminal 2 and thus the common line of the emitter follower circuit are detached from the line 7 leading to the cathode K, but remain with this line a low-resistance resistor 21 is connected.

The changeover switch 12 is now fed from a separate source 22 of 250 V, with the normally closed contact 15 on line 7 and normally open contact 14 are at +250 V « The main contact 13 is via the capacitor 16 by means the mentioned common line connected to the emitter follower scarf device.

By switching the switch 12, the desired erase voltages across the resistor 21 with a RC time determined by the capacitor 16 and the resistor 21 is obtained. These voltage jumps are in series with the setting voltage for the screen, which is above the Emitter resistance 9 can be measured. Since, as seen from the display tube, the resistor 21 increases the internal resistance of the DC voltage source, this is it selected low resistance. Since the erase current to the display tube must flow through the emitter follower circuit due to the series arrangement, it is ensured that this Current cannot cause a voltage drop, which could possibly damage the transistor, waa is achieved in that the internal resistance of the supply part a e.g. with the help of the buffer capacitor 20 is chosen low, while the transistor 8 is within

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is controlled such wide limits that it does not block and the extinguishing current as a modulation component on its setting current is permissible "

In Fig. 3, by means of the switch 12

and the voltage jump obtained by the capacitor 16 is formed directly on the line 6 leading to the screen S.

In the case of the negative-going pulse, the capacitor 16 and the changeover switch 12 form a load for the emitter follower «the latter will deliver a large current, causing the transistor 8 by the presence of the base resistor 10 with the voltage divider 3, ^ »5 and the collector resistor 11 saturates. as The equivalent resistance for the RC time should now be the parallel connection of the resistors mentioned with an emitter resistor 9 and the shield load can be used.

The positive pulse causes the blocking of the diode connected between the emitter connection of the transistor 8 and the emitter circuit, so that the transistor is shielded from the high voltage. Since the RC time for this pulse must be of the same order of magnitude as for the negative pulse, there is a parallel path in the form of a resistor 25 and a diode 2k , which ensures that the resistor 25 has the same value as the parallel resistance of the resistors 10 and 11. In FIG. K , the connection point of the capacitor 16 lies between the base resistor 10 and the transistor 8.

In the case of the positive-going impulse the follows

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* Emitter follower of the supplied voltage, but due to the larger current it reaches the saturation state through the Presence of collector resistance 11. One between the base and the collector arranged diode 26 is thereby switched in the forward direction, so that at the transistor electrodes no high voltages occur. For the RC time, the parallel connection of the resistor 10 and the voltage divider 3f ** »5 and the resistor 11, of the resistor 9 and the screen load must be taken into account.

The negatively directed pulse is also passed on by the emitter follower, the collector-emitter voltage of the transistor rising to a value at which the diode 26 breaks down in the reverse direction. This breakdown voltage is such that the highest permissible voltages across the transistor are not exceeded. The diode 26 can, for example, be a Zener diode of a certain type. In this circuit according to FIG. U , it will be desirable that the RC time for negative and positive step voltages is essentially determined by the resistor 10 and the voltage divider 3 # ^ i 5. The time obtained for the positive jump will be slightly shorter than the time obtained for the negative jump due to the parallel connection of the resistors 11, 9 and the screen load, which latter jump probably has the same RC time at the beginning of the pulse as long as the diode 26 operated in the reverse direction.

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However, it has been found that the

Writing the screen full with the positive voltage pulse is not as critical a process as the actual deletion with the negative pulse, so that the sizing of the circuit can be matched to the latter erasing phase should.

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Claims (1)

PHN.5365, PATENT CLAIMS ι extinguishing circuit for a display tube with a Storage screen which is a DC voltage source connected between the storage screen and the cathode circuit Storage of information and contains a first capacitor, which is connected to a square wave voltage source with a coating is connected, whereby through the interaction of this condensation ψ sators with resistors an erasing voltage is obtained which is the differential of the square wave voltage which is superimposed on the voltage of the DC voltage source and which erases the information on the screen, characterized in that the DC voltage source contains an emitter follower circuit, the emitter circuit of which is between the storage screen (s) and the cathode circuit is switched on and which contains at least one transistor (8) whose maximum permissible collector-emitter voltage is lower than the sum of the k is DC voltage and the peak value of the erase voltage, that means are provided with the help of which the smitter follower circuit is protected from excessively high voltages, and that the connection point of the other surface of the first Capacitor (16) is arranged in the circuit in such a way that, in combination with the means mentioned, the resistances (3f 4, 5 »10) and the capacitor (16) together the deliver correct time constants. 2, quenching circuit according to claim I ₁ dadurdti characterized in that the mentioned connection point between a base resistor (lO) and the base input of the emitter 209831/0593 - 15 - PHN.5365. follower transistor (8), and that the means mentioned are formed by a collector resistor (ii) and a second capacitor (17), one of which is coated with the collector of the emitter follower transistor (8) and the other is coated with a square-wave voltage source (12, 13 » 1 ^ _f 15) is connected, while the time constant of the base equivalent resistors (3, ^ »5» 10) and the first capacitor (16) is at least equal to the constant of the parallel connection of the collector resistor (11), the emitter circuit equivalent resistors (9, 5) and the second capacitor (17). 3. Quenching circuit according to claim 1, characterized in that a resistor (9) is arranged in series with the emitter circuit between the storage screen (s) and the cathode circuit (κ), one end of which is connected to the common feed line (7) of the DC voltage source is, and that the mentioned connection point is on the mentioned common line of the DC voltage source, while the mentioned means are formed by a low internal resistance of the supply part of the DC voltage source and by a sufficient current control of the emitter follower transistor (8), so that the voltage corresponding to the extinction voltage across the DC voltage source Extinguishing current builds up almost no voltage drop « 4, extinguishing circuit according to claim 1, characterized in that that the means for protecting the emitter follower circuit (8) have a first between the emitter terminal of the Emitter follower transistor and the emitter circuit in the same forward direction as the base-emitter circuit of this 20 983 1/0593 - 16 - PHN.5365. Transistor (8) arranged diode (23), furthermore a base resistance (10) and a collector resistance (11), and that in parallel with the circuit consisting of the base resistance (10), the base-emitter circuit and the first diode ( 23), a series arrangement of a resistor (25) and a second diode (2k) is connected with a current flow direction opposite to that of the first mentioned circuit, while the mentioned connection point is between the first diode (23) and the emitter circuit « 5. Extinguishing circuit according to claim 1, characterized in that that the means for protecting the emitter follower circuit has a collector resistor (11) and an between the base and the collector of the emitter follower transistor (8) arranged third diode (26) contain which diode the same forward direction as the base-collector diode and at a high voltage, which is the highest j | permissible base-collector voltage of the transistor falls below, in the other direction lets current through, while the mentioned connection point between a base resistor (1O) and the base input of the emitter follower transistor lies. 6. Storage device for storing information by means of a display tube with a storage screen, characterized in that an erasing circuit for Clearing the memory screen according to any of the preceding Claims is applied, 209831 / 0B93 Lee r se 11 e