US2459602A

US2459602A - Circuit for stabilizing focus of magnetically focused cathoderay tubes

Info

Publication number: US2459602A
Application number: US718196A
Authority: US
Inventors: Lynn J Ulman
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1946-12-24
Filing date: 1946-12-24
Publication date: 1949-01-18
Anticipated expiration: 1966-01-18

Description

Jan. 18, 1949. J ULMAN 2,459,602

CIRCUIT FOR STABILIZING FOCUS OF MAGNETICALLY FOCUSED CATHODE-RAY TUBES Filed D80. 24, 1946 Regulated B+ INVENTOR Lynn J. Ulmqn.

A'TTORN W Patented Jan. 18, 1949 UNITED STATES CIRCUIT FOR STABILIZKNG FOCUS OF MAGNETICALLY FOCUSED CATHODE- RAY TUBES Lynn J. Ulman, Baltimore, Md, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 24, 1946, Serial No. 718,196

3 Claims.

raybeam. This is usually done by means of a focusing coil traversed by direct current which is substantially coaxial with the major axis of the cathode ray tube. The drawing attached hereto illustrates the general form of such an arrangement.

With a given voltage impressed on the accelerating anode of the cathode ray tube to impart a definite velocity to the electrons impinging on the screen, a definite value of current must flow through the focusing coil in order to maintain a sharp focus of the cathode ray beam on the fluorescent screen. In practical cases, both the value of the voltage on the accelerating anode and the value of current through the focusing coil are likely to vary in undesired fashion be cause of random variations in the voltage of the sources supplying direct current, to changes in resistance with temperature of the focusing coil, andrto similar causes. If the current through the focusing coil is adjusted to the necessary value to focus the cathode ray beam accurately at the beginning of operations, the causes above-mentioned are likely to cause defocusing of the beam from time to time during subsequent operation. It is always expensive, and in many cases impracticable, to provide voltage regulators to maintain the direct current voltage sources so free of random and undesired variations as to maintain the desired degree of focus of the cathode ray beam at all times.

One of the objects of my invention is to provide a novel circuit in which the effects of voltage variation on the above-mentioned direct-current sources on defocusing of the cathode ray beam are substantially eliminated.

Another object of my invention is to provide a cathode ray tube and circuit therefor of such type as to maintain substantially constant focus in the cathode ray beam at all times during use.

' Still another object of my invention is to provide an arrangement economical in first cost and simply adjustable to maintain constant the size of the trace of the cathode ray beam on the screen of a fluorescent cathode ray tube.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawing in which the single figure is a schematic diagram of "a circuit supplying accelerating voltage and focusing coil currents to a cathode ray tube, in accordance with the principles of my invention.

Referring in detail to the drawing, a cathode ray tube l, which may be of any desired conventional type, has at one end a fluorescent screen 2 on which impinges an electron beam produced by acceleration of electrons from an electron emissive cathode 3 by an accelerating anode 4. Such arrangements are conventional in the art and require no extended description. In order to focus the trace of the electron beam toan area of small cross-section on the screen 2, I provide a focusing coil '5 which may also be of a type well known in the art. The voltage for the accelerating anode 4 is furnished by the output of a full-wave rectifier 6 of a type convention-al in the art which may be provided by any suitable filtering arrangements, this rectifier being supplied with alternating voltage from a conventional alternating-current supply transformer primary winding 1 connected to an ordinary alternating-current power source (not shown).

Direct current for the focusing coil 5 is supplied from a second rectifier 8 energized from the same source, which likewise may be suitably filtered by arrangements conventional in the art. The current flowing through the focusing coil 5 from the rectifier 8 flows serially through an electron tube 3 of a type conventional in the art, which is provided with a control electrode I i and a screen grid 12, also conventional in the art. The amount of current through the focusing coil may be initially adjusted by Varying the potential impressed on the control grid I l, which is derived from a suitable potentiometer supplied with direct current from the positive terminal l3 of a regulated voltage supply of any type well known in the art to be suitable.

It can be shown readily that the maintenance of a constant size of the focal spot of the electron beam on the screen 2 requires that the following equation shall be fulfilled:

H=magnetic field strength in the tube due to the focus coil.

V=-accelerating potential of the cathode ray tube.

K=a constant.

Suppose now that the-electron beam has been adjusted to the desired focus initially with a voltage V0 impressed from the voltage source 6 and with a magnetic field strength of H0. Now suppose that the alternating-current supply connected to winding 1 undergoes an increase so that the voltage impressed by the source 6 on accelerating anode 4 increases by a small fraction a.

3 Then thefollowingquantity, corresponding to the; left-hand side of Equation 1, will represent the condition in respect to the focus of the cathode ray beam on the screen 2:

Since a is small, we may neglect a and higher powers in the following discussion.

Since the same source which furnishesvoltage to rectifier 6 likewise furnishes Voltage to rectifier 8, it is evident that the voltage impressed by rectifier 8 on the circuit through focusing coil will likewise increase; and since the circuit through that focusing coil embodies a non-linear impedance (i. e: electron tube 9.), the current through the focusing coil 5: will increase by a small fraction. 1) which will, in general, be different fromv a. Hence the conditions at the focus of; the. cathode ray beam in tube I will be represented by a: modification of the left-hand member of. Equation 1 which will take the form Since: the'f-raction b is a small one, 19 may beneglectedfrom further consideration.

Itwill; be evident that Equation 1- may then be rewritten; in l the form It isevident that if we can make: 2b equal to a, the conditions at the cathode ray beam focus, being represented by Ho Vo, will be independent of both b and a. In other words, the focus of the cathode ray beam will be maintained constant, regardless. of. any small fluctuations which may take place in the voltage supplied from the power source.

Since the electron tube 9 is a non-linear resistor, it is readily possible, by. properly adjusting the value of the voltage impressed on the screen. electrode E2, to insure that the percentage changein current flowing through the focusing coil 5 is exactly one-halfof the voltage change impressed by the voltage source 8.: to cause that current flow. The voltage impressed on the screen. electrode I2 is determined by therelative magnitudes of the resistors M and I5, and by properly vary-- ing: these resistors, the condition that the rate of current change in the plate circuit of tube 9 shall be one-half as. great as the direct-current voltage supplied to that circuit by source 8 may be readily fulfilled.

The initial. focusing of the cathode ray beam. on the screen 2 may be effected by varying the potential impressed by the potentiometer It on the control electrode II. The potentiometer i6 is supplied with voltage from a regulated direct-- current source, which may be of small cost because it merely supplies voltage to a control electrode.

The time constants of the circuit supplyingvoltage to the screen grid and the circuit supplying accelerating voltage to the anode 4 should be as nearly as possible the same in order that voltage fluctuations shall be quickly compensated for.

I claim as my invention:

1. In combination with means for producing an 4 electron beam bymeansmf an accelerating anode, a focusingmagnet fOr said electronbeam, a power supply and means for impressing voltage therefrom on said accelerating anode, means for impressing voltage. on. said focusing magnet from a. current source energized by said power supply in series with said focusing magnet and with an electron tube having an anode, a cathode, a first control electrode adjacent said cathode and asecond control electrode between said first control electrode and said anodes, means for impressing voltage derived from said supply on said second. control electrode, and means for so adjusting the voltage impressed on said control electrodes that the fractional change in current energizing said focusing magnet due to changes in said power'source'is substantially one-half'the fractional change in" the voltage impressed on saidaccelerating-a-node.

2. In combination with means for producing an electron beam by means of an accelerating anode, a focusing magnet forsaid electron beam, a power supply and means for impressing voltage therefrom on said accelerating anode, means energized by said supply for impressing voltage on said focusing magnet from a current source in;

series relation with said focusing magnet. and with an electron tube having an anode, a cathode,

a first control electrode adjacent said cathode and.

a second control electrodebetweenl said first control electrodeandsaid anodes, means forimpressing voltage derived from-'saidsupply'on said sec-- ond control electrode, and means for so: adjusting:

the voltage impressed on saidi control electrodes that thefractional change in current energizing;

said focusing magnet'dueto changesiinasaidpower. source is substantially one-half the fractional changein the-voltage impressedion. said accelerating anode.

3'. In. combination with means for producingaam electronbeanl by; means ofan acceleratinganode,

a focusing: magnet for' said. electron beam, a. power supply andimeans for impressing. voltage:

therefrom. on said accelerating anode, means: for" impressingvoltage on saiict focusing'magnet from; a current: source. energized bysaidtpower supply:

inseri'es with said focusing magnet and: with. an electron: tube having an anode,,a cathode, a first control electrode.- ad'jacent said; cathode. and a. second control: electrode between said first con. trolelectrode and: said anodes; means for. impressing; voltage from said current supply on one ofsaid control. electrodes, and means for'so ad'- justing the voltageimpressed onsaid control elec trodesthat the-fractional changein current energizing saidl IEOCLlSiBg, magnetdue to changes. in said power source is. substantially one-half the fractional change in. the voltage impressed on said. accelerating, anode.

LYNN.J..ULMAN.

REFERENCES CITED The following references are of record in, the file of this patent:

, STATES PATENTS Number Name Date 2;219;l94' Mynall Oct, 22,1940 2,291,682 Blumlein'et' a1. Aug. 4', 1942