US2300127A

US2300127A - Protective device

Info

Publication number: US2300127A
Application number: US360764A
Authority: US
Inventors: Maddock Alan Julian
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1939-10-20
Filing date: 1940-10-11
Publication date: 1942-10-27
Anticipated expiration: 1959-10-27

Description

oct. 27, 1942. A, J. Mmmm f 2,300,127

` I RoTEcTIvE' DEVICE Filed oct. 11. 1940 F ig. 2.

lIl l fIToRNeY Patented Oct.' 27, 1942 PROTECTIVE DEVICE Alan Julian Maddock, London, England, assigner to lVestern Electric Company, Incorporated, New York, N. Y., a corporation of New York Application October 11, 1940, Serial No. 360,764 In Great Britain October 20, 1939 (Cl. Z50-'17) Claims.

It is the purpose of this invention to provide protection for apparatus involving the use of electron discharge valves employed as ampliers of electric waves or as generators thereof. A particular object of the invention is the provision of protection for radio transmitters against damage by lightning or breakdown of the apparatus and for low frequency amplifiers against overload. i

It is a further object of this invention to provide such protection with the minimum delay of action and a circuit is employed which obviates the necessity for any relays or contactors.

In a radio transmitter, for example, supplying radio frequency power to an antenna system, serious damage may be caused to the equipment if a lightning discharge occurs to the antenna. The lightning may initiate an arc or flashover between some part of the electrical circuit and earth and through the arc so initiated the transmitting equipment will then supply considerable radio frequency power and it is this power which often causes the greatest damage to the apparatus. Means must therefore be provided for removing the anode voltage supply to some part of the transmitter or to render at least a part of the amplifying chain inoperative as an amplier in order to extinguish this power arc. Once extinguished, normal operation may then be resumed since the lightning discharge has ceased and so the arc will not be initiated again.

According to the invention, the arc initiated by the lightning discharge or iiashover acts as a form of electronic switch to connect into the circuit of one or more of the valves a source of voltage which renders inoperative the normal amplifying or oscillating action of the valve cr valves. In one arrangement for carrying out the invention a negative bias is applied to a grid of one or more valves in the amplifying chain, preferably one of the earlier stages in the amplifier. By this means the said valve or valves and the succeeding valves are rendered inoperative as ampliers so that no power lis generated to feed into the arc. The arc thus ceases, the

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bias thereby being removed and the amplifiers are then able to operate again normally.1

The invention will be better understood from a consideration of the following descriptiony in conjunction with the accompanyingdrawing, Figs. l, 2 and 3 of which show three methods of applying the invention to a radio transmitter. Referring to Fig. 1 V is a thermionic valve acting as an amplifier in the radio frequency amplifying chain and is shown for convenience as a triode but it will be clear from the following description that the system may be applied equally well to a tetrode, pentode or other multielectrode tube, said tube acting either as ampliiier or as oscillator.

T represents the point at which protection against a discharge is required and may be the antenna system connected to the ampliers or the transmission line or concentric feeder connecting said amplifier to the antenna system: in any case G represents a point Vwhere a discharge to earth may occur and a definite point may be arranged for G if desired such as horn or ball gaps though it is clear that the system will function no matter where the arc occurs so long as there is a connection for direct current to the point of iiashover.

The grid of V is supplied with its normal D. C. potential by the source B1 through a resistance R whose function will be described later. In the diagram the supply is shown for convenience as being connected to the grid through a radio frequency choke L3 but obviously any known scheme may be employed. The source B1 and cathode of V are connected to earth.

From the grid side of resistance R is connected a source of supply B2 with itsv negative pole connected to R and its positive pole connected to the antenna system, transmission line or other point where protection is required (designated by T) through a radio frequency filter system such, for example, as shown consisting of chckes and condensers L1, L2 and C, the purpose of this being to provide a D. C. connection to T whilst preventing radio frequency voltage being communicated to B2 and the remainder of the system.

The method of operation may now be described as follows:

Under normal conditions valve V has impressed upon its grid the normal D. C. voltage supplied by the source B1 but when an arc occurs between 'I' and earth, i. e., across the gap G, the arc acts as an electronic switch and connects the positive pole of B2 to earth so that the voltage now existing between the grid and cathode of V is negative by the difference between the supply voltage of B2 and the arc drop across G. If the value of B2 is correctly chosen the grid of V will be made such an amount negative, or substantially Zero if preferred, with respect to the cathode that V ceases to operate as an amplifier and the whole of the succeeding stages receive no radio frequency driving voltage and so no power Vis delivered to the point T. The main power arc across G thus ceases and the only current that might tend to persist across G is that determined by sources B1 and B2 acting in series (opposing in the case illustrated) and flowing through resistance R. The value cf R and voltage of B2 are so chosen that the current from B1 and Bz through the arc is of such value that the arc is not maintained- As soon as the arc ceases B2 is automatically disconnected from earth and only the normal grid voltage from B1 is applied to the grid of V and the amplifier then again functions normally. The protective circuit is then ready to function when any further discharge occurs across G.

It will be seen from the foregoing that the system operates by the application of a zero or negative potential to the grid of a thermionic valve during the time of passage of an arc between one point of the system and earth. It is clear, therefore, that the system is not limited to the use of a triode valve as described above but may be applied to a tetrode, pentode or other multielectrode tube and may be applied to control of voltage not only of the control grid but of the screen grid, suppressor grid, or any other electrode where the application of a substantially zero or negative voltage renders the valve inoperative in its normal manner including an electrode specifically added to the valve for use in the manner described.

It is clear also that the scheme is not limited to use in radio frequency amplifiers, which latter may be of any type and amplifying modulated waves or not, but may be used equally well for so called audio-frequency or vision-frequency amplifiers or any amplifiers or oscillators where thermionic valves are employed as amplifiers or oscillators to provide protection against damage by flashover or overload. In this case the protective filter would be made suitable for the frequencies involved. An extension of the scheme is also possible where it is desired to suppress both radio and audio-frequency amplification at the same time, for example in transmitters employing final stage anode modulation where considerable audio-frequency power is dealt with and both amplifying chains may be rendered inoperative.

The scheme may also be extended to cases where it is desired to secure protection against discharges that may occur over any of a number of points, for example, in an antenna array consisting of several antennae from which discharges to earth may occur, said antennae being fed by one transmitter. In this case separate R. F. filters such as L1, Lz, C, may be connected to each antenna on the one side and on the other all connected to the source Bz as shown in Fig. 2: in this way a discharge to earth on any of the antennae will cause the transmitter to cease functioning momentarily.

A still further application is to the case where two or more transmitters are connected to the same antenna system in which case a separate suppressor source of voltage B2 may be used for each transmitter but the positive pole of all these supplies may be connected to the same R. F. filter system L1, L2, C. By this means all transmitters are rendered inoperative when a discharge occurs from the antenna system to earth. Such a scheme is shown diagrammatically in Fig. 3.

If desired the rate at which restoration of the amplifying chain returns to normal can be adjusted to a certain extent by suitable choice of the value of resistance R and by connecting a condenser across this resistance.

What is claimed is:

1. In amplifying apparatus to be protected against discharge damage by lightning or overload, a thermionic valve having a grid, a source of voltage and a connection from said grid over said Source of voltage to a point in said apparatus where protection is required, whereby the initial arc or flashover acts as a form of electronic switch to connect said source of voltage into the circuit of said valve to render inoperative the normal amplifying action of said valve.

2. In amplifying apparatus to be protected against discharge damage by lightning or overload, a thermionic valve having a grid and a cathode, said cathode being grounded, a source of voltage and a connection from said grid over said source of voltage to a point in said apparatus where protection is required, whereby an arc or flashover to ground acts as a form of electronic switch to connect said source of voltage into the grid-cathode circuit of said valve to render inoperative the normal amplifying action of said valve.

3. In amplifying apparatus to be protected against discharge damage by lightning or overload, a thermionic valve comprised in said apparatus, a grid and a cathode in said valve, a source of voltage, a connection from said grid to the negative side of said source of voltage, and a connection from the positive side of said source of voltage to a point in said apparatus where protection is required, whereby the initial arc or fiashover acts as a form of electronic switch to connect said source of voltage into the circuit of said valve to render said grid of negative potential with respect to said cathode and make inoperative the normal amplifying action of said valve.

4. In amplifying apparatus including a thermionic valve having a grid and a cathode, a source of voltage for rendering said grid sufficiently negative to make inoperative the normal amplifying action of said valve, said grid being connected over said source of voltage to a point in said amplifying apparatus where protection from discharge damage is required, a normal source of grid voltage for said tube and a resistance connected in series with said normal source of grid Voltage.

5. The combination in accordance with claim 4 in which said resistance and said normal source of grid voltage are serially connected between said grid and cathode, and said first-named source of voltage has its negative side connected to the grid end of the resistance and its positive side connected to the point where protection is required.

6. The combination in accordance with claim 4 in which said resistance and said normal source of grid Voltage are serially connected between said grid and cathode, said cathode being ground* ed and said first-named source of voltage has its. negative side connected to the grid end of the resistance and its positive side connected to a point where protection is required against an arc. or flashover to ground.

7. Amplifying apparatus including a thermicnic Valve having a grid, a source of voltage and a protective filter, said grid being connected over said source of voltage and said protective filter to a point in said amplifying apparatus Where protection against discharge damage is required.

8. Amplifying apparatus including a thermionic, valve having a grid, a source of voltage, a pro-4 tective lter and an antenna, said grid being connected over said source of voltage and said protective lter to a point of said antenna where protection from damage due to arc or flashover is required.

9. A radio transmitter including a thermionic valve having a grid, a source of Voltage, a plurality of protective filters and a plurality of antennae, said grid being connected over said source of voltage and through a plurality of said protective lters respectively to said antennae.

10. A plurality of radio transmitters, each having a thermionic valve with a grid, an antenna system, a plurality of sources of voltage, and a protective filter, connections from each of said radio transmitters to said antenna system, a. connection from each of said grids to the negative side of a separate one of said sources of voltage,v