US2320958A - Thyrite protective device - Google Patents

Description

June 1, 1943. G. USSELMAN THYRITE PROTECTIVE DEVICE Original File d Oct. 2, 1956 16 osc/lm 70R rig INVENTOR 0. L. USSELMAN 361! M ATTO N EY Patented June 1, 1943 UNITED STATES PATENT OFFICE THYRITE PROTECTIVE DEVICE George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware 4 Claims.

This invention relates to an improved protective circuit and, more particularly, to a protective circuit for the protection of filament bypass condensers and filament transformers generally employed in a radio transmitter.

This application is a division of application No. 103,643, filed October 2, 1936, now Patent #2,201,211, granted May 21, 1940.

Filament bypass condensers and transformers have heretofore been manufactured with a sufficiently high safety factor above its normal working voltage so that it was not generally necessary to employ any protective device to protect the condensers and transformers against occasional increased voltages. However, with the advent of mercury vapor tube rectifiers now employed in the higher powered transmitters, trouble with condenser and transformer failure has greatly increased due to higher voltage surges. Several attempts have been made to protect the condensers and transformers, such as to shunt spark gaps in rare gas across the condensers for protection, but this method has not proven to be reliable.

In the present invention, the filament bypass condensers and filament transformers are protected by novel circuits employing a particular form of material, sold under the name Thyrite, which is described more fully in United States Patent #1322342, to McEachron, of September 8, 1931, and the General Electric Review for February, 1930, pages 92 to 100. (vol. 33, No. 2).

The use of this Thyrite protective device permits the use of lower voltage bypass condensers, and consequently larger capacity can be obtained in the same size condenser case.

The present invention will best be understood by referring to the accompanying drawing which is a wiring diagram of a portion of a radio transmitter circuit wherein the bypass condensers are protected by Thyrite discs.

The drawing shows an abbreviated transmitting circuit employing a protective device I, which is made up of alternate plates or discs 2 of Thyrite and metallic electrodes 3, 4 and 5, the electrodes 3 and 4 each being attached to a terminal of filament bypass condensers 6 and l which are to be protected. The other electrodes 5 and 5' are connected to ground. From the circuit, it will be noted that the Thyrite is in parallel with the apparatus to be protected. In this case, the center points of the condensers 6 and l are connected to ground. The two outside terminals of the condensers are connected to the filament leads of the tube 8. The purpose of the bypass condensers 6 and l is to act as a radio frequency bypass to prevent the radio frequency from entering and damaging the filament trasformer 9. It is also necessary that the filament leads be grounded for radio frequency currents for the proper operation of a transmitter.

During the operation of high power transmitters employing mercury vapor tubes there frequently occurs an arc-over, due to various causes, across the neutralizing condenser l0 or in the tube 8, itself. In either case, the result is an instantaneous dischargeof the smoothing condenser H in a high voltage rectifier 12. This discharge enters the filament of tube 8 and when protective devices are not used the current of the discharge must flow through the secondary winding of the filament transformer 9, through the impedance 13 to ground. The impedance 13 may be such apparatus as overload relays, meters, resistors, etc.

In any case, the impedance I3 is usually high enough to cause abnormal voltages to occur from the terminals of condensers 6 and 1 to ground. It is not practical for certain reasons, such as space requirement and high manufacturing cost, to make the voltage rating of the condensers high enough to sustain more than about one-tenth of the full rectifier voltage. However, in practical operation, condensers 6 and l are subject to several times their voltage rating when an arc-over, as mentioned above, occurs. This results in frequent condenser and filament transformer failures which continue to occur unless proper means of protection is used. A modulated source of high frequency oscillations I4 is connected to the grid coupling condenser l5 and ground l6.

'Ihyrite has the property of a decrease of resistance with an increase of current through it, which follows the law given more fully in the February, 1930, issue of General Electric Review, and the McEachron Patent #1,822,742, of current=KV where V is the voltage across the Thyrite material, and K is some constant depending upon the size and type of the Thyrite plate. When the voltage is doubled, the current through the Thyrite plate increases about twelve times so that it will be seen that this material is ideal for a condenser protective device, and with proper mounting will protect the bypass condensers and. filament transformers from dangerous voltages which might cause insulation failures of the transmitter, and as a consequence interruption of service.

Details of the mechanical construction of the Thyrite protective structure are shown and described in more detail in the application of which this case is a division. Reference may be made to that case for a detailed description.

While I have have particularly shown and described several modifications of my invention, it is to be particularly understood that my invention is not limited thereto but that modifications may be made Within the scope of my invention.

I claim:

1. In a radio device, a thermionic discharge tube having at least a pair of filament terminals,

an energizing circuit connected thereto, means for effectively connecting each of said terminals to ground for radio frequency comprising a pair of condensers, each having one plate connected to one of said filament terminals and the other plate grounded, and means for protecting said condensers from voltage surges comprising an alternate assembly of Thyrite discs and metallic electrodes in intimate contact with each other, the central and end electrodes of said assembly being connected together and con nected to ground, and connections from each of the intermediate electrodes to said filament terminals.

2. In a radio device, a thermionic discharge tube having at least a pair of filament terminals, an energizing circuit connected thereto, means for effectively connecting each of said terminals to ground for radio frequency comprising a pair of condensers, each having one plate connected to one of said filament terminals and the other plate grounded, and means for protecting said condensers from voltage surges comprising an alternate assembly of Thyrite discs and metallic electrodes, the central and end electrodes of said assembly being connected together and connected to ground, and connections from each of the intermediate electrodes to said filament terminals.

3. In a radio device, a thermionic discharge tube having at least a pair of filament terminals and a plate, a source of high potential having one terminal connected to said plate and. the other connected to a point of common reference potential, a condenser connecting each of said filament terminals to said point of reference potential, said condenser being capable of withstanding normal potentials thereacross but incapable of withstanding substantially the full potential of said source and means for protecting each of said condensers including a resistor connected thereacross, said resistor having a negative resistance-voltage characteristic.

4. In a radio'device, a thermionic discharge tube having at least a pair of filament terminals and a plate, a source of high potential having one terminal connected to said plate and the other connected to a point of common reference potential, a condenser connecting each of said filament terminals to said point of reference potential, said condenser being capable of withstanding normal potentials thereacross but incapable of withstanding substantially the full potential of said source and means for protectmg each of said condensers, said means including an alternate assembly of Thyrite discs and metallic electrodes in intimate contact with each other, the central and end electrodes of said assembly being connected together and to said point of reference potential and connections from each of the intermediate electrodes to said filament terminals.

GEORGE L. USSELMAN.

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