US2668930A - Protective system - Google Patents

Description

Feb. 9, 1954 C. G. SUITS PROTECTIVE SYSTEM Filed Oct. 18, 1950 Fig. 8

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Patented Feb. 9, 1954 PROTECTIVE SYSTEM Chauncey G. Suits, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application October 18, 1950, Serial No. 190,692

4 Claims.

My invention relates to protective systems and, more particularly, to electric circuits for protecting thermionic cathodes of electron discharge devices.

An object of my invention is to provide a new and improved electrical circuit for protecting thermionic cathodes of electron discharge devices.

Another object of my invention is to provide a new and improved circuit for controlling the electron-emission from thermionic cathodes, especially during the warm-up period of the cathode.

A further object of my invention is to provide a new and improved circuit for protecting thermionic cathodes during the warm-up period and also for reducing the time required for the cathode to be brought up to normal operating temperature.

A still further object of my invention is to provide a new and improved circuit for controlling the current to electrical devices having temperature varying characteristics.

In the attainment of the foregoing objects, I employ a resistor which has a negative temperature-resistance coefficient, 1. e., the resistance varies inversely with the temperature. This resistor is connected to a device so as to limit the current thereto until the temperature of the resistor exceeds a determinable value. The resistor is physically located in close relation to the device such that the resistance of the resistor is directly dependent upon the temperature of the device. In one embodiment, the circuit is arranged to protect a directly heated cathode, while in another embodiment it is arranged to protect an indirectly heated cathode.

In a third embodiment, there is shown a circuit arrangement employing a pair of resistors having negative temperature-resistance characteristics. This circuit employs one resistor to protect the cathode in the same manner as do the resistors of the first two embodiments and the other resistor is used to limit the heating current when the temperature of the cathode exceeds a predetermined value. In all of these embodiments, the resistors are mounted near to the cathode such that the temperature of the cathode controls the temperature and, hence, the resistance of the resistors.

While the invention is described as applied to protection of cathodes employed in ionic atmospheres, it is to'be understood that it is not limited to such application, as the disclosure in this respect is merely illustrative for purposes of explaining the inventive concept.

For further objects and advantages and for a better understanding of my invention, attention is now directed to the following description and accompanying drawing and also to the appended claims. In the drawing, Figs. 1, 2 and 3 are separate embodiments of my invention.

Referring particularly to Fig. 1, a fluorescent lamp I is provided with a hermetically sealed envelope 2, a pair of heating filaments 3 and 4, and a pair of electron emission surfaces, hereinafter called cathodes, 5 and 6. A protective resistor 1 is serially connected between cathode 6 and one terminal of a source of alternating voltage. A ballast reactor 8 is connected between cathode 6 and the source of alternating power for reasons Well known in the art and cathode 5 is directly connected to another terminal of said source of alternating power. When power is first supplied to the lamp, it is undersirable for a discharge to occur between cathodes 5 and 6, because emission therefrom before they have been sufliciently heated reduces the life of the cathodes and, consequently, the life of the lamp. To protect these cathodes from emission before they have been sufiiciently heated, a protective impedance 1 is serially connected between the source of power and one of these cathodes. Resistor 1 is physically positioned close to heating filament 4 such that it derives its temperature from the same source as does cathode 6. Resistor I has a high negative temperature-resistance coefiicient, which causes resistor I to have a high resistance when it is cold and to have a low resistance when it is heated. While cathode 6 and, hence, resistor 1 is at a relatively low temperature, resistor I has a high value which necessarily limits the current to cathode 6 and, consequently, prevents a discharge between cathode 5 and cathode 6. As the temperature of the cathodes increases to the normal operating level, the resistance of resistor I suddenly changes from a very high resistance value which would not permit a discharge between cathodes 5 and 6 to a very low resistance which does not interfere with the passage of normal lamp current. Thus, in starting, the lamp is prevented from operating until its cathode is at a suitable temperature and automatic protection against cold starting is provided. I Referring particularly to Fig. 2, a portion of a fluorescent tube employing a directly heated cathode 9 is shown. A protective resistor ll! of the same type of material as resistor 1 is serially connected between cathode 9 and a source of alternating voltage. Resistor II] is located in close physical relation to cathode 9 such that the temperature and, hence, the resistance of resistor I0 is directly dependent upon the temperature of cathode 9. In this arrangement, cold starting of the fluorescent lamp is avoided because emission cannot occur from cathode 9 until resistor H! has been heated to a predetermined value.

Referring to Fig. 3, an electron discharge device I l, which contains an ionizable medium, and which is known in the art as a thyratron, is provided with an indirectly heated cathode l2. Cathode I2 is heated by a conventional filament l3 and a pair of resistors l4 and I5 are located.

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