US3005161A

US3005161A - Electronic timer

Info

Publication number: US3005161A
Application number: US44347A
Authority: US
Inventors: Bruck George
Original assignee: Avco Corp
Current assignee: Avco Corp
Priority date: 1960-07-21
Filing date: 1960-07-21
Publication date: 1961-10-17
Anticipated expiration: 1978-10-17

Description

Oct. 17, 1961 ca. BRUCK 3,005,161

ELECTRONIC TIMER Filed July 21, 1960 Y I? i l 1 .6 24 1.. i; b) g l V I2 i 22 I m? I E 1 M l -L- 7\ Rs n? INVENTOR.

GEORGE BRUCK.

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This invention relates to a simple, inexpensive electronic timer suitable for controlling large currents of short duration.

In many applications, such as fusing, delayed sequential switching, and other operations where relatively large, short-term currents are controlled timewise, the practice has been to employ gaseous discharge tubes. The instant of firing of the tube serves as the controlling function.

While gaseous discharge tubes are capable of handling large currents of short duration, the commercial manufacturing tolerances of these tubes are such that the firing voltage of one tube will vary to a considerable extent from that of another supposedly identical tube and, hence, such gaseous discharge tubes are not satisfactory in those applications where great precision is required for the timing operation.

Therefore, the primary object of this invention is to provide a gaseous discharge tube and circuitry wherein is provided extremely accurately timed, high speed switching of large currents of short duration.

Another object of this invention is to provide an extremely accurate timer using components having only commercial tolerances.

Another object of this invention is to provide circuitry for a gaseous discharge tube wherein the instant of firing is independent of tube parameters.

Other objects and a better understanding of the precise nature of this invention may be obtained by reference to the following detailed specification and to the accompanying drawings, in which:

FIG. 1 represents a preferred embodiment of my invention and FIG. 2 is a simplified representation of the RC components of FIG. 1.

FIG. 1 shows a discharge tube filled with an inert gas, such as neon or radioactive krypton. The tube 10 contains three electrodes, including an auxiliary anode 11, a cathode 12, and a main anode 13. Bias for the auxiliary anode 11 is provided by means of two

resistors

14 and 15 which are connected across a large storage capacitor 17. In this application the storage capacitor 17 is charged by a battery 16 or other suitable power supply which is momentarily connected across the capacitor by a switch S. Bias for the cathode 12 is provided by means of a relatively small resistor 18 shunted for surge currents by a large capacitor 19 and a damper circuit including a capacitor 20 and a resistor 21 for preventing undesired oscillations. The values of

resistors

14 and 15 are selected so that the voltages across them are equal to approximately one-half of the voltage across the capacitor 17.

Bias for the main anode 13 is provided by means of an R-C network including a capacitor 22 and a resistor 23 in series across the

resistors

14 and 15. The main anode 13 is connected to the junction of resistor 23 and capacitor 22 through a very low impedance load, in the instant application, a fuse 24.

It is known, that gaseous discharge devices, such as tube 10, show a constant flip-over or switching voltage point, once the gas chamber is ignited or ionized by the discharge through the auxiliary anode. Advantage is taken of this characteristic by the use of circuitry which makes the time of firing of the main anode 13 dependent almost solely on the charging of the capacitor 22 to a 3,605,161 Patented 0st. 17, 1961 fixed value, namely, the voltage of the auxiliary anode 11.

In operation, the capacitor 17 is charged to the required voltage upon the momentary closing of the switch S, and equal voltages are established across the resistors 14- and 15 so that the voltage on the auxiliary anode 11 is at approximately the midpoint of the capacitor 17. As will be seen, the capacitor 17 must be established large enough to supply the required discharge currents without appreciable changes in its voltage level during the firing period of the tube.

Upon the charging of capacitor 17, a discharge results almost instantly from the auxiliary anode 11 to the cathode 12, and the resultant current flows through the resistor 18, elevating the voltage at the cathode 12. At the same time the capacitor 17 begins to discharge through the resistor 23 to charge the capacitor 22.

A nominal time, dependent upon the value of the R-C parameters, is required for charging condenser 22 to bring the voltage of the main anode 13 up to that of the auxiliary anode 11. When this potential is reached and slightly exceeded, the discharge in the valve will flip over, i.e., transfer conduction to the main anode 13, and the fuse 24 or similar device incorporated in series with this anode will ignite.

Thus, by means of the resistors 14 and 15 I provide a predetermined and fixed voltage at the auxiliary anode 11 sufficient to cause a discharge from the anode 11 to the cathode 12 and through the cathode resistor 18. Cur rent flowing through the resistor 18 raises the voltage of the cathode, thereby maintaining the appropriate discharge conditions of the particular tube used in the circuit. That is to say, the voltage at the auxiliary anode 11 remains fixed, no matter what the voltage drop is between the auxiliary anode 11 and the cathode 12, and variations among tubes are compensated by the increase in cathode voltage due to the current flow through resistor 18. In addition, a discharge path is established between the main anode 13 and the cathode 12 when the voltage at the main anode 13 equals or just slightly exceeds the voltage at the auxiliary anode 11. Since the voltage at the anode 13 is fixed, the time when such equality exists may be precisely predetermined by selection of the proper parameters for the resistors and the capacitors. Hence, the variations in characteristics among supposedly identical gaseous discharge tubes are irrelevant, and tubes having extremely wide commercial tolerances are perfectly acceptable since the instant of firing of the tube is substantially independent of tube parameters.

For the purpose of determining circuit parameters, the circuit of FIG. 1 may be shown in simplified form as illustrated in FIG. 2, wherein resistor R is equivalent to

resistors

14 and 15 in parallel. The instantaneous voltage conditions existing across capacitor 17 and capacitor 22 are defined by the equation:

E =E (e" l -e"= 2 where E =the voltage appearing across capacitor 17; E =the voltage appearing across capacitor 22;

where R C R and C are the impedances of the respective elements.

It will be apparent to those skilled in the art that the accuracy of this system resides largely in the selection and maintenance of suitable parametric values. Satisfactory system parameters may be arrived at empirically aided by calculations assuming certain values initially based on knowledge and experience. in a general sense, typical applications will require that:

(1) Capacitor 17 be very much larger than capacitor 5 (.2) Capacitor 19 be large with respect to capacitor (3) Resistor 2 3 be very much larger than the summation of resistors 14- and i5; and

(4) The product of R and C be very much larger than the product of R and C The circuitry described was developed primarily as a short-term control device suitable for arming explosives, but it is nowise limited to such utility. The principle is applicable to functions requiring accurately timed repetitive switching. Suitable parameters need be chosen for the desired application. And their selection may be guided by calculations made according to established circuit theory.

What is claimed is:

1. An electronic timer comprising: a gaseous discharge tube having a main anode and auxiliary anode and a cathode; means for applying a predetermined voltage between said auxiliary anode and said cathode sutficient to establish conduction between said auxiliary anode and said cathode; a condenser connected between said main anode and said cathode; means for charging said condenser to a voltage slightly greater than said predetermined voltage at a predetermined rate; and means for maintaining the voltage at said auxiliary anode substantially fixed whereby conduction between said main anode and said cathode results at a time determined by the rate said condenser is charged.

2. The invention as defined in claim 1 wherein said means for maintaining said voltage includes a resistor in series with said cathode.

3. An electronic timer comprising: a gaseous discharge tube having a main anode, an auxiliary anode and a cathode; a cathode resistor in series with said cathode; means for applying a predetermined substantially fixed voltage across said auxiliary anode, said cathode and said cathode resistor in series, sufficient to establish conduction between said auxiliary anode and said cathode; a condenser connected across said main anode, said cathode and said cathode resistor in series; and timing means for charging said condenser at a predetermined rate to a voltage level just slightly greater than said predetermined substantially fixed voltage, whereby a conduction path between anode and said cathode is established at a time dependent on said predetermined rate.

4. The invention as defined in claim 3 wherein said means for applying a predetermined substantially fixed voltage comprises a charged storage capacitor connected across first and second resistors, said auxiliary anode, said cathode and said cathode resistor in series being connected across one of said resistors, and wherein said timing means is a third resistor, said third resistor and said condenser being connected across said first and second resistors, said time being established in accordance with the R-C parameters of said storage capacitor, said resistors and said condenser.

5. The invention as defined in claim 4 wherein a large by-pass condenser is connected across said cathode resistor.

No references cited.