US2548818A

US2548818A - Thermionic overvoltage protection circuit

Info

Publication number: US2548818A
Application number: US634098A
Authority: US
Inventors: William R Rambo
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-12-10
Filing date: 1945-12-10
Publication date: 1951-04-10
Anticipated expiration: 1968-04-10

Description

April 10, 1951 w RAMBO 2,548,818

THERMIONIC OVERVOLTAGE PROTECTION CIRCUIT Filed Dec. 10, 1945 FlG.| LOW OVERLOAD cmcurr VOLTAGE W IOJ 7/ l 8 5 HIGH TRANSMITTER VOLTAGE 3% F53 'ww3vwv m 35 26- SUPPLY \VOLT-AGE CONTROL VOLTAGE INVE/VT'OR WILLIAM R. RAMBO A TTORNE Y Patented Apr. 10, 1951 THERMIONIC OVERVOLTAGE PROTECTION cmourr William R. Rambo, Cambridge, Mass., assignor to the United States of America as represented by the Secretary of War Application December 10, 1945, Serial No. 634,098

6 Claims.

My invention relates in general to protective devices and more particularly to circuits for control of excess voltages and currents in electronic apparatus.

In many types of electrical equipment it is necessary to guard against voltages and currents higher than those normally present in the equipment. This is especially true in circuits employing extremely high voltages and currents in which a small percentage deviation in the input energy may produce a large variation in the voltages present in the system. Hence, to avoid arcing, fiashovers, and burnout in circuit components, it is customary to employ automatic apparatus to break high voltage or high current circuits when there is danger of overload. These devices may take a number of forms ranging from simple circuit breakers to almost completely electronic circuits.

Accordingly, one of the objects of the present invention is to provide a protective device for electrical apparatus.

Another object is to provide a sensitive and reliable electronic protective circuit.

Still another object is to provide an electronic protective circuit which may be readily constructed with standard components.

These and further objects of my invention will be apparent to those skilled in the art upon reference to the following specification, claims,

and to the drawings in which:

Fig. 1 is a blockdiagram showing the application of my invention to a transmitter.

Fig. 2 is a schematic diagram of a preferred embodiment of my invention.

Referring now to Fig. 1, one use of a protective circuit such as my invention comprises is shown in block diagram form. A high voltage supply source 5 is connected to a transmitter 6 through a variable resistor l and a switch 8 which form a part of overload circuit 9. A low voltage supply provides a power source for overload circuit 9. When the voltage of source increases to a point where damage is likely to occur in transmitter 6, the voltage across resistor l attains a value sufficient to cause operation of overload circuit 9 which opens switch 8, removing the high voltage from transmitter 6.

The details of an overload circuit constructed according to the principles of my invention are shown in Fig. 2, to which reference is now made. Voltage from a source such as low voltage supply III in Fig. l is applied across a voltage divider network consisting of serially connected resistors l5, l6, and [1. Connection is made from the junction of resistors I5 and I6 through coil I9 of relay 20 to anode 2| of vacuum tube 22. Screen grid 23 of vacuum tube 22 is connected to the more positive end of resistor 16 while cathode 24 is connected to the less positive end of the same resistor, thereby maintaining a voltage differential between screen grid and cathode. Capacitor 26 and resistor 21 are connected in parallel between control grid 28 and cathode 24 of the tube. Connection is made from cathode 24 through resistor 29, having capacitor 30 in parallel therewith, and reset switch 3| to one side of variable resistor 32, the other side of which is connected directly to the negative terminal of the input supply voltage. A glow discharge tube is connected, in series with a resistor 36, between control grid 28 and the junction of switch 3| with resistor 32. Capacitor 31 is connected in shunt with the series combination of resistors 32 and 36. Contact 39 of relay 2D is connected to the more positive end of resistor I! and arm 40 of the relay is connected to the less positive end of the same resistor. Contact 42 is connected between resistors 32 and 36 and arm 43 is connected to terminal 44 which may be adapted for external connection to a circuit which it is desired to protect. A similar terminal 45 is connected to the same end of resistor 32 to which capacitor 31 is tied.

Before the circuit is energized, the contacts of relay 20 are open so that the high voltage circuit of which contact 42 and arm 43 form a part is broken. When the supply voltage is applied to the protective circuit, the voltage appearing across resistor I! of the series voltage divider causes the neon glow tube 35 to ignite. The current through glow tube 35 must flow through resistor 21, thereby setting up a voltage drop sufficient to bias grid 28 of the tube beyond cut-ofi potential with respect to cathode 24. Therefore, no current flows in coil IQ of relay 20 and the contacts of the relay remain open. To apply high voltage to the protected circuit, reset switch 3| is closed. When this is done, the voltage which maintains ionization in tube 35 is temporarily short-circuited through capacitor 39, which may be several microfarads in capacity, thus extinguishing tube 35. When the grid bias on the tube is removed in this fashion, plate current flows in the tube, the initial surge of current charging capacitor 30 and closing the contacts of relay 20. The closing of

contacts

39 and 40 short-circuits resistor I! which initially supplied ionization voltage for glow tube 3 35 so that the latter remains nonconducting until there is an abnormal change in circuit conditions. Contacts 42 and A3 close simultaneously with

contacts

39 and 40 thus completing the high voltage circuit.

When the protected circuit draws current from the high voltage supply, a small voltage drop occurs across resistor 32. This voltage is impressed on glow tube 35 since no current is now flowing in

resistors

36 and 21. If for any reason there is an abnormal increase in supply voltage or a short circuit in the protected device so that the current through resistor 32 increases beyond a predetermined point, the voltage impressed on glow tube 35 will be sufiicient to initiate ionization therein. Thereupon, tube 22 is again biased beyond cut off allowing the contacts of relay 2% to open, breaking the high voltage circuit. Ionization voltage for tube 35 is then supplied by the drop across resistor ii, maintaining vacuum tube 22 in a nonconductive state until reset switch 31 is closed. It will be noted that resistor 32 is variable 50 that the tripout point may be selected at will.

If an overload should still exist When reset switch 3| is closed, capacitor 38 will be charged by the surge of current through tube 22 which also closes relay 20. Contacts 39 and ill shortcircuit resistor ll, however the voltage across resistor 32 is sufficient to re-establish ionization in glow tube 35. Therefore vacuum tube 22 will again be blocked and the contacts of relay will open. If switch 3| is still held closed, conduction cannot occur in tube 22 until the charge on capacitor 36 has leaked ofi. This takes place relatively slowly so that normally pressure on switch 3| will be released before relay 29 closes again. Thus my circuit provides protection even during the time in which the reset switch is in the closed position.

In many cases, momentary overloads are not sufilcient to damage the protected equipment, and hence it is desirable that relay 20 should not be operated unless an overload persists longer than a predetermined duration. When momentary surges of voltage occur across resistor 32 they serve only to charge capacitor 3'! and hence do not provide ionization voltage for tube 35. The length of time for which an overload must endure before the high voltage circuit is broken is determined by the time constant of resistor 36 and capacitor 31.

Capacitor 30 provides an automatic time delay before which relay 20 may not be reset after having opened. When switch 3| is closed and capacitor 38 is charged by the surge of current through tube 22, the overload may have persisted, in which case the relay contacts will immediately open and cannot be reset until capacitor 30 is discharged sufficiently for glow tube 35 to be extinguished. This time delay is chiefly dependent upon the capacity of capacitor 32 and the value of resistor 29.

It has been pointed out hereinbefore that an outstanding advantage of my circuit lies in the fact that the relay is operated by a sudden current differential rather than a gradual one. Other advantages which have not previously been mentioned are: maintenance of the operating accuracy of the circuit regardless of changes in tube characteristics caused by ageing; the practicability of utilizing a relatively heavy duty relay which requires a large current differential for operation; the possibility of resetting the control relay when the current in the protected circuit 4 has decreased as little as 1 percent below the trip-out value.

While there has been described hereinabove what is at present considered to be a preferred embodiment of the present invention, it will be obvious to those skilled in the art that changes and modifications may be made therein without exercise of inventive ingenuity. Hence, I claim all such modifications and adaptations as may fall fairly within the spirit and scope of the hereinafter appended claims.

What I claim is:

1. A protective device for electrical apparatus including a normally conducting thermionic vacuum tube having anode and cathode electrodes and at least one control electrode, a normally energized electromechanical relay having circuit switch contacts in a control voltage circuit, means for connecting the energizing coil of said relay in the anode-cathode circuit of said vacuum tube for normal energization and activation of said control voltage thereby, a normally non-conducting glow-discharge tube, means for connecting said glow-discharge tube in the control electrodecathode circuit of said vacuum tube so that said glow-discharge tube when conductive causes said control electrode to be biassed beyond cut-off potential with respect to said cathode, means for developing voltages substantially proportional to voltages in the electrical apparatus to be protected, means for utilizing the voltages thus developed to effect ionization in said glow-discharge tube, and means intermediate said developing means and said glow tube for delaying the utilization of the developed voltages, thereby deenergizing said electromechanical relay to remove the applied voltage from said apparatus to be protected when said voltages increase beyond a predetermined value only after a given interval.

2. Apparatus in accordance with claim 1 wherein said means for developing voltages substantiall proportional to voltages in the electrical apparatus to be protected comprise resistive means forming a portion of the circuit for conducting energy to said electrical apparatus to be protected.

3. In a protective circuit for electrical apparatus a thermionic vacuum tube having anode and cathode electrodes and at least one control electrode, a gaseous glow-discharge tube, means for connecting said glow-discharge tube in the con trol electrode-cathode circuit of said thermionic tube so that when said glow-discharge tube is conductive said control electrode is biassed negatively with respect to said cathode so that current flow in said thermionic tube is blocked, voltage developing means, means for utilizing the developed voltage to render said glow-discharge tube conductive, switching means, and means for connecting said switching means so that when said thermionic tube is conductive said voltage developing means is rendered inactive.

4. In a protective apparatus for limiting the voltage impressed onto a utilizing means, thermionic means having anode, cathode and at least one control electrode, voltage divider means connected to the input circuit of said thermionic tube for governing the bias between the cathode of said tube and at least on of the control electrodes, switching means responsive to a predetermined current fiow in said tube, said switching means being normally engaged when the voltage to be limited is less than a predetermined value, and means responsive to the voltage to be limited and influencing the current flow n said thermionic tube, said responsive means blocking the current flow in said thermionic tube when the voltage to be limited exceeds a predetermined value whereby said switching means is actuated substantially simultaneously with the blocking of current flow through said thermionic tube, and whereby the resistive value of at least a portion of said voltage divider means is altered when the switching means is actuated.

5. A device according to claim 1, further including means for supplying initial ionization voltage to said glow tube, reset switch means for temporarily short circuiting said ionization voltage supplying means to thereby establish conductivity through said thermionic tube, and switch means controlled by said relay for normally short circuiting said initial voltage supplying means.

6. A device according to claim 5, further including a time delay circuit intermediate said reset switch means and said glow tube to permit said relay to open if a voltage in said voltage developing means persists.

WILLIAM R. RAMBO.

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

UNITED STATES PATENTS Number Name Date 1,297,188 Langmuir Mar. 11, 1919 15 2,147,781 Ward Feb. 21, 1939 2,153,202 Nichols Apr. 4, 1939 2,250,202 Matusita July 22, 1941 2,404,001 Smith July 16, 1946