US3821594A

US3821594A - Two step magnetron start circuit

Info

Publication number: US3821594A
Application number: US00345678A
Authority: US
Inventors: R Webb; J Roberts
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1973-03-28
Filing date: 1973-03-28
Publication date: 1974-06-28
Anticipated expiration: 1991-06-28

Abstract

A control arrangement for stepped energizing of a magnetron in a microwave oven includes means to delay the application of the full high voltage to the magnetron to insure operation in the desired oscillating mode. Such delay is accomplished by providing a thermistor coupled serially with an input terminal of a voltage doubler, such that upon magnetron conduction, the current flowing through the thermistor causes a decrease in resistance therein which in turn produces an increase in the doubler output voltage, whereupon the reaching of a predetermined level thereof, a relay effects switching to apply full line voltage to the voltage doubler input terminals.

Description

[11] 3,821,594 June 28, 1974 [5.4] TWO-STEP MAGNETRON START CIRCUIT [75] Inventors: Robert F. Webb; John J. Roberts,

both of Louisville, Ky.

[73] Assignee: General Electric Company,

. Louisville, Ky.

[22] Filed: Mar. 28, 1973 21 Appl. No.: 345,678

[52] US. Cl. 315/104, 219/1055, 315/3951, 331/86 [51] Int. Cl. H0111 7/06 [58] Field of Search 219/1055; 315/102, 104; 331/86; 321/37 [56] References Cited 3/1972 Tingle-y 315/102 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Richard A. Rosenberger Attorney, Agent, or Firm-Richard L. Caslin; Francis H. Boos, Jr.

[57] ABSTRACT A control arrangement for stepped energizing of a magnetron in a microwave oven includes means to delay the application of the full high voltage to the magnetron to insure operation in the desired oscillating mode. Such delay is accomplished by providing a thermistor coupled serially with an input terminal of a voltage doubler, such that upon magnetron conduction, the current flowing through the thermistor causes a decrease in resistance therein which in turn produces an increase in the doubler output voltage, whereupon the reaching of a predetermined level thereof, a relay effects switching to apply full line voltage to the voltage doubler input terminals.

6 Claims, 1 Drawing Figure 1 TWO-STEP MAGNETRON START CIRCUIT BACKGROUND OF THE INVENTION The present invention relates to a control arrangement for a magnetron and more particularly to a twostep starting circuit for a domestic microwave oven magnetron, for protection of the magnetron against failure to oscillate such as often occurs when a magnetron is immediately connected to a full wave high voltage supply, and particularly when the high voltage is applied simultaneously with application of filament current.

Such failure to oscillate may be highly destructive to the magnetron tube. It is preferable that the cathode of a magnetron be heated to its proper operating temperature before full anode voltage is applied. If the anode voltage is applied before the cathode surface has been uniformly and thoroughly heated to its operating temperature, one possible result is that a very high density current flows from the cathode and application of the full anode voltage at this time subjects the cathode to the stress of a very strong electric field which may cause cathode material to be pulled from the cathode surface. Another possible result is that the magnetron may operate improperly in a low current, high voltage mode which may eventually cause its self-destruction. A delay then is necessary between the application of the cathode heater voltage and the application of the full anode voltage to allow the cathode emitting surface to reach its operating temperature.

One solution to the delay problem is to provide a timer motor to control activation of a relay having contacts arranged that, upon their closing, a currentlimiting series connected resistor will be by-passed, thereby allowing the full line voltage to be applied to the primary of a transformer supplying power to the mangetron. Such a solution is taught in U.S. Pat. No. 3,392,309 Hickman. Another solution is taught by U.S. Pat. No. 3,398,371 Lamb, wherein is taught the use of a relay coil in series with the magnetron, which coil is responsive to magnetron current to control a set of relay contacts so as to by-pass a current-limiting serially connected resistor in the supply line to the magnetron power transformer primary. U.S. Pat. No. 3,591,826 Valles teaches the use of hot-wire relays connected across a power transformer low voltage secondary winding used for cathode heating, wherein the relay contacts upon energization by-pass a currentlimiting series connected resistor in the high voltage portion of the transformer secondary winding. U.S. Pat.

No. 3,651,371 Tingley teaches the use of a relay having its coil serially connected to a magnetron anode such that the relay is responsive to magnetron current flowing through the relay coil to switch relay contacts for by-passing a series resistance. When such relay contacts are in the normally-open state, substantially all the current supplied to the magnetron is in the form of half-waves derived from the first half-wave voltagedoubler circuit used therein. When the load currents imposed by the magnetron are such as to cause the relay to be energized to close its contacts, the series limiting-resistor is by-passed, essentially allowingboth halves of the voltage doubler to be switched into the circuit to effect the high power mode; in other words, applying full voltage to the magnetron tube. The aforementioned references in their various forms suffer from age applied to the magnetron by switching the input voltages to the voltage doubler circuit.

A reliable control arrangement is needed, therefore, for delaying the application of the full anode voltage to the magnetron tube and which control arrangement is less complex and less expensive of manufacture than those presently available.

It is', therefore, an object of the present invention to provide a control arrangement for starting a magnetron, and more particularly, to provide a two-step start circuit for a magnetron tube in a microwave oven which is reliable and positive of operation and less ex pensive to manufacture than those presently available.

SUMMARY OF THE INVENTION In accordance with one form of the present invention, a control arrangement for energizing a magnetron comprises a full-wave voltage multiplier having a pair of input terminals and a pair of putput terminals, one of the terminals being coupled with a first line voltage terminal. A magnetron having an anode and a cathode is coupled with the output terminals of the voltage multiplier. Relay means responsive to voltage to become energized at a predetermined level thereof is coupled serially with said magnetron, the relay means having a set of contacts, such that in the de-energized state of the relay means said contacts will serve to couple the second of said multiplier input terminals to a neutral line terminal and in the energized state to couple the second multiplier terminal to a second line voltage ter minal. A delay means is coupled serially with said neutral terminal and said relay means contacts for providing a time delay before said relay means becomes energized, thereby allowing the magnetron to warm up before full voltage is applied thereto.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing there is shown a schematic representation of the preferred embodiment of the control arrangement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, there is illustrated schematically, a control arrangement for a magnetron oscillation tube such as is used in a microwave oven. Supplied by terminals 10, 12 and 114 denoted first line voltage, neutral and second line voltage terminals respectively, the circuit, connected to the usual 240 volts AC source, feeds a magnetron tube 20 having an anode 22, a cathode 26 and a cathode heating filament 30. The filament 30 may be supplied from a transformer (not shown).

For the production'of the high voltage necessary to operate the magnetron tube, a voltage multiplier such as full-wave voltage doubler 40 is provided comprising a pair of

diodes

42 and 44 and a pair of

capacitors

48 and 50. Input terminal 54 of the doubler 40 is coupled to first line voltage terminal through a current limiting resistor 64. Coupled across the

output terminals

58 and 60 of voltage doubler 40 are the anode 22 and the cathode 26 of magnetron 20, respectively. Coupled serially with mode 22 and output terminal 58 is field coil 70,. the purpose of which is to provide the necessary magnetic fields for the operation of magnetron tube 20.

In accordance with the present invention, relay means, such as a relay switch 80 comprising a coil 82 and a switch 84, is provided for switching the input voltage to doubler 40. Coil 82 is coupled in parallel with field coil 70 and is responsive to the voltage thereacross to become energized at a predetermined level of that voltage. Switch 84 comprises a set of contacts 86 and 88 and a switch arm 90 such that, in the deenergized state of relay coil 82, switch arm 90 will be in coupling arrangement with contact 86. Coupled serially between contact 86 and neutral terminal I2 is delay means, such as thermistor 94 having a negative temperature coefficient, the thermistor 94 serving to delay the application of full power to the magnetron to allow for warm-up thereof. Contact 88 is coupled with second line voltage terminal 14.

As is shown in the drawing, in the starting condition, voltage doubler 40 has its input terminals 54 and 56 coupled to terminals 10 and 12 respectively, and, as a result, approximately 118 volts AC is applied thereto. However, since thermistor 94 is connected serially between input 56 and neutral terminal 12, there is a relatively large voltage drop thereacross, with a resultant lowered output of voltage doubler 40 across

output terminals

58 and 60. With the application of power to terminals 10, 12 and 14, it may be assumed that power is likewiseapplied to filament 30; in other words, filament 30 is activated at the same time that voltage is applied to the magnetron anode 22 and cathode 26. As cathode 26 begins to heat, current is begun to be drawn through magnetron 20 and, likewise, through thermistor 94 and as current is drawn therethrough, the effect is to cause the gradual heating up of thermistor 94 which in turn causes the resistance thereof to fall. As this happens, the voltage drop across thermistor 94 also falls and as a further result, the input voltage to voltage doubler 40 rises, followed by a higher voltage output from the doubler 40 to be applied to magnetron 20.

There is a point then, for which the voltage output of the voltage doubler, and as a result, the voltage across field coil 70, reaches such a value that relay coil 82 becomes energized. With the energization of coil 82, the switch 84 is activated to switch arm 90 over to couple with contact 88. Now the full line voltage from terminals 10 and 14 is caused to be applied to the input terminals-54 and 56 of voltage doubler 40. There is a resultant full wave ouput at

terminals

58 and 60 of the voltage doubler 40 reaching the 500-600 volt DC area.

At this point, the magnetron tube 20 has had 'sufficient time to warm up thoroughly, and the application of the approximately 600 volts DC places the magnetron in a full operating condition ready to deliver the necessary microwave energy to the microwave oven interior.

Following are listed values for the several components which have been used to place the present embodiment of this invention in operational condition:

Resistor 64 0.75 ohms [3.2 to 16.0 volts DC pull-in 400 ohms at room temperature 365 uf Relay coil 82 Thermistor 94 Capacitors 48, S0

It should be understood that such a control arrangement for a magnetron will function properly without the field coils as discussed herein. In other words, a permanent magnet or magnets may be employed to produce the necessary magnetic fields for the operation of the magnetron. In such a situation, the relay coil will become energized to effect pull-in upon the reaching of a predetermined level of current flow therethrough. It has also been considered that the thermistor may be replaced by a resistive heating element of an appropriate value, and coupled in parallel therewith and responsive to the heat generated thereby, a normally-open bimetal switch. Upon the drawing of a predetermined amount of current through the magnetron tube, and as a result through the heating element, the bimetal switch will close, effectively by-passing the resistive heating element to allow the approximately 1 18 volts AC line voltage to be applied to the inputs of the voltage doubler.

It is to be further understood that the relay may be supplied with additional sets of poles or switches to perform other'circuit functions. For example, one pole might be used to cause switching to effect reduced magnetron filament voltage, and another might be used to turn on a timer or a light.

It should be apparent to those skilled in the art that the embodiment described heretofore is considered to be the presently preferred form of this invention. In accordance with the Patent Statutes, changes may be made in the disclosed apparatus and the manner in which it is used without actually departing from the true spirit and scope of this invention.

What is claimed is:

l. A control arrangement for energizing a magnetron comprising:

a neutral terminal and a pair of line voltage terminals for supplyig electric power;

a full-wave voltage multiplier having a pair of input terminals and a pair of output terminals, one of the input terminals being coupled with a first of said line voltage terminals;

a magnetron having an anode and a cathode coupled respectively with the output terminals of said multiplier;

relay means responsive to voltage to become energized at a predetermined level thereof, said relay means being coupled serially with said magnetron and further, having a set of contacts, in the deenergized state of the relay means to couple the second of said multiplier input terminals to said neutral terminal, and in the energized state to couple said second multiplier terminal to said second line tenninal; and

delay means coupled serially with said neutral terminal and said relay means contacts for allowing a time delay before said relay means becomes energized, thereby allowing the magnetron to warm up before full voltage is applied thereto.

2. The control arrangement of claim 1 wherein a field coil is coupled in parallel with said relay means for providing the necessary magnetic field for the operation of said magnetron tube.

3. The control arrangement of claim 1 wherein the delay means is a thermistor.

4. The control arrangement of claim 1 wherein the delay means is a heating element and coupled in parallel therewith and responsive thereto is a normally-open bimetal switch.

5. In a microwave oven, a control arrangement for energizing a magnetron comprising:

a neutral terminal and first and second line voltage terminals for supplying electric power;

a voltage multiplier having a pair of input terminals and a pair of output terminals, one of said input terminals being coupled with said first line voltage terminals;

a field coil coupled serially with said magnetron across said multiplier output terminals;

a relay switch having a set of contacts and further having a coil coupled in parallel with said field coil, the switch being responsive to the voltage across said field coil to become energized at a predetermined voltage level, in the de-energized state of the relay to couple the second multiplier input terminal to said neutral terminal, and in the energized state to couple said second multiplier terminal to the second line terminal; and

a thermistor coupled serially with said neutral terminal and said switch contacts for allowing a time delay before said switch becomes energized to effect gradual warm-up of said magnetron.

6. In a microwave oven, a control arrangement for energizing a magnetron, said arrangement comprising:

a neutral terminal and first and second line voltage terminals for coupling to a source of electric power;

a full-wave voltage doubler having a pair of input terminals and a pair of output terminals, one of said input terminals being coupled with said first line voltage terminal;

a current limiting resistor coupled serially with said first line terminal;

a magnetron having an anode and a cathode coupled serially with the output terminals of said doubler;

a field coil coupled serially with said magnetron across said doubler output terminals;

a relay having a coil and a set of normally-open contacts, the coil being coupled in parallel with said field coil and responsive to the voltage thereacross-to become energized at a predetermined voltage level, in the de-energized state of the relay to couple the second doubler input terminal to said neutral terminal, and in the energized state to couple said second doubler input terminal to the second line terminal;

and a thermistor coupled serially with said neutral terminal for allowing a time delay before said relay becomes energized for effecting gradual warm-up

Claims

1. A control arrangement for energizing a magnetron comprising: a neutral terminal and a pair of line voltage terminals for supplyig electric power; a full-wave voltage multiplier having a pair of input terminals and a pair of output terminals, one of the input terminals being coupled with a first of said line voltage terminals; a magnetron having an anode and a cathode coupled respectively with the output terminals of said multiplier; relay means responsive to voltage to become energized at a predetermined level thereof, said relay means being coupled serially with said magnetron and further, having a set of contacts, in the de-energized state of the relay means to couple the second of said multiplier input terminals to said neutral terminal, and in the energized state to couple said second multiplier terminal to said second line terminal; and delay means coupled serially with said neutral terminal and said relay means contacts for allowing a time delay before said relay means becomes energized, thereby allowing the magnetron to warm up before full voltage is applied thereto.

3. The control arrangement of claim 1 wherein the delay means is a thermistor.

5. In a microwave oven, a control arrangement for energizing a magnetron comprising: a neutral terminal and first and second line voltage terminals for supplying electric power; a voltage multiplier having a pair of input terminals and a pair of output terminals, one of said input terminals being coupled with said first line voltage terminals; a magnetron having an anode and a cathode coupled respectively with the output terminals of said multiplier; a field coil coupled serially with said magnetron across said multiplier output terminals; a relay switch having a set of contacts and further having a coil coupled in parallel with said field coil, the switch being responsive to the voltage across said field coil to become energized at a predetermined voltage level, in the de-energized state of the relay to couple the second multiplier input terminal to said neutral terminal, and in the energized state to coUple said second multiplier terminal to the second line terminal; and a thermistor coupled serially with said neutral terminal and said switch contacts for allowing a time delay before said switch becomes energized to effect gradual warm-up of said magnetron.

6. In a microwave oven, a control arrangement for energizing a magnetron, said arrangement comprising: a neutral terminal and first and second line voltage terminals for coupling to a source of electric power; a full-wave voltage doubler having a pair of input terminals and a pair of output terminals, one of said input terminals being coupled with said first line voltage terminal; a current limiting resistor coupled serially with said first line terminal; a magnetron having an anode and a cathode coupled serially with the output terminals of said doubler; a field coil coupled serially with said magnetron across said doubler output terminals; a relay having a coil and a set of normally-open contacts, the coil being coupled in parallel with said field coil and responsive to the voltage thereacross to become energized at a predetermined voltage level, in the de-energized state of the relay to couple the second doubler input terminal to said neutral terminal, and in the energized state to couple said second doubler input terminal to the second line terminal; and a thermistor coupled serially with said neutral terminal for allowing a time delay before said relay becomes energized for effecting gradual warm-up of said magnetron.