CA1069184A - Power controller for microwave magnetron - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A power supply for a microwave magnetron includes a transformer having a secondary winding, a capacitor connected to one terminal of the winding, a magnetron connected between the other terminals of the capacitor and of the winding, so as to form a first unidirectional current-conducting path capable of conducting current in one direction and a second circuit connected in shunt of the magnetron having unidirectional current-conduction characteristics and being switchable to an "on" or "off" condition, the second circuit being electrically poled opposite to the polarity of the magnetron. Means are provided for controlling the switching of the second circuit into its current-conducting condition as a function of the magnitude of current through the magnetron.

Description

10~i9184 This invention relates to regulated adjuætable electrical power supplie and, in a re specific context, to a regulated aajustable power supply for a microwave magnetron of a microwave oven.
The microwave oven is a well-known appliance used to heat or cook foods by exposure to microwave energy. For this purpose, conventional microwave ovens employ an electronic vacuum tube, known as a magnetron. Simply stated, the magnetron is a device having unidirectional current carrying characteristics which converts DC power into energy within the microwave frequency range, such as the frequency permitted by United States law for that purpose, namely 2,450 megahertz. To provide that DC voltage, various additional electrical components are included as a power supply to convert normal household line voltage, typically 120 or 240 volts at 50 to 60 hertz into the high voltages, in the order of 3,000 to 4,000 volts DC, required in the operation of pre-sently available magnetrons. In its essentials, a typical microwave oven power supply contains a tran~former for stepping up the 120 volt ~C to the level of 3,000 to 4,000 volts, depending upon the particular voltage requirement of any specific model of microwave magnetron, a rectifying means, or a voltage doubler-rectifier, and the magnetron itself. Moreover, a source of low voltage is provided for the heater of the magnetron.
Microwave energy generated by the magnetron is taken from the magnetron output and transmitted either directly, or through a waveguide, into the oven chamber.
The average power supplied to the magnetron is set within limits by the design of both the power supply and the magnetron and i~ generally directly related to the microwave output power generated thereby. It is known that the adjustment

-2-.

~ MCP-76-17 1(~6~184 of the microwave power can be made within limits by adjustment of the DC current level through the magnetron. Present microwave oven power supplies generally employ a high leakage reactance transformer, in combination with a modified half-wave voltage doubler, known also aq a "Villard" circuit, to rectify and double the voltage output of the high voltage transformer, and apply a high voltage DC to the magnetron. Exampleq of such circuits appear in United States Patents Nos. 3,396,342, 3,651,371; and

3,684,978. These circuit~ provide satisfactory operation and use a minimum number of components.
Recent practice i9 to provide additional elements within the oven power supply which permit the user to adjust the average power of the magnetron. This has been accomplished as either a two-step "high" or "low" power arrangement or as an adjustable level device allowing continuou~ adjustment. In the first type of device, the value of the capacitance in the voltage doubler circuit was made variable in order to permit adjusting the current, see United States Patent No. 3,684,978. Another expedient is to employ a triac control in the primary circuit of the transformer, in order to regulate the average of current into the power supply, but this circuit requires a separate filament transformer because of the interrelated voltage into the primary of the high voltage transformer, so that the expedient of having the filament winding combined on the same transformer core upon the high voltage transformer, but as a separate winding, cannot be employed. Additionally, the use of pulse techniques, inherent in this known method, in the primary winding of the tran~former creates additional stresses on the transformer insulation, which should preferably be avoided.

In the case of current control in the secondary winding .
~. .

~069184 circuit, the approach set forth in United States Patent No.
3,684,978 does not provide sufficient variety of adjustment, and an adjustable resistor approach, disclosed in United States Patent No. 3,760,291, appears somewhat impractical and wasteful, in that a resistor consumes electrical energy as it generates heat.
The present invention relates to the control of the average power output level of a magnetron by controlling the voltage in the secondary winding of the transformer. More particularly, the invention provides a control which can be used to allow the user to selectively adjust the power level of the magnetron within a certain range, or, in an alternative applica-tion, which may be established adjusted to a fixed level at the factory. Then, the filament voltage may be supplied with power from an additional winding on the same high voltage transformer as that which is used to provide the high voltage to the magne-tron. The primary pulse techniques of the above-mentioned known technique is avoided, the reliability of the transformer i9 improved, and any surges caused by lightning hitting the input line, as might destroy semiconductor type control devices connected in the primary circuit, are minimized.
The invention provides a power supply of the type having a transformer for generating high voltages AC in a secondary winding, having a capacitor in series with the secondary winding and one terminal of a unidirectionally current-conducting load, such as a magnetron, and a diode connected in shunt of the load, or magnetron, electrically oppositely poled thereto. Means are included electrically in series with the diode to selectively control the average current through the diode during the half-cycle of AC in which the magnetron is not conducting current and :. . : . .~ . . ..

~069184 .~ .

thus control the level of charge and voltage on the capacitor.
In this way, the level of voltage and current applied to the magnetron on alternate half-cycles, during which the magnetron conducts current, may be selectively adjusted in level.
Suitably, in accordance with a specific feature of the ; invention, means are provided to monitor the current through the magnetron or other unidirectionally current-conducting load and means responsive to the level of current during the one half-cycle of AC for controlling the average value of current through the diode during the other AC half-cycle. Ideally, the load current is held at or near the selected current level. Thus, when the current thr~ugh the magnetron falls below a certain ~i selected level, the control means permits the current through the diode to increase, so as to permit a greater charge to be stored in the capacitor during one, i.e. the first, half-cycle, to thereby cause greater current to the magnetron on each alternate, i.e. second, half-cycle. Conversely, if the level of current through the magnetron is above a selected level, the control means reduces the average value of current through the diode on the other AC half-cycle to lessen the charge on the capacitor and thus to reduce the level of voltage which appears across the magnetron during the next succeeding alternate half-cycle, to thereby cause a lesser current through the magnetron.
Suitably, the reference level can be adjusted by the user to any desired level so as to permit the user to control the power supplied to the magnetron. Alternatively, the reference level can be adjusted at the factory to one level or for two separate levels to be obtained, depending on the alternative positioning of a switch accessible to the user.
In a more specific aspect of the invention, the control ;

,: . . . - : . . ~ - .

10t;~184 , means comprises a semiconductor controlled switch, such as a triac, means for providing, i.e. setting, a reference signal representative of the desired magnetron current level, means for comparing the magnitude with that set as the reference and providing an output to the gate of the semiconductor controlled switch at a time before or during the next AC half-cycle com-puted to properly charge the series capacitor.
In a still more detailed aspect of the invention, the means for determining the magnetron current level comprises a first resistance means connected in an electrical series circuit with said magnetron. In addition, a series circuit comprising a resistor and a capacitor is connected across said first resis-tance means, to form a conventional RC circuit. Preferably, the RC circuit has a time constant T, which is the product of capa-citance (farads) multiplied by resistance (ohms), (secondswhich is approximately 1/2F seconds), where F is the frequency in cycles per second of the AC high voltage appearing across the transformer secondary winding. A comparator, such as an operational, or differential amplifier, including a noninverting input and an inverting input forms part of the circuit. The reference voltage is applied to the noninverting input of the comparator and the voltage across the capacitor in the RC net-work is applied to the inverting input thereof. When the voltage across the timing capacitor exceeds a certain level, the output of the comparator switches from low to high, to shut off the triac. Thus, during the alternate half-cycle in which the voltage applied to the diode is correctly poled, so as to nor-mally conduct current, current is blocked since the triac is not gated on. However, at some time during such half-cycle, the voltage across the timing capacitor discharges to a level less 1069~84 than the reference level, which, in turn, causes the comparator to switch to its "on" state and provides a low gate pulse to the triac. Once the triac conducts current, current flows through the capacitor and the diode during the remainder of that AC
half-cycle to thus charge the capacitor. It is noted that, on the next succeeding AC half-cycle, the triac is essentially shut off, no current flows through the diode, and the magnetron is properly poled with respect to the applied voltage and again conducts current. The voltage across the magnetron at that half-cycle equals the sum of the voltage across the capacitor and the AC voltage across the secondàry winding of the transformer.
The invention will become better understood from the following detailed description of one embodiment thereof, when taken in conjunction with the drawings, wherein:
Figure 1 illustrates one embodiment of a power supply circuit for a magnetron, in accordance with the invention, Figure 2 illustrates characteristic ideal wave-shapes of voltages or currents inherent in the operation of the circuit of Figure 1, and Figure 3 is a power supply circuit for a magnetron in accordance with another embodiment of the invention.
The embodiment illustrated in Figure 1 includes a trans-former T having a primary winding Tp, a high voltage secondary winding TSl, a low voltage secondary winding TS2, a capacitor Cl, a magnetron M, a rectifier diode D2, a triac TR1, a comparator A1, resistors R4 and R5, a capacitor C3, a diode Dl, a capacitor C2, further resistors Rl and R3 and a potentiometer resistor R2.
Transformer T is of any conventional type, as it has a laminated iron core, with the primary and secondary windings located thereon and with the primary formed from a predetermined number of turns of insulated wire adapted for connection to a source of current, suitably 120 volt 60 hertz. The high voltage, secondary winding TSl is formed of a large number of turns of relatively fine insulated wire, so as to define a large turns ratio between the secondary TSl and primary Tp and place the secondary in a step-up voltage relationship with the primary, while the other secondary winding TS2 is of a relatively few number of turns of insulated wire, defining a turns ratio with the primary less than one to place the secondary in a voltage step-aown relationship with the primary, to provide the low voltage suitable for application to a magnetron heater. A tap 3 in secondary winding TSl provides a low voltage relative to the reference point to be obtained. The transformer may be one in which the primary winding is "loosely" coupled to the high voltage secondary, known as a high leakage reactance type trans-former found in existing commercial ovens. It is noted, however, that the transformer may be a ferrite core transformer used in electrical inverter-type power supplies providing high-voltage high-frequency power in the order of 20 kilohertz or above.
Secondary winding TS2 is connected across the two heater-cathode terminals of the magnetron. One terminal 1 of secondary winding TSl is connected electrically in series to one terminal ; of capacitor Cl and the other terminal of the capacitor is connected in circuit to one of the heater-cathode terminals of ; magnetron M. The magnetron anode is connected to one terminal of resistor R5. The other terminal of resistor R5 is connected to ground potential, as shown. The other terminal 2 of secondary ,: . . - . - -: -:

TSl is also connected to ground. Tap 3 is located at winding TSl nearer the terminal 2, so as to be at a low voltage point rela- -tive to ground. Tap 3 is connected to the cathode terminal of rectifier diode Dl. The anode terminal of rectifier diode Dl is connected to one terminal of capacitor C2 and, in turn, the remaining terminal of capacitor C2 is connected to electrical ground potential to form a rectifier-filter circuit. Resistors Rl, R2 and R3 are connected in electrical series circuit between the ungrounded~terminal of capacitor C2 and ground potential, to form a resistive voltage divider network. Potentiometer resistor R2 contains a conventional movable tap R2t, by means of which the resistance value between an end terminal and the tap may be selectively adjusted in value. This tap is connected electrically to the noninverting input of comparator Al as indicated by the plu8 (+) sign. The V- power input terminal of comparator Al is connected in circuit with the ungrounded terminal of capacitor C2 and the other power terminal of comparator Al is connected to ground. The output terminal of comparator Al is connected to the gate of triac TRl and the inverting input of comparator Al, as represented by the minus (-) qymbol, i8 connected in circuit between resistor R4 and capacitor C3. Resistor R4 and capacitor C3 are connected in electrical 3eries circuit and that series circuit is connected in circuit across resistor R5.
The anode of rectifier diode D2 is connected in circuit with the heater-cathode terminal of the magnetron and the cathode of diode D2 i~ connected to one power terminal of triac TRl. The remaining triac power terminal is connected to ground potential. The magnetron is situated in circuit between capa-citor Cl and ground potential, so that, since the magnetron has self-rectifying properties, passing DC current in a direction ` MCP-76-17 from its anode to its cathode-heater, the circuit has unidirec-tional current-conducting properties. The diode D2, similarly, can pass current only in a direction from its anode to cathode.
Since the diode is connected in a second electrical circuit that shunts the first circuit, this second circuit has unidirectional current-conducting characteristics also, but, inasmuch as the diode anode is connected to capacitor Cl, this second circuit conducts current only in a direction opposite to the current direction in the first circuit and i8 therefore in essence "oppositely electrically poled".
With a source of AC voltage, ~uch as I20 volts 60 cycle AC, connected to primary winding Tp of the transformer, by transformer action, the primary voltage is ~tepped up and appears across secondary winding TSl as a high voltage AC. By design, this AC voltage i9 less than that specified for operation of the magnetron. Additionally, also by transformer action, the primary voltage is stepped down and appears across secondary winding TS2 as a low AC voltage which, by design, is needed for the magnetron heater which heats the magnetron cathode to make it more electron emi~sive, as is known to those skilled in the art.
A low voltage AC voltage is also produced at tap 3 of the high voltage secondary winding TSl. This voltage is rectified ; by rectifier Dl and the resultant DC current charges capacitor C2 to a predetermined low voltage level to form a low voltage DC
power supply for the control circuits. This DC voltage is applied from the ungrounded side of capacitor C2 to the V- input of comparator Al. By selection of the suitable tap location on the transformer secondary winding TSl, the DC voltage established ; is of the proper level to provide DC power for the comparator Al.
In addition, the DC is applied across the voltage divider formed ' . . : ' - ' ' :'' '' ' ' , : ~ ' ~06~184 by resistors Rl, R2 and R3. The DC current through the resistors produces a voltage or IR drop thereacross and a portion of the voltage drop appears between tap R2t and ground. The potenti-ometer tap R2t is movable to any one of various locations along the resistor R2 and hence allows selection of different levels of reference voltage. The tap location may be connected by means of a shaft, indicated by the dashed line, to a rotatable knob that may be located on the control panel of the microwave oven.
A calibration legend to inform the operator of the various power levels which may be attained in the oven through rotation of the knob to one position or any other position may be conveniently provided. On the other hand, the tap may be adjusted to a selected position at the factory for a predetermined power level and fixed in that position. It is noted that a pushbutton selected resistor network or equivalent may be substituted for the potentiometer arrangement.
Magnetron M conducts electrical current only when the voltage at its heater-cathode assembly is negative with respect to the magnetron anode and then only when the magnitude of that voltage reaches a predetermined triggering level, such as 3,500 volt~ by way of example, as in the case of a specific model magnetron that has a normal operating voltage of about 4,000 volts. Thus, the circuit including magnetron M has unidirectional current-carrying characteristics. Diode D2 conducts current in a direction from its anode to its cathode and blocks current from passing in the opposite direction. Moreover, since diode D2 is in series with the triac TRl, current flows only when the triac is switched into its current-conducting condition. A triac is a well-known semiconductor switching device having two current-carrying terminals and a gate terminal. Functionally, the triac 1069~84 conducts only after a high positive or negative voltage relative to the grounded terminal is applied to its gate electrode and thereafter continues to conduct current even after removal of the potential from its gate electrode, as long as the current through the device does not reduce to zero, i.e. as long as current flows through it.
When the current does 90 reduce to zero, the triac reverts, i.e. switches, to its noncurrent-conducting condition and can return to its current-conducting condition only in re~ponse to another inîtiating voltage applied to its gate electrode. Thus, the series circuit of diode D2 and triac TRl is switchable and has unidirectional current-conducting charac-teristics. When the AC voltage at winding terminal 1 i8 in the negative AC half-cycle, as is shown theoretically and graphically in Figure 2a, relative to the other winding terminal considered as the voltage reference, the AC voltage at the terminal of capacitor Cl which i8 connected to diode D2 and to the magnetron i~ negative.
Assuming for the moment that the output of operational amplifier Al applied to the gate of triac TRl i9 voltage low, that the AC voltage at winding terminal 1 is positive, and that the triac TRl i8 in its "on" or current-conducting condition, then current flows from the ~econdary winding through capacitor Cl, through diode D2, through the triac to ground and thus to the other terminal, i.e. terminal 2, of the transformer secondary winding TSl. This current charges capacitor Cl to +V. Within one half-cycle of alternating current, the current and voltage reach a peak and then decrease to zero. At zero current, the triac TRl restores to its "off" or noncurrent-conducting condi-tion and remains off during the next half-cycle. As is noted, the current through the secondary circuit has charged capacitor Cl to a positive potential +V, theoretically speaking to the peak level of voltage which appeared across secondary winding TSl.
On the next or alternate half-cycle, the AC voltage across winding TSl reverse~ and makes the voltage at terminal 1 nega-tive with respect to the grounded terminal 2. As this occurs, the voltage on the winding is in additive relationship with the voltage on capacitor Cl and, theoretically speaking, assuming no electrical load, the voltage measured between the anode of diode D2 and ground would attain a level of twice the peak voltage across secondary winding TSl or -2V. mi8 is recognized as a voltage multiplication effect. However, in reality, as the voltage across ~econdary winding TSl is building up in the reverse direction, at some point in time the voltage level between the capacitor C2 and ground exceeds that required to initiate current conduction in magnetron M. Inasmuch as the voltage across the magnetron is properly poled, i.e. biased, 90 that the cathode iq negative with respect to the grounded anode, current flows during this half-cycle in a path from winding terminal 2, ground, resistor R5, the magnetron, capacitor Cl, winding terminal 1, and through winding TSl to winding terminal 2.
The magnetron, as is known, converts the DC energy into high frequency microwave energy which is taken from output Mo and conventionally routed to the cooking chamber of a microwave oven, not illustrated, the details of which are not necessary to the understanding of the present invention.
Consider now the operation of the control circuits and the selective switching of triac TRl. The DC current flowing thr~ugh the magnetron on one, i.e. a first, half-cycle AC also passes through resistor R5 producing a voltage drop across -~ MCP-76-17 resistor RS, such as might be represented by any of curves A, B
or C of Figure 2b. This voltage i9 of a pulsating nature having a peak value equal to that representative of the peak current through the magnetron.
This voltage is applied through resistor R4 to charge capacitor C3 up to essentially peak voltage. Resistor R4 and capacitor C3 are seen to form a well-known RC circuit. For example, during the half-cycle when magnetron M is conducting current, the voltaqe across resistor R5 creates a charging current through resistor R4. During the alternate half-cycle in which the magnetron is not conducting current, the charge on capacitor C3 discharges through resistor R4 and resistor R5 in series.
The time constant of the circuit R4, RS and C3 is preferably about equal to one AC half-cycle, so that the charge on a capacitor C3 is partially discharged during the alternate half-cycles in which magnetron M is not conducting current. For example, the AC half-cycle may be 8.3 milliseconds and the time constant may be 8 milliseconds. Thus, during an alternate half-cycle, the level of voltage on capacitor C3 is representative of the current level through magnetron M during the preceding half-cycle. This voltage i9 ideally illustrated in Figure 2c.
Obviously, the greater the voltage drop across resistor R5 during the half-cycle in which the magnetron is conducting current, the higher is the level of charge on capacitor C3, as is evident by comparing curves A, B and C in Figure 2c. In effect, the RC circuit stores the information on the current level through the magnetron in the preceding half-cycle for use during the subsequent half-cycle. The voltage from capacitor C3 is coupled to the inverting input indicated by a minus sym~ol (-) of operational amplifier Al. It is noted that this time , ~- MCP-76-17 constant can be made longer or shorter than one half-cycle, with concurrent change in value of other circuit components, to obtain similar but less satisfactory results.
An operational amplifier is a well-known circuit device and is here used as a comparator by means of which the voltage level representative of current through the magnetron i9 com-pared to the predetermined voltage level established at the reference input thereof from the tap R2t of potentiometer resistor R2. me operational amplifier or comparator, as may be variously termed herein, is illustrated by a conventional symbol.
However, the circuit in fact is a somewhat complicated component having numerous transistors, diodes and resistors on a single integrated circuit chip. Such types of devices are available from semiconductor manufacturers, one type being marketed under the designation CA 741, although an equivalent may be used.
Briefly, in operation, the output V0 of the operational amplifier is negative low when the level of voltage at its inverting input is less than the voltage level at its reference input. When the voltage at the inverting input exceeds the voltage level at the reference input, the output is zero. As is shown, the output of the comparator Al i8 coupled to the gate electrode of the triac TRl. The triac blocks current flow through the path from one terminal of capacitor Cl, diode D2, the triac to ground and thence to the other side of the secondary TSl until the voltage at the inverting input of comparator Al falls below the reference voltage level.
Returning now to the state of charge on capacitor C3, it is apparent that capacitor C3 was charged during the preceding AC half-cycle to a certain voltage which is representative of the current through the magnetron. This voltage is greater than the ~ MCP-76-17 1069~84 reference voltage applied to the reference input of the compar-ator. However, during the half-cycle the voltage on capacitor C3 reduces to below thi~ reference level and the output of comparator Al turns negative and turns on the triac. This occurs during the next AC half-cycle when magnetron M is not conducting current and when voltage applied to the anode of diode D2 is proper for conduction. Considering curve A of Figure 2c as representative of the voltage on capacitor C3 at this time, triac switching occurs when curve A and the dashed line VREF inter~ect. With triac TRl in the "on" condition and diode D2 properly biased, current commences to flow through that circuit to charge the capacitor Cl to the level of voltage of AC
at the time the circuit conducts current.
Con~ider next the case where the current through the magnetron is excessive. In that event, capacitor C3 is charged to a higher voltage than in the preceding case as represented in curve C of Figure 2c. Accordingly, during the alternate half-cycle, the discharge of this capacitor takes a longer period of time and does not fall to the reference level VREF until a later time within the next alternate half-cycle, as compared to the preceding case, as may be represented by curve C of Figure 2c.
Thus, amplifier Al switches on at a later time and allows triac TRl to switch on at a later time to initiate current through the path including diode D2 and triac TRl to charge capacitor Cl to a lower voltage level.
Conversely, when the magnetron cQnducts less than the desired current in one AC half-cycle, the voltage across resistor R5, represented ideally by curve B of Figure 2b, is lower, capacitor C3 is charged to a lower value, the voltage on capa- -citor C3 in the next succeeding half-cycle will drop more rapidly, ~ MCP-76-17 i.e. in a shorter time, to the reference voltage level during the next half-cycle of AC, as ideally represented by curve B of Figure 2c, and the operational amplifier Al switches on triac TRl at an earlier time within the next ~ucceeding half-cycle, so that capacitor Cl is charged to a higher voltage. It is of course understood that current of greater magnitude will flow through the magnetron during the next succeeding AC half-cycle, since the voltage on the capacitor will be additive with the voltage of winding TSl.
By a judicious choice of voltage levels and circuit time constants, the current lével of the magnetron is monitored, i.e.
~ampled during each half-cycle in which the magnetron conducts.
mi8 information is used in the next AC half-cycle to gate or ungate the shunt current path across the magnetron in order to regulate the level of charge on the series capacitor Cl, which, in turn, affects the current level of the magnetron in the next succeeding half-cycle in which the magnetron again conducts current.
In essence, there are a high-voltage secondary winding, a series capacitor and a magnetron (which is a unidirectionally current-conducting device) connected in a series circuit. There is an additional circuit for conducting current in shunt of the magnetron during the half-cycles in which the magnetron i9 not properly poled, i.e. biased, for conducting current, so as to provide a voltage doubling effect by charging up the capacitor during these alternate half-cycles.
The shunt current path is effectively blocXed during the half-cycles in which the magnetron is conducting and remains blocked until the beginning of, or at some point in time within, the next half-cycle of AC, when the control means again unblocks the shunt current path to allow the capacitor to receive some electrical charge and the control means is responsive to the level of current through the magnetron in the preceding half-cycle as the determinant of the time at or within the next succeeding half-cycle in which the control means unblocks the current.
If the average current to the magnetron is increased, the average power is increased. By adjusting the reference voltage at the reference input terminal of the comparator Al, i.e. by setting it for higher or lower power levels, the magnetron power ; level may be easily adjusted.
Within the spirit of the invention, and viewed in a broader context, it is apparent that other electrical loads having a unidirectional current-conducting characteristic can be utilized or sub~tituted for the magnetron, to achieve the same effect and advantage.
Moreover, other obviou~ variations are apparent simply from Figure 1. As indicated above, the magnetron include~ a heater, with the low-voltage secondary winding TS2 of the tr~ns-former T supplying heater current. A~ is well known, however, asecond transformer or other means can be used for this purpose, even though the~e alternatives are more expensive. Specifically, the novel arrangement as applied to a magnetron permits the ' heater winding to be employed on the ~ame transformer core as the high-voltage ~econdary winding. Similarly, while a DC source i~ shown to supply the reference and power supply voltages for operational amplifier Al as a rectifier-filter combination con-~isting of diode Dl and capacitor C2 coupled to a low voltage tap on secondary TSl, it is equally possible to accomplish this function either by substituting for that a separate DC source 106~184 or, alternatively, providing a low voltage AC winding separate from the secondary TSl, such as is the case with the heater winding, or providing a separate transformer entirely to provide the low voltage AC into the rectifier Dl. These are obviously more expensive alternatives. While the triac is used in the mode where it initially blocks current, it is equally possible for a device to be used in accordance with the teachings set forth herein to normally allow current to flow initiaLly and then to block the current. In effect, it is possible to implement a reversal of the sequential arrangement of parts, as compared to that illustrated, but for the same purpose of regulating the magnitude of charge and the voltage to which the capacitor Cl is charged during alternate half-cycles when the magnetron is not poled, or biased, for conducting current.
Ideally, a single silicon controlled rectifier can be substituted for the diode and the triac in Figure l, to perform the same functions which can be achieved with a regular SCR
voltage, if available, for operation with a negative gating or by incorporating an inverter in the circuit to invert the ampli-fier output to a positive voltage. ~owever, this substitution i8 presently permissible only in a low voltage power supply by which some other electrical load, i.e. other than a magnetron, is driven. A magnetron operates in the voltage range of 3,000 to

4,000 volts or more, 80 that currently available semiconductor control devices are not acceptable, as they are not capable of withstanding back voltages at that level, although diodes, such as one useful as diode D2, are available which can withstand such high voltages. Moreover, tne triac is somewhat self-protecting against reverse voltage transients and i6 thus preferred in com-parison to presently available silicon controlled rectifiers.

.

1C~69184 The serious problems connected with the operation of magnetrons is that of moding or mismoding. This means that, under certain circumstances when power is applied to the magnetron, the magnetron may go into oscillation at a frequency at which the magnetron was not designed to operate, instead of the correct frequency. This phenomenon is the cause for an inherent diffi-culty in using magnetrons and occurs chiefly in operation of magnetrons in the pulse mode, typically in radar systems where the full voltage is applied almost instantaneously, such as like a step function. If the anode voltage is applied gradually to the desired level, such as at audio frequencies or below, the phenomenon of moding does not usually occur. Conversely, were the magnetron circuit to be gated on and off rapidly, as by a device such as a silicon controlled rectifier or triac, which have very fast operating times, namely in the order of nano-seconds, a mismoding problem could arise. However, as is seen in the circuit, the rapid turn on of the triac, or a substituted semiconductor controlled rectifier, occurs during the charging of the capacitor and does not directly gate on or off the magne-tron. In this specific context, there is provided a uniquefeedback control circuit for regulating the current level of a magnetron in which the rapid switching of current occurs during the alternate half-cycles in which the magnetron is not operating.
It is anticipated that any voltage transients that may arise and appear to cause difficulties can be cured by judicious insertion of suitable protéctive devices, such as Zener diodes.

It is noted that with present magnetrons it is not necessary to adjust the voltage applied to the magnetron over a full range from zero volts to a maximum operating voltage, but that only a limited range of voltage variation is satisfactory.

:` -` MCP-76-17 By way of example, a specific model of a magnetron may provide an output of'600 watts with an applied voltage of 4,000 volts, but may provide only 100 watts output with an applied voltage of ~,500 volts. mus, only 500 volts less result in a six-fold reduction in power output.
Therefore, it is only necessary to reduce the charge or voltage on the main capacitor by a small percentage to obtain a large percentage reduction in power output.
Reference may be made to Chapter 5, Clamper Circuits, pages 65-71 of the book entitled "Semi Conductor Pulse Circuits", Mitchell, Holt, Rinehart ~ Winston, 1970, for background principle relevant to the clamping of voltages to different levels and which formed part of the helpful knowledge from which the present invention evolves.
A second embodiment of the invention is presented in Figure 3. For clarity, those elements in this embodiment which are essentially the same as those described in connection with Figure 1 are identified by the same reference character, to identify the element. Moreover, inasmuch as most of the elements in Figure 3 are connected in the same manner and have the same function as in the above-described embodiment, they are not described again, in the i,nterests of clarity and conciseness, so that the following description of structure is confined to those changes or modifications in the c~rcuit which are significant.
Thus, in the embodiment shown in Figure 3, the anode of magnetron M is connected electrically to, ground. The terminal 2 of secondary winding TSl is connected in series with resistor R5 to ground. The high-voltage diode D2, used in Figure 1, is not used in this embodiment. Diode Dl is connected in circuit with its anode terminal to secondary winding tap 3 and the cathode ., . ~.

~c~69~84 terminal to the circuit junction of capacitor C2 and resistor Rl and is thus reversed in polarity from the diode circuit orienta-tion in the embodiment of Figure 1, to rectify and produce a positive polarity voltage at the circuit junction of the capacitor C2 with resistor Rl and hence at the noninverting input of operational amplifier Al.
The anode of a rectifier diode D3 is connected in circuit with ~econdary winding terminal 2 and one terminal of resistor R5. Re~istor R6 and capacitor C4 are connected electrically in ~erie~ circuit between the anode of diode D3 and ground. A
bleeder re~i~tor R7 is connected electrically in shunt of capa-citor C4. The circuit junction of resistor R6 with capacitor C4 is connected to the inverting input, designated by the minus symbol (-), of amplifier Al to apply any voltage on capacitor C4 to the amplifier inverting input.
Resistor R7 and capacitor C4 essentially perform the function of an R-C circuit, which i9 that performed by resistor R4 and capacitor C3 in the embodiment of Figure 1. However, because rectifier diode D3 is included in this circuit and blocks current flow in one direction, the resistor R7 is connected to provide a path for current out of the capacitor, so that capaci-tor C4 may be discharged.
The mode of operation of the circuit is the same as that of the embodiment of Figure 1 in most aspects, except as follows.
Diode Dl rectifies the low voltage AC that appears at low voltage tap 3 of secondary TSl and provides a positive volt-age to charge up capacitor C2 and thus the rectifier and capacitor function as a positive DC voltage source. This volta~e is applied to the Vl input of the comparator, i.e. operational amplifier Al, to provide power to the amplifier and to the ~0691B4 .

resistive voltage divider network Rl, R2 and R3, so that a positive voltage is applied via potentiometer tap R2t to the noninverting input + of the amplifier Al.
m e magnetron current flows in a series circuit including capacitor Cl, secondary winding TSl, resistor R5, and ground.
Hence, the voltage drop across resistor R5 during the AC half-cycle in which the magnetron conducts current is proportional to the magnetron current, as in the first embodiment. The polarity of the voltage drop is positive with respect to ground and this is then applied through resistor R6 to charge up capacitor C4.
During the alternate half-cycles in which magnetron M is not conducting current, the voltage drop across resistor R5 is representative essentially of current to charge capacitor Cl.
Since the latter current is opposite in direction to the current through the magnetron in the preceding half-cycle, the voltage at resistor R5 i9 negative in polarity. Diode D3 acts to block that negative voltage from being applied to capacitor C4.
Thus, capacitor C4 is charged to a voltage of positive polarity during one half-cycle and that voltage level is repre-~entative of the DC current through magnetron M. During the next half-cycle of AC during which the power circuit capacitor Cl is to be charged, capacitor C4 commences to discharge through resistor R7. At any given period of time during this half-cycle, the voltage on C4 is a function of the time constant RC of the circuit formed by resistor R7 and capaci~or C4, and the voltage level to which capacitor C4 was initially charged. As in the embodiment of Figure 1, this time constant is preferably equal to 1/2F, where F is the line voltage frequency, such as 8 milli-seconds.

~069184 As in the first embodiment, the voltage at the inverting input of comparator Al decreases to a level equal to that of the reference voltage applied via tap R2t to the inverting input.
At that time, the differential amplifier Al, used as the compara-tor, switches it~ output from zero to a high positive voltage toapply an enabling input to the gate of triac TRl and the triac switches into its current-conducting condition.
As in the embodiment of Figure 1, the point in time during a half-cycle of AC at which the amplifier Al switches shifts back and forth, depending on the magnetron current level during the preceding half-cycle, to automatically ideally, by virtue of this feedback mechani~m, adjust the voltage to which capacitor Cl charges.
Setting of tap R2t, by means of knob K allow~ the same adjustment as in the embodiment of Figure 1.
- It i8 noted that, since the embodiment of Figure 3 employs a po~itive output voltage to trigger triac TRl into its "on" condition, as contrasted with a negative trigger voltage in the embodiment of Figure 1, those electronic semiconductor switch-ing devices which require a positive trigger voltage, such as asilicon controlled rectifier, or a series circuit of a rectifier diode and a silicon controlled rectifier, both electrically poled in the same direction, may be directly substituted for triac TRl.
By omitting, in the embodiment of Figure 3, the diode D2, u~ed in the embodiment of Figure 1, and thus as a result of the absence of a diode in series with the triac in the embodiment of Figure 3, the risk of damage due to certain high-voltage tran-sients is eliminated. The triac is selected to have a voltage breakdown characteristic of between 1.1 to 2 times the magnitude of the normal operating voltage of the magnetron, as specified by .~ ' ' .. . ~ -the magnetron manufacturer. This voltage breakdown is the characteristic voltage at which the semiconductor switch goes into its conducting state, irrespective of any gating voltage.
By way of example, it was found suitable to select a triac having a breakdown voltage of 4,500 volts for a magnetron having a normal operating voltage of 4,000 volts.
In the event that a high transient voltage commences to develop and reaches 4,500 volts, the triac in Figure 3 will break down and conduct the current, shunting the current from the magnetron and the triac continues to conduct the current until the current passing through it falls to zero. The triac then restores to its "off", i.e. noncurrent-conducting state.
Thus, although in normal operation of the circuit the triac conducts current in only one direction to charge the main series capacitor, in the abnormal or voltage transient suppress-ing de, the triac also conducts current in the opposite direction to discharge the main capacitor and protect the circuit again~t high voltage transients.
It is believed that the foregoing detailed description of two embodiments of the invention is sufficient to enable one skilled in the art to make and use the invention. However, it is expressly understood that the specific details presented for that purpose are not intended in any way to limit the invention, inasmuch as, in accordance with the teachings contained herein, numerous modifications, alterations or substitutions of equiva-lents, such as those in part described herein! may be made by those skilled in the art.

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.. . . ..

Claims (24)

THE EMBODIMENTS OF THE INVENTION IN WHICH
AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE
DEFINED AS FOLLOWS:

1. A microwave oven power supply, comprising:
transformer means, said transformer means having a primary winding for connection to a source of AC and a secondary winding for providing a high voltage AC transformed from said primary winding across the secondary winding terminals, said AC voltage having a generally cyclically varying sinusoidal waveform including a first half-cycle in which the voltage level over a period of time, T, rises from zero to a maximum in one polarity direction and then reduces to zero followed by an alternate half-cycle in which the voltage level over, a time, T, rises from zero to a maximum in an opposite polarity direction and then reduces to zero;
capacitor means;
a magnetron, said magnetron having unidirectional current-carrying characteristics so as to conduct current only on one half-cycle of AC;

means connecting said secondary winding, said capacitor means and said magnetron in an electrical series circuit;
semiconductor controlled switch means of the type having a gate input and a pair of current-conducting main terminals, said switch means further being of the type having an electrically nonconductive state and responsive to application of a control voltage to its said gate input for substantially instantaneously, in a time substantially less than T, switching to a current-conducting condition to pass current between said main terminals and responsive to the current between said main terminals reducing effectively to a level of zero and absent a control voltage at said gate input for substantially instantaneously, in a time substantially less than T, restoring to the electrically nonconductive state;
said semiconductor controlled switch means being connected in circuit essentially in series with said capacitor means and said secondary winding and essentially in shunt of said magnetron for conducting current in shunt of said magnetron responsive to the application of a control voltage to the gate input thereof;
current-monitoring means, comprising resistor means connected in electrical series circuit with said magnetron, for providing an output signal representative of the magnetron current during said one half-cycle;

timing means coupled to said current-monitoring means for providing a time-varying signal that changes in level with lapse of time during the alternate half-cycle of AC and has an initial level representative of the current level in said magnetron during the preceding one half-cycle, whereby said time-varying signal attains a predetermined level as a function of both the magnetron current level and lapse of time;
said timing means comprising:
a resistor and a capacitor electrically connected in series circuit across said resistor of said current monitoring means, said circuit having a time constant, Tc, where Tc equals the product of the capacitance measured in farads and R is the value of resistance measured in ohms, with said Tc being less than 1/2 F, where F is the frequency of the AC supplied by said transformer; and control means coupled to said timing means responsive to the level of said time-varying signal attaining a predetermined value during said alternate half-cycle for thereupon providing a control voltage pulse to said gate input of said semiconductor controlled switch means, said control means comprising: comparator means, said comparator means having a reference input, an inverting input and an output; reference voltage source means;
means connecting said reference voltage source means to said reference input of said comparator means;

means connecting the output of said comparator means to the gate electrode of said semiconductor controlled switch means; and means coupling the voltage from said capacitor of said timing means to the inverting input of said comparator means.

2. The power supply as defined in Claim 1, wherein said reference voltage source means includes means for adjusting the reference voltage output thereof.

3. The power supply as defined in Claim 1, wherein said reference voltage source means comprises:
a source of DC voltage; and resistor means connected across said source of voltage, said resistor means including a tap, whereby a reference voltage is produced at said tap.

4. The power supply as defined in Claim 3, wherein said tap is selectively positionable, so as to permit adjustment of said reference potential.

5. The power supply as defined in Claim 4, wherein said DC voltage source comprises:
tap means on said secondary of said transformer; and rectifier and filter means connected in circuit therewith to provide a DC voltage at said capacitor.

6. The power supply as defined in Claim 1, wherein said semiconductor controlled switch comprises a triac.

7. The power supply as defined in Claim 1, wherein said control means includes:
comparator means, said comparator means having a reference input, an inverting input and an output;
a source of reference voltage coupled to said reference input, said source being adjustable in level; and means coupling said time-varying signal to said inverting input.

8. A power supply for a microwave magnetron comprising:
a transformer having a primary winding for connection to a source of electrical power and a high voltage secondary winding adapted to have an AC voltage of frequency F induced thereacross from said primary winding;
capacitor means, said capacitor means having one terminal connected to one terminal of said secondary winding;
diode rectifier means;
triac means, said triac means and said diode rectifier means being connected in series circuit between the remaining terminal of said secondary winding and the remaining terminal of said capacitor means;
a magnetron;
resistor means;
means for connecting said resistor means and said magnetron in series circuit between said remaining terminal of said secondary winding and said remaining terminal of said capacitor means;
said magnetron being electrically poled in circuit in opposite electrical polarity relationship to the direction in which said diode is poled, so that the circuit containing said magnetron conducts current only in one direction and the circuit containing said diode conducts current only in the opposite direction;

an R-C circuit means connected across said resistor means, said R-C circuit comprising a resistor and capacitor connected in series and having a time constant, T, less than 1/2 F;
comparator means, said comparator means having a reference input, an inverting input and an output;
means for applying enabling voltages to said gate electrode of said triac under control of said comparator output;
means for applying a reference voltage from a reference voltage source to said reference input of said comparator means, said reference voltage source being selectively adjustable; and means for coupling the output of said R-C circuit to said inverting input of said comparator means;
whereby during one half-cycle of AC
across said secondary winding current will flow through said winding, said series capacitor, said magnetron and said resistor to develop a voltage across said resistor proportional to the current through said magnetron during that half-cycle and during the opposite half-cycle said magnetron is nonconducting;
whereby said R-C network receives and stores a voltage proportional to the voltage across said first resistor means through the alternate AC
half-cycle and wherein said comparator means enables said triac only at some predetermined time within such alternate half-cycle in which said diode is poled to conduct current;
whereby said capacitor will be charged only to a voltage level existing at the time within said alternate half-cycle in which said triac is enabled.

9. The power supply as defined in Claim 8, wherein said comparator means comprises an operational amplifier.

10. The power supply as defined in Claim 8, wherein said means for applying enabling voltages to said gate electrode of said triac under control of said comparator output comprises electrical conductor means coupled between said comparator output and the gate electrode of said triac.

11. A regulated power supply for an electrical load of the type having unidirectional current-conducting characteristics, comprising:
transformer means for producing an AC
voltage across a secondary winding;
capacitor means, one end of said capacitor means being connected with one terminal of said secondary winding;

Claim 11....continued.

means for connecting said unidirectional current-conducting load in series circuit between the remaining terminal of said capacitor means and the remaining terminal of said secondary winding;
a second electrical circuit connected between said remaining terminal of said capacitor and said remaining terminal of said secondary winding in parallel of said first circuit, said second electrical circuit having unidirectional current-carrying characteristics and oppositely poled with respect to said first circuit for conducting current only during alternate half-cycles, said second electrical circuit being switchable between a current-conducting condition and a noncurrent-conducting condition for regulating the average value of current therethrough and thereby regulating the charge on said capacitor during the half-cycle in which said first circuit is in the noncurrent-conducting condition;
first resistor means connected in circuit with said first circuit so as to produce a voltage thereacross representative of current through said first circuit;
timing circuit means comprising second resistor means and capacitor means connected in series across said first resistor means, said timing circuit having a product of resistance in ohms x capacitance in microfarads equal to or less than 8.3 milliseconds for charging to the level of voltage across said first resistor means during said first half-cycle and slowly discharging during the next half-cycle;
comparator means having a reference voltage input, an inverting input and an output;
means for applying a reference voltage to said reference input;
means for applying the voltage across said capacitor in said timing circuit to said inverting input; and means for coupling the output of said comparator to said second circuit for switching said second circuit between the "off" and "on"
condition;
whereby during an alternate half-cycle the voltage across the capacitor in the timing circuit drops to the reference voltage level and said comparator provides an output which allows said second circuit to conduct current for the remainder of said alternate half-cycle.

12. The power supply as defined in Claim 11, wherein said timing circuit means comprises a resistor and a capacitor connected electrically in series circuit.

13. A microwave oven power supply, comprising:
transformer means having a primary winding for connection to a source of AC voltage and a high voltage secondary winding for providing a high AC
voltage transformed from said primary across the secondary winding terminals; said AC voltage having a generally sinusoidal, cyclically varying waveform including a first half-cycle in which the voltage level over a period of time, T, rises from zero to a peak in one polarity direction and then decreases to zero followed by an alternate half-cycle in which the current level over a period of time, T, rises from zero to a peak in an opposite direction and then decreases to zero;
capacitor means having first and second terminals, said capacitor means having one terminal connected to a terminal of said secondary winding;
means for connecting a magnetron in circuit between the remaining terminal of said capacitor means and the remaining terminal of said secondary winding to define an electrical series circuit in which the voltage across said capacitor means is in additive relation with the voltage of said secondary winding during the first half-cycle of AC, said magnetron having unidirectional current-carrying characteristics so as to conduct current only during the first half-cycle of said AC voltage;

current-monitoring means for providing an output representative of magnetron current during the first half-cycle; and control circuit means connected in shunt of said magnetron and coupled to said current-monitoring means for initiating current flow through said capacitor, by-passing said magnetron, at a first predetermined instant of time within the alternate half-cycle responsive to said output of said current-monitoring means being at a predetermined level and for initiating current flow through said capacitor and by-passing said magnetron at a second predetermined instant of time within said alternate half-cycle greater or less, respectively, than said first predetermined instant of time responsive to said output of said current-monitoring means being above or below, respectively, said predetermined level and for terminating said current at the conclusion of said alternate half-cycle.

14. A microwave oven power supply, comprising:
transformer means, said transformer means having a primary winding for connection to a source of AC and a secondary winding for providing a high voltage AC transformed from said primary winding across the secondary winding terminals, said AC voltage having a cyclically-varying, substantially sinusoidal waveform including a first half-cycle in which the voltage level over a period of time, T, rises from zero to a maximum in one polarity direction and then reduces to zero followed by an alternate half-cycle in which the voltage level over a time, T, rises from zero to a maximum in the opposite polarity direction and then reduces to zero;
capacitor means;
a magnetron, said magnetron having unidirectional current-carrying characteristics, so as to conduct current only on one half-cycle of AC;
means connecting said secondary winding, said capacitor means and said magnetron in an electrical series circuit;
semiconductor controlled switch means of the type having a gate input and a pair of current-conducting main terminals, said switch means further being of the type having an electric-ally non-conductive state and responsive to application of a control voltage to its said gate input for substantially instantaneously, in a time substantially less than T, switching into a current-conducting condition to pass current between said main terminals and responsive to the current between said main terminals reducing effectively to a level of zero in the absence of a control voltage at said gate input for sub-stantially instantaneously, in a time substantially less than T, restoring to the electrically non-conductive state;

said semiconductor controlled switch means being connected in circuit essentially in series with said capacitor means and said secondary winding and essentially in shunt of said magnetron, for conducting current in shunt of said magnetron responsive to the application of a control voltage to the gate input thereof;
current-monitoring means for providing an output signal representative of the magnetron current during said first half-cycle;
timing means coupled to said current-monitoring means for providing a time-varying signal that changes in level with lapse of time during the alternate half-cycle of AC and has an initial level representative of the current level in said magnetron during the preceding first half-cycle, whereby said time-varying signal attains a predetermined level as a function of both the magnetron current level and lapse of time; and control means coupled to said timing means responsive to the level of said time-varying signal attaining a predetermined value during said alternate half-cycle for thereupon providing a control voltage pulse to said gate input of said semiconductor controlled switch means, whereby said switch means conducts current in shunt of said magnetron to said capacitor means for the remaining duration of said alternate half-cycle.

15. In a microwave oven power supply, the combination comprising:
transformer means, said transformer means having a primary winding for connection to a source of AC voltage and a high voltage secondary winding for providing a high AC voltage transformed from said primary across the secondary winding terminals;
said AC voltage having a substantially.
sinusoidal, cyclically-varying waveform including a first half-cycle in which the voltage level over a period of time, T, rises from zero to a peak in one polarity direction and then decreases to zero followed by an alternate half-cycle in which the current level over a period of time, T, rises from zero to a peak in the opposite direction and then decreases to zero;
capacitor means having first and second terminals, said capacitor means having one terminal connected to one terminal of said secondary winding;
means for connecting a magnetron in circuit between the remaining terminal of said capacitor means and the remaining terminal of said secondary winding to define an electrical series circuit in which the voltage across said capacitor means is in additive relation with the voltage of said secondary winding during the first half-cycle of AC, said magnetron having unidirectional current-carrying characteristics so as to conduct current only during the first half-cycle of said AC voltage;

charge control means responsive to magnetron current during the first half-cycle of AC voltage for permitting electrical charge to be applied to said capacitor means during an alternate half-cycle and controlling such charge to a level dependent upon and automatically determined as a function of said magnetron current level during the immediately preceding first half-cycle whereby a voltage appears across said capacitor;
said charge control means including:
semiconductor controlled switch means, said semiconductor controlled switch means being of the type including a gate input and a pair of main terminals, said controlled switch means being normally in an electrically nonconducting state in which no current may pass between said main terminals and, responsive to a control voltage applied to its gate input, for substantially instantaneously, in a time substantially less than said T, switching into a current-conducting state in which current may pass between said two main terminals and, responsive to current through said terminals attaining a level effectively of zero and in the absence of a control voltage at said gate input for automatically sub-stantially instantaneously, in a time substantially less than said T, restoring to its electrically nonconductive state;

said semiconductor controlled switch means being connected in electrical circuit in shunt of said magnetron for conducting current to said capacitor means by-passing said magnetron responsive to the application of a control voltage to said gate input;
current-monitoring means responsive to the level of current through said magnetron during the first half-cycle for providing an output signal of a level proportionate to said magnetron current; and control circuit means coupled to said current-monitoring means and said semiconductor controlled switch means for equating said level of said output signal of said current-monitoring means with a corresponding one of a plurality of elapsed time intervals, each of which time intervals being equal to or less than said time T, and for providing a control voltage pulse to said gate input of said semiconductor controlled switch means at the expiration of a time interval, commencing with the alternate half-cycle which follows the said first half-cycle, equal to said corresponding one of said elapsed time intervals;
whereby said semiconductor controlled switch means may conduct current by-passing said magnetron and passing to said capacitor means for the duration of the alternate half-cycle.

16. A power supply for the magnetron of a micro-wave oven, comprising a transformer whose secondary cir-cuit includes the magnetron and a capacitor in series connection, the capacitor being thus connected to dis-charge through the magnetron during first half-cycles of operation in which the magnetron is conducting, there being provided a unidirectionally conductive shunt cir-cuit branch parallel to the magnetron, which causes the capacitor to be charged during second half-cycles when the magnetron does not conduct, thereby to achieve the effect of voltage addition to the secondary voltage dur-ing the first half-cycles of operation, the shunt cir-cuit branch comprising switch means for controlling the duration of current flow which charges the capacitor during second half-cycles as an inverse function of cur-rent flow through the magnetron in a preceding first half-cycle.

17. Power supply according to Claim 16, compris-ing circuit means for generating a signal which is repre-sentative of current flow through the magnetron during a first half-cycle of operation and for controlling the period of conductivity of the switch means during the sub-sequent second half-cycle as a function of the signal.

18. Power supply according to Claim 16, wherein the switch means is a semiconductor device having a control electrode.

19. Power supply according to Claim 16, wherein the switch means is a triac whose control electrode has applied to it a time-varying signal whose duration is a function of current flow through the magnetron in the preceding first half-cycle.

20. Power supply according to Claim 19, comprising a resistance-capacitance circuit whose capacitor is charged during first half-cycles of operation, thereby to generate the signal which is representative of current flow through the magnetron during first half-cycles of operation.

21. Power supply according to Claim 20, comprising a comparator to which are applied the voltage across the capacitor of the resistance-capacitance circuit as well as a reference voltage, the output signal from the comparator being applied to the control electrode of the triac.

22. Power supply according to Claim 21, comprising resistor means in another, unidirectionally conductive branch circuit of the secondary circuit, the resistor means having a tap from which the reference voltage is derived.

23. Power supply according to Claim 22, comprising a rectifying diode and a filtering capacitor in the other, unidirectionally conductive branch circuit for providing a substantially constant-level, direct-current reference voltage.

24. Power supply according to Claim 22 or Claim 23, wherein the reference voltage is adjustable by adjusting the position of the tap.