High-voltage SCR circuit for microwave oven and the like

An SCR circuit is provided in a high-voltage supply for energizing a magnetron that is assembled in a microwave oven. The high-voltage supply circuit includes a voltage step-up transformer. This transformer is arranged with a low-voltage secondary winding which is connected across the heater-cathode electrode of the magnetron and a high-voltage secondary winding which is coupled in series with a capacitor. The series combination of the capacitor and the high-voltage secondary is connected across the cathode and anode electrodes of the magnetron; and importantly, the high-voltage SCR circuit includes a number of series-connected SCR's electrically in shunt of the magnetron. A triggering circuit gates the high-voltage SCR circuit to the current-conducting state during a selected phase portion of an applied AC potential, in order to control the amount of power being supplied to the magnetron. The high-voltage SCR circuit also advantageously includes a diode and a coil disposed in electrical series with the series-connected SCR's respectively to cooperate with the SCR's in blocking reverse voltage and to limit the rate of current rise in the forward direction. A coupling circuit is associated with the gate terminal of each of the individual SCR's for coupling the triggering circuit gating signal to all of the SCR gates simultaneously, while preventing current leakage back through the coupling circuits.

BACKGROUND OF THE INVENTION 
This invention relates generally to high-voltage SCR arrangements and more 
particularly to a high-voltage SCR circuit that is incorporated in a 
high-voltage supply for energizing a magnetron of the type commonly 
employed in a microwave oven. 
Microwave ovens have been developed as domestic appliances for heating and 
cooking foods by exposure to the energy of microwave radiations. According 
to modern commercial practice, these domestic microwave ovens employ a 
magnetron, basically an electronic vacuum tube which converts a DC 
electrical input into an electromagnetic output in the microwave frequency 
range. Magnetrons of this type generally include a cathode-heater 
electrode and an anode electrode and exhibit a unidirectional current 
carrying characteristic. Such a magnetron further requires a DC potential 
across the electrodes of on the order of 3000 to 5500 volts to bias the 
tube into conduction for producing the microwave energy. 
In the past, high-voltage power supplies for providing this operating 
potential ordinarily have included a transformer for stepping up 
conventional household 120-volt AC line power, together with a rectifier 
and doubler circuit for generating the required level of DC voltage. 
Generally, a separate source of low-voltage AC potential is supplied to 
the heater electrode of the magnetron. 
Adjustability of the microwave power level is both desirable and a user 
convenience; and according to one conventional practice, microwave oven 
power supplies have been provided with a high leakage reactance 
transformer and a halfwave voltage doubler or villard circuit. The latter 
circuit comprises a high-voltage capacitor in series with the high-voltage 
secondary coil of the transformer and a high-voltage rectifier for 
blocking reverse current to the capacitor. Moreover, various circuits have 
been employed heretofore as a control in the primary of the transformer 
for regulating the amount of current applied thereto, thereby affording a 
degree of regulation over the power being delivered by the secondary and 
doubler circuit to the magnetron tube and, consequently, a degree of 
control over the microwave power output. An alternative prior art control 
arrangement utilizes a capacitor having two selectable values as the 
series capacitance of the villard circuit or voltage doubler, thereby 
providing two selectable power levels to the magnetron. Another control 
arrangement relies on a variable resistor in the current path to the 
magnetron for adjusting the amount of current supplied thereto. In the 
former case, only two selectable power levels are available. Moreover, the 
special dual value capacitor is a relatively expensive device. In the 
latter case, a limited range of adjustment is available, and considerable 
power must be dissipated in the resistor. This requires a relatively 
expensive resistor and one which is capable of consuming a relatively 
large current. As will be appreciated the consumption of current generates 
undesirable heat; and this may have a deleterious effect on other circuit 
components. 
A further prior art arrangement for variable control of the microwave 
output electrically connects a semiconductor triac is joined to a 
triggering circuit adapted for selectively varying the portion of the AC 
cycle during which the triac goes into conduction. The triggering circuit 
is fed by an additional low-voltage tap on the high-voltage secondary of 
the transformer and includes either an RC phase shifting network and a 
semiconductor diac in series between the transformer tap and the control 
terminal of the triac or, alternatively, a multivibrator circuit connected 
between the tap and the triac control terminal. In the case of a 
multivibrator, an additional diode is required in series with the triac. 
This arrangement therefore contemplates a number of added circuit elements 
and devices as well as a transformer with a supplementary low-voltage tap 
on the high-voltage secondary, thus adding considerably to the expense and 
labor required to produce the high-voltage magnetron supply. 
OBJECTS AND SUMMARY OF THE INVENTION 
A general object of the present invention is to provide a new and improved 
high-voltage circuit for supplying selected, different amounts of power to 
a microwave magnetron tube. 
A more specific object of this invention is to provide a high-voltage 
circuit of the type described which eliminates the need for either a 
high-voltage rectifier or a separate filament transformer for heating the 
cathode-heater electrode of the magnetron tube. 
Another object of this invention is to provide a high-voltage SCR circuit 
for use in a high-voltage supply circuit, which SCR circuit is relatively 
simple and inexpensive, can be produced as a unit to facilitate its 
connection in the high-voltage circuit, and yet is rugged and reliable in 
operation. 
Yet another object of this invention is to provide a high-voltage SCR 
circuit in accordance with the foregoing object, which is capable of 
handling a considerably higher range of voltage than a conventional SCR in 
both the forward and reverse direction, and is responsive to a trigger or 
gate pulse for going into conduction, yet is highly efficient in blocking 
large reverse voltages and is relatively free of leakage current 
therethrough in response to forward voltage, in the absence of a trigger 
or gate pulse. 
Briefly, and in accordance with the foregoing objects, a high-voltage SCR 
circuit includes a plurality of individual SCR's connected electrically in 
series. Coupling circuit means are joined to the gate terminals of each of 
the plurality of SCR's for simultaneously coupling a trigger pulse thereto 
while substantially preventing leakage current through the circuit. 
In a preferred embodiment, DC reverse blocking means are provided in series 
with the plurality of SCR's to cooperate in opposing an applied reverse 
voltage. Also, in another preferred embodiment, limiting means are 
provided in series with the plurality of SCR's to restrict the rate of 
rise of applied current in the forward direction. 
Other objects, features and advantages of the present invention will become 
apparent upon a consideration of the following detailed descriptions, 
together with the accompanying drawing wherein like reference numerals are 
used throughout to designate like elements and components.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
Referring now in detail to the drawing and initially to FIG. 1, a 
high-voltage SCR circuit indicated generally by the reference numeral 10 
is shown to include a selected number of substantially similar individual 
SCR's 12, 14, 16 and 18 that are electrically in series. In particular, 
the cathode of SCR 12 is joined to the anode of SCR 14 the cathode of 
which is joined to the anode of SCR 16 and so on in the series, the 
cathode of the final SCR 18 comprising a cathode terminal 20 of the entire 
high-voltage SCR circuit 10. As illustrated in FIG. 1, the high-voltage 
SCR circuit 10 includes at least four individual SCR's. However, in 
accordance with the principles of this invention, as few as two or as many 
as ten or more similar, series-connected individual SCR's may be utilized 
as is required by the level of the voltage to be handled in a particular 
application. 
In accordance with a feature of this invention, gating or coupling circuits 
22, 24, 26 and 28 are associated respectively with the individual SCR's 
12, 14, 16 and 18. The gating or coupling circuits 22-28 are substantially 
identical. Thus, only the coupling circuit 22 associated with the SCR 12 
need be described in detail. 
The coupling circuit 22 includes a diode 30 whose anode is connected with 
the anodes of the diodes of the other gating circuits and to a common 
terminal 32 which may be termed the trigger terminal of the high-voltage 
SCR circuit 10. The cathode electrode of the diode 30 is fed in series 
with a resistor 34 to the gate electrode of the SCR 12. The resistor 34, 
as well as the like resistor in each of the coupling or gating circuits 
22-28, has its value chosen to limit the gate circuit current to a 
desirable level. These resistors (e.g. resistor 34) are also chosen so as 
to make each gate circuit 22-28 of sufficiently high impedance to allow 
turn-on of the succeeding SCR (i.e., SCR 12, 14, 16 or 18). 
In a preferred arrangement, the high-voltage SCR circuit 10 additionally 
includes a DC reverse blocking element comprising a diode 36 electrically 
connected in series with the plurality of SCR's 12-18. Specifically, the 
anode and cathode electrodes of the diode 36 are oriented in the same 
polarity configuration as the anodes and cathodes of the individual SCR's 
12-18. So arranged, the diode 36 cooperates with the SCR's 12-18 in 
blocking reverse DC voltage. 
Moreover, a rate of current rise limiting element, specifically a coil 38, 
is joined electrically in series with the diode 36 and the SCR's 12-18. 
The remote end of coil 38 defines a terminal 40 of the high-voltage SCR 
circuit 10 and may be characterized as the anode terminal thereof. It will 
be appreciated that the coil 38 tends to limit the rate of current rise in 
the forward direction, thus preventing damage to the SCR's 12-18 as a 
result of a rapid change in current, such as might occur during SCR 
breakdown upon exposure to overvoltage. 
It will also be appreciated that the diodes in the respective coupling or 
gating circuits 22-28, such as the diode 30, substantially prevent current 
leakage between the anode 40 and cathode 20 of the circuit 10 at high 
voltage levels. Specifically, it has been found that without such diodes, 
leakage current is often experienced at high voltages, emanating from the 
anode of the first SCR 12, through the gate or coupling circuit 22 down to 
the gate of the last SCR 18 and through its cathode, thus effectively 
short-circuiting the intervening SCR's. The addition of diodes, such as 
the diode 30, substantially prevents current flow in this direction. 
One important use of the high-voltage SCR circuit 10 is in high-voltage 
supply circuits where it is desired to control the amount of power 
supplied to a load. As a specific example, and as illustrated in FIG. 2, 
the SCR circuit 10 may be advantageously utilized in a high-voltage 
circuit for a microwave magnetron tube 42 of a microwave oven. It is not 
intended to limit the applications and uses of the high-voltage SCR 
circuit 10 of this invention by making reference to the exemplary 
application. 
Turning now in detail to FIG. 2, the microwave magnetron tube 42 includes 
and anode electrode 44 which is electrically grounded and a heater-cathode 
electrode 46 which is joined across a pair of leads 48, 50. The 
high-voltage SCR circuit 10, illustrated in detail in FIG. 1, is connected 
electrically in shunt of the magnetron tube 42, the anode terminal 40 of 
the high voltage SCR circuit 10 being joined with the lead 48 and the 
cathode terminal 20 thereof being coupled to ground. The coil 38 and diode 
36 of the high-voltage SCR circuit 10 find special advantage in this 
application. However, in other applications, these elements may be omitted 
if desired. 
The high-voltage circuit illustrated in FIG. 2 advantageously includes a 
transformer 52 comprising a primary coil 54, a low-voltage secondary coil 
56, and a high-voltage secondary coil 58. The low-voltage secondary coil 
56 is connected across the leads 48, 50 of the heater-cathode electrode 46 
to provide a suitable low-voltage AC current thereto for heating purposes. 
Briefly, it will be appreciated that the magnetron tube comprises a vacuum 
tube device which requires some heating of its cathode in order to release 
sufficient electrons for proper operation. It will also be noted that, in 
this regard, many prior art high-voltage circuits require a separate 
filament transformer for this purpose. The present invention effectively 
eliminates the need for this extra component. 
Continuing with reference to FIG. 2, the high-voltage secondary coil 58 is 
joined at one side to a capacitor 60 that is in series relationship with 
the lead 48 of the heater-cathode electrode 46; and a shunt resistor 62 is 
provided across the capacitor 60. The opposite side of the high-voltage 
secondary coil 58 is connected via a resistor network 64 to ground. 
In accordance with a feature of this invention, a triggering circuit 
designated generally by the numeral 66 feeds the gate or trigger terminal 
32 of the high-voltage SCR circuit 10. The trigger circuit 66 itself 
includes a pair of timer integrated circuits 68, 70 that are connected in 
sequence with the terminal 32. Specifically, an output terminal 72 of the 
timer circuit 70 is connected with the terminal 32, while an input 
terminal 74 thereof is connected with an output terminal 76 of the timer 
circuit 68. An input terminal 78 of the timer circuit 68 is fed from a 
suitable source of AC power, such as the AC power source connected across 
the primary coil 54 of the transformer 52. Specifically, a voltage 
divider, comprising a pair of resistors 80, 82 and a current limiting 
resistor 84, is connected between the AC source and the input terminal 78 
of the timer circuit 68. A diode 86 has its cathode electrode joined to 
the junction of the resistors 80 and 82 and its anode electrode coupled to 
ground. The timer integrated circuits 68 and 70 are preferably of the type 
designated generally "555". 
The timer integrated circuit 68 includes a trigger terminal 88 and a reset 
terminal 90 connected together at the terminal 78. The circuit 68 also 
includes a control voltage terminal 92 connected via a capacitor 94 to 
ground, a reference terminal 96 joined directly with ground, a discharge 
terminal 98, and threshold terminal 100 that is connected via a capacitor 
102 to ground. A positive DC voltage is empressed on a terminal 104 of the 
timer circuit 68 and on a resistor 106 which is connected in series with a 
variable resistor or potentiometer 108, resistors 106 and 108 being 
connected, in turn, between the terminal 104 and the terminal 98 of the 
timer integrated circuit 68. 
With reference to the timer integrated circuit 70, a ground terminal 110 
thereof is connected to ground, a control voltage terminal 112 is 
connected via a capacitor 114 to ground; and discharge and threshold 
terminals 116 and 118 are coupled to ground via a capacitor 120, in 
similar fashion to the timer circuit 68. In addition the input terminal 74 
of the timer circuit 70 comprises its trigger terminal, while a reset 
terminal 122 is connected in common with a voltage supply terminal 124 to 
a source of positive DC potential. A resistor 126 is desirably connected 
between the voltage supply terminal 124 and the discharge terminal 116. 
In operation, the trigger circuit 66 functions substantially as follows. 
The timer integrated circuit 68 responds to a triggering signal applied at 
the terminal 78 by producing an output pulse of predetermined amplitude 
and duration. The amplitude of the output pulse is determined by the value 
of the DC voltage supplied at the terminal 104. The duration of the output 
pulse is determined, in turn, by the values of the fixed resistor 106, the 
variable resistor 108, and the capacitor 102. 
The timer integrated circuit 70 functions as a monostable circuit and in 
similar fashion as described for the timer integrated circuit 68. 
Specifically, responsive to the falling edge of the output pulse from the 
timer circuit 68, at the input terminal 74, the timer circuit 70 produces 
a pulse output at the terminal 72; and this pulse is fed to the gate or 
trigger terminal 32 of the high-voltage SCR circuit 10. The duration of 
the output pulse minimum at the terminal 72 is determined by the values of 
the fixed resistor 126 and the capacitor 120. The amplitude of the output 
pulse is dependent upon the value of the positive DC potential applied to 
the terminal 122. 
It will be appreciated from the foregoing description, that a trigger pulse 
will be applied to the trigger or gate terminal 32 of the high voltage SCR 
10 at a predetermined point in the phase of each cycle of the AC signal 
appearing at the input of the triggering circuit 66; and the variable 
resistor or potentiometer 108 effectively provides an adjustment for 
selecting the point in the phase of the AC cycle at which the trigger 
pulse will be produced. 
It will be apparent that the high-voltage SCR circuit 10 functions in the 
manner of a single SCR of high value, which is to say that the circuit 10 
behaves as a rectifier upon voltage being applied in the reverse 
direction, that is with a positive potential at the terminal 20 with 
respect to the terminal 40, and behaves as an electronic switch and a 
rectifier in series in the forward direction, that is with the potential 
positive at the terminal 40 with respect to the terminal 20. 
With the gate or trigger current at the terminal 32 at null, a relatively 
high breakover voltage value is necessary to cause the circuit 10 to go 
into conduction with current flowing from anode to cathode. In the 
presence of the trigger or gate pulse at the terminal 32, however, the SCR 
circuit 10 readily goes into conduction. The circuit 10 functions 
unidirectionally, effectively blocking the flow of any current in the 
opposite direction, that is from the terminal 20 to the terminal 40. The 
provision of the diode 36 aids in such reverse blocking, as previously 
described. With the gate or trigger signal applied thereto, the circuit 10 
will continue to conduct in the direction between the terminal 40 and 
terminal 20 as long as positive potential is present at the terminal 40 
with respect to the terminal 20. Thus, on the presence of the positive AC 
half-cycle at the terminal 40 via the capacitor 60, conduction will 
continue from the point in the phase of the AC cycle at which the trigger 
pulse is applied to the terminal 32 until the beginning of the following 
negative AC half-cycle, as illustrated by FIG. 3. 
Turning now more specifically to FIG. 3, the energy supplied to the 
capacitor 60 is dependent upon the proportion of the positive AC 
half-cycle during which the SCR circuit 10 is conducting. If the 
potentiometer 108 is adjusted to produce a gate pulse relatively early in 
the AC positive half-cycle, a correspondingly higher amount of energy will 
be supplied to the load, capacitor 60, while a comparatively lower amount 
of energy is supplied to the load, capacitor 60 when the potentiometer 108 
is adjusted to produce the gate pulse later in the AC positive half-cycle. 
The shaded portions of the diagram illustrate the amounts of energy 
supplied in each instance. 
It will be appreciated that the magnitude of the DC charge or potential on 
the capacitor 60 will be dependent upon the energy supplied thereto in the 
positive AC half-cycle from the high-voltage secondary 58 during the 
periods of conduction of the high-voltage SCR circuit 10. The voltage 
across the secondary 58 during the negative half of the AC cycle is 
blocked by the SCR circuit 10, and therefore is additive to the capacitor 
voltage with respect to the magnetron 42. Consequently, the amount of 
power supplied to the magnetron 42 varies in accordance with the power 
supplied to the capacitor, thereby controlling the amount of microwave 
energy ultimately produced. Thus, the variable resistor 108 functions as a 
control for selecting the amount of resultant microwave energy. In other 
words, the provision of the high-voltage SCR circuit 10 and trigger 
circuit 66 as described provides an adjustable control for the microwave 
power delivered to a microwave oven with which the magnetron tube 42 is 
associated. Since this power control is obtained in the secondary of the 
power transformer, there is little effect thereof on the voltages or power 
available at the heater winding 56 as often occured in prior art devices 
wherein the power control was located in the primary coil 54 of the 
transformer 52, whereby many prior art designs required separate filament 
or heater transformers. 
What has been shown and described herein is a high-voltage SCR circuit that 
is useful in many applications for alternatively gating or blocking 
current in voltage ranges considerably higher than heretofor possible with 
the use of a single SCR semiconductor device. The circuit is particularly 
advantageous when used with the trigger circuit according to this 
invention for controlling the amount of power delivered to a load, as for 
example in the illustrated microwave magnetron high-voltage supply 
circuit. Moreover, the provision of the SCR circuit 10 as a unit, having 
only three external terminals (20, 32 and 40) facilitates its 
interconnection in any desired application. 
The specific examples illustrated and described herein are to be taken as 
exemplary only. Various changes beyond the embodiments described may occur 
to those skilled in the art and are to be understood as forming a part of 
a the present invention insofar as they fall within the spirit and scope 
of the appended claims.