US2788521A - Selective control apparatus - Google Patents

Description

April 9, 1957 G, E, UNDY SELECTIVE CONTROL APPARATUS Original Filed Nov. 17, 1948 L oad flpparafus INVEN TOR.

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F/zaurncy MMQ A United States Patent fiice 2,788,521 Patented Apr. 9, 1957 SELECTIVE CONTRGL APPARATUS Gustav E. Undy, Detroit, Mich, assignor to Matti-Products Co., Hazel Park, Mich, a corporation of Michigan Continuation of abandoned application Serial No. name, November 17, 1948. This application April 39, 1954, Serial No. 426,656

9 Claims. (Cl. 343-225) This application is a continuation of my United States application, Serial No. 60,610, filed November 17, 1948, now abandoned.

This invention relates generally to radio controlled remotely operated apparatus and more specifically to new and improved radio receiver for actuating a controlled apparatus.

An object of this invention is to provide such a receiver which will not be actuated by signals other than that to which the receiver is tuned.

Another object of this invention is to provide such an apparatus in which transient or static signals will not serve to actuate the controlled apparatus.

Another object of this invention is to provide such a receiver which is operable to actuate the controlled apparatus solely when a signal of a certain carrier frequency modulated by a predetermined lower frequency is supplied to the radio receiver.

Another object of this invention is to provide in such a receiver a network having a pair of impedance elements in which the relative voltage therebetween is dependent upon the modulation frequency of the carrier signal.

Another object of this invention is to provide a control apparatus sensitive to the relative voltages set up between the two impedance elements.

Another object of this invention isto provide in such an apparatus time delay means whereby the controlled apparatus is actuated solely subsequent to a predetermined time during which the actuating signal is supplied thereto.

Other objects of this invention will be apparent from the specification, the appended claims, and the drawings in which drawings,

Figure 1 is a diagrammatic view showing a transmitter for use in connection with the invention;

Fig. 2 is a digrammatic view of a radio receiver embodying the invention; and

Fig. 3 is a diagrammatic view showing certain operating characteristics of the invention.

Referring to the drawings by characters of reference, the numeral 1 designates generally a radio transmitter energized from a suitable source of electrical energy such as a motor vehicle storage battery B1 which is connected to a transformer T1 through the vehicle ignition switch IS and a selectively controlling pushbutton SW1. A vibrator V of the usual type controls the current ilow whereby alternating current of the desired frequency and voltage is supplied by the output winding of the transformer T1 for energizing the transmitter. The transmit ter comprises a vacuum tube V2 having two triodes within a single envelope and which may be of the type readily purchased in the trade as the 6SN7. The main electrode of the left-hand triode of the valve V2 is connected across the output winding of the transformer T1 and serves as a half-wave rectifier for supplying direct current to the positive bus B2 and negative bus B3. The anode of the right-hand triode of the valve V2 is connected through the primary Winding of a transformer T2 to the bus B2. The cathode of the right hand triode of the valve V2 is connected to the bus B3 to complete the anode circuit. The secondary of the transformer T2 is connected between the cathode and control grid of the right-hand triode 0f the valve V2. The usual grid resistor and grid condenser are provided between the grid of the righthand triode of the valve and the secondary winding of transformer T2. The secondary winding of the transformer T2 is also connected between the screen grid and cathode of the valve V3. The transformer therefore acts as a feed back for the valve V2 whereby the righthand triode of the valve V2 operates to oscillate at a desired modulating frequency which may be in the order of magnitude from two and a half to five and a half kilocycles. The modulation frequency on the screen grid of the valve V3 acts to modulate the signal generated thereby.

The anode of the valve V3 is connected through a coupling condenser to the transmitting antenna A1 and also to an auto transformer T3 to the positive bus B2. It will be noted that the cathode of the valve V3 is connected to the negative bus B3 whereby the anode circuit for the valve V3 is completed. The controlling grid of the valve V3 is connected to the secondary tap of the auto transformer-T3 whereby the valve V3 is made to oscillate at the desired transmission frequency. A tuning condenser Cl is coupled across the auto transformer T3 for tuning the output frequency of the valve V3. As indicated by the reference character x, the filaments of the valves V2 and V3 are connected to one terminal of the battery B1 through the ignition switch is and by means of the ground connection to the other terminal thereof whereby closure of the switch IS will immediately ener" gize the filaments.

It will now be seen that with the ignition switch 151 closed, closure of the switch SW1 energizes the transformer Ti whereby it will supply anode potential to the valves V2 and V3. As stated above, the left-hand portion of the valve V2 serves as a rectifier for supplying the anode voltages of the right-hand portion of the valve V2 and of the valve V3. The right-hand portion of the valve V2 operates in the usual manner to provide a modulating signal for the valve V3 which is applied to the screen grid of this valve V3. The valve V3 is itself made to oscillate at the desired carrier frequency which may be between three hundred and live hundred kilocycles. The output signal of the antenna A1 is-therefore a carrier wave having a'frequency which may be between three hundred and live hundred kilocycles modulated by a lower frequency which is preferably from two and a half to five and a half kilocycles.

Referring now to Fig. 2, the numeral 10 indicates generally a radio receiver having an antenna A2, a radio frequency amplifying network 12 and an audio frequency detection and amplifying network 14 for supplying an amplified audio-frequency signal to a transformer T4. The output of the transformer T4 is fed into an output network comprising a resistor R19 and an inductance I16 and variable capacitance C19 arranged in parallel with each other and in series with the resistor Rift. The capacitor C10 and inductance Ill) are tuned so that their resonating frequency is the same frequency as the modulation frequency of the carrier Wave to which the receiver 10 is to respond. The voltage appearing across the resistor R10 is applied across a series network comprising the left-hand anode and cathode of a valve V4, and resistor R11 connected in parallel with capacitor C11. The voltage appearing across the parallelly connected inductance I14) and capacitor C11 is applied across a series network comprising the right-hand anode and cathode of the valve V4, and the resistor R12 connected in parallel with capacitor C12. The capacitors C11 and C12 which are arranged in parallel across the resistors R11 and R12 B11 through a resistor R13 while the free terminal of the resistor R12 is connected to the controlling grid of the right-hand triode of the valve V5. Furthermore the resistive values of the resistors R11 and R12 are high with respect to the value of the resistor R10 so that the magnitude of current flow through R11 and R12 is a small fractron of the magnitude of current flow through .the resistor R10 so that the networks including the valve V4 serve primarily as voltage measuring networks. Each of-the cathodes of the valve V5 is connected to ground through the usual resistor and parallelly connected capacitor. The ground potential will be somewhat above that of the negative bus B11 due to the voltage drop through the resistor R13b. Therefore the right-hand grid of the valve V5 will normally be held, in the absence of a voltage across R11 and R12, at a sufficiently negative value with respect to its res ective cathode to hold the right-hand section of the valve V5 in a cutoff or nonconducting condition.

The anode of the right-hand triode of the valve V5 is connected through the controlling winding of a relay Kr and resistor R14 to the positive bus B10. Due to the rectifying action of the valve V4 and the way in which the main electrodes are connected, the voltages appearing across the resistors R11 and R12 will tend to oppose one another. They are so polarized that the voltage appearing across the resistor R11 tends to render .the grid of the right-hand triode of the valve V5 ne ative with respect to the cathode thereof while the polarization of the resistor R12 is such that voltage a pearing thereacross tends to render the grid of the right-hand triode o; the valve V5 positive with respect to the cathode there- When the relative potentials across the resistors R11 and R12 are substantially balanced or equal the algebraic total value thereof is zero and any subsequent increase in value of the potential across the resistor R12 will cause the right-hand portion of the valve V to conduct. Conduction of the richt-hand triode of the valve V5 causes current to flow from the bus B through the resistor R14. the energizing winding of the rela Rr, the anode of the right-hand triode of the valve V5, its associated cathode, the resistance capacity network to ground, and the resistor RBI) to the negative terminal B11 of the direct current power su ply.

In order that the relay Rr will not operate until the signal has been placed thereacross for a predetermined time, a condenser C14 is arranged in parallel with the energizing winding thereof. The value of the ca acitor C14 and of the resistor R14 will of course determine the time lag between passage of current through the righthand triode of the valve V5 and energization of the relay Rr. It is believed that it will be understood that as the value of the resistor R14 or of the capacitor C14 or of both is increased,v the time delay for operation of the relay Rr will be increased.

Upon energization of the relay Rr, its contacts a will be closed completing an obvious circuit through the energizing winding of a relay Rr2 whereby its contacts a will close to complete obvious energizing circuits for the controlled apparatus Ap. It will usually be desirable to have the apparatus Ap energized for a period of time upon momentary closure of the contacts a of the relay Rr and therefore the relay Rr2 is provided with front contacts b whereby once the relay R12 is energized, the front contacts b will close completing a by-pas-s circuit about the contacts a of the relay Rr whereby subsequent de-energization of the relay Kr and the opening of the contacts a thereof will not act to tie-energize the relay Rr2. The relay Rr2 may be selectively de-energized by breaking the circuit through the front contacts I) thereof, for example, as is diagrammatically shown by the normally closed manually operable switch SW2. It will, of course, be obvious to those skilled in the art that the switch SW2 may be controlled by any suitable means such as a time-actuated switch energized concurrently with the closure of the contacts a of the relay Rr2 or may be mechanically actuated as a result of the energization of the apparatus Ap.

It is believed that the remainder of the details of construction may best be understood by a reference to the operation of the receiver which is as follows: To prepare the receiver for operation, the line switches LS1 and LS2 are closed to energize the lines L1 and L2 for energizing the power transformer T2 of the receiver. The transformer T2 has a first secondary winding for energizing the filaments of the valves V4, V5, V8, V9 and V10 as indicated by the reference character yy on the filaments and this secondary winding and the filaments of these valves are then immediately energized. A second or power secondary winding is connected through a full wave rectifying valve V10 in the usual manner to supply anode potential between the positive and negative busses B10 and B11 respectively, whereby these busses are also immediately energized to place the receiver in operation to receive radio signals. It is to be assumed that the transmitter to which the receiver of Fig. 2 is responsive has been set to generate a radio frequency signal of, for example, 400 kilocycles modulated by a frequency of 4 kilocycles. Upon operation of the transmitter this signal is picked up by the antenna A2 and is supplied across the primary winding of the antenna input transformer T5. The secondary winding of this transformer T5 has parallelly connected therewith a variable capacitor C25 and this parallel network is adjusted to resonate at a frequency of 400 kilocycles. The secondary winding of the transformer T5 is connected between the controlling grid and cathode of the radio frequency amplifying valve V8 of the radio frequency amplifying network 12. The anode and cathode of the valve VS are connected across the primary winding of the output transformer T6 of the radio frequency amplifying network so that the input signal of the antenna A2 is fed through the valve V8, amplified therein, and fed across the primary winding of the transformer T6.

The signal appearing across the secondary winding of the transformer T6 is applied between the grid and cathode of the left-hand triode of a valve V9 which serves as a detector. The grid of the detector triode is normally biased to maintain the detector nonconductive and is rendered positive in the usual manner during the positive half cycles of the output of the transformer T6 whereby a pulsating direct current signal is fed therefrom to the right-hand triode of the valve V9 which serves in the usual manner to amplify the rectified output of the detector. The output of the right-hand triode of the valve V9 is capacitively coupled to the grid of the lefthand triode of the valve V5 through the coupling con denser C26. The detected signal is then again amplified by the left-hand triode of the valve V5 and fed to the primary winding of the aforementioned transformer T4.

As stated above the resonant network comprising the capacitor C10 and inductance is tuned to 4 kilocycles so that, under the presently assumed conditions, the amplitude of the voltage thercacross and that of the voltage across resistor R12 are at limit values, which in this case, are maximum values. As will be explained in more detail later, under the assumed conditions, the voltage across the resistor R10 will also be at a limit value, in this case a minimum, so that the algebraic sum of the voltage drops across the resistors R11 and R12 will be suflicient to render the grid of the right-hand triode of the valve V5 in condition to permit conduction of this triode whereby arise; ant

current is supplied to the energizing winding of the relay Rr.

It will be understood that this application, the term limit value is employed in its usual sense as connoting either maximum or minimum conditions, and thus as describing the amplitude of a voltage when it reaches its most positive or its least positive or its most negative or its least negative value. After this current has been fed to the energizing winding of the relay Rr a sufficient time as determined by the magnitudes of the resistor R14 and capacitor C14,. the relay Rr will be actuated to close its contacts a which immediately energize the relay RrZ through an obvious energizing circuit causing the contacts a and 1; thereof to close. Closure of the contacts b of the relay Rid. completes an obvious holding circuit for the relay Rr2 while closure of its contacts a completes an obvious energizing circuit for the apparatus A Referring now more specifically to the operation of the selector network, it will be obvious that this network comprises essentially a resistor R arranged in series with parallelly connected capacitor C10 and inductance I10 and supplied by a voltage across the secondary winding of the transformer T 4'. As stated above the capacitor C10 and inductor 110 are tuned so that they are resonant at a frequency corresponding to the modulating audio frequency received by the antenna A2 which in this instance was assumed to be 4 kilocycles. It will be evident to those skilled in the art that at other frequencies the voltage drop across the parallelly connected capacitor C10 and inductor 110 will be very low since the inherent resistance thereof is very low and substantially all of the voltage drop will appear across the resistor R10.

Referring now to Fig. 3, the ordinate represents voltage while the abscissa represents the frequency of the moduiating audio-frequency signal. The line E11 represents generally the plot of the voltage appearing across the resistors RM) and R11 when plotted against frequency. it will be noted that as the frequency approaches the resonant frequency of the circluit, the voltage across the resistors Rltl and R11 drops in negative value and as the frequency then increases beyond resonance, it again increases in negative value in the usual form of a resonant curve. The line E12 represents generally the voltage appearing across the resonant network including the capacitor C10 and inductor I10 and also the direct voltage appearing across resistor R12 when plotted against frequency and it will be noted that it takes the shape of the usual peaked resonant curve. The line 1313b indicates the negative bias imparted by the resistor R131) to lower the potential of the bus B11 below ground potential Egg. The line Eg-c designates generally the algebraic sum of the voltages appearing across the resistors R11, R12, and R1311. The value of the resistor R13b has been so chosen that it will produce a voltage drop thereacross substantially equal to the negative cutoff voltage Eco of the valve V5. Referring to Fig. 3 at all values of Eg-c above the line E132; the righthand portion of the valve V5 will conduct and at values therebelow it will remain nonconductive. it will be evident that for all frequencies below 3 kilocycles the volt-age represented by the line Eg-c is below the cutoff voltage whereby the right-hand triode of the valve V5 will be maintained blocked. At about the time the line Eg-c crosses the line E1312 which may be at the point A3, the voltage at the grid of the right-hand triode of the valve V5, while still negative with respect to the respective cathode, is sufiiciently small to permit the valve V5 to conduct so that the lower frequency limit of conduction of the valve V5, with the assumed values of the resistor R10, will be 3 kilocycles. Similarly the line Eg-c will cross the line E13b at an upper frequency above the 4000 cycle frequency at a point indicated as A4. It will be evident from reference to Fig. 3 that for all frequencies above 5 kilocycles the value of. the voltage represented by the line E11 exceeds that represented by the line E12 and the right-hand triode of the valve V5 will be maintained nonconductive.

Summarizing it will now be seen that with the present values of the resistor R10, the right-hand triode of the valve V5 will conduct at all frequencies above 3 kilocycles and below 5 kilocycles but will not conduct at frequencies below 3 or above 5 kilocycles per second. Therefore unless the input signal is within this range, the relay Rr will not be operated. It will be apparent that the sharpness of the curves E11 and E12 will be dependent to a great extent upon the Q of the electrical selection circuit. For a higher Q, the curve will become narrower and more peaked and conversely for a lower Q of the circuit, the curves will become wider. It is desired to maintain the value of the resistor R10 so that the Q of the circuit is quite high but yet is sufficiently low to provide a sufficient range of operating frequencies to take into account any slight inaccuracies in the sending characteristics of the transmitter or any slight changes in frequency of the receiver resonating circuit which might result from manufacturing tolerances.

It is believed that it will be evident from the foregoing that this invention provides a selection circuit whereby the controlled circuit will not be actuated by extraneous signals such as might be received by the receiver due to static or other local interference but will be easily actuated upon continued sending of the signal to which. the receiver is tuned.

What is claimed and is desired to be secured by United States Letters Patent is as follows:

l. In apparatus of the character described, means responsive to a radio carrier wave modulated by a constanofrequency audio-frequency signal, means for detecting said wave and having an output audio-frequency signal the frequency of which is a function of the modulation frequency and the average amplitude of which is subject to variation, a network, means including audiofrequency coupling means connecting said network to said detecting means, said network comprising a resistor in series with a parallelly arranged capacitor and inductor, the combination of said capacitor and said inductor resonating at a preselected frequency, said network having three terminals, said resistor and said capacitor and said inductor having a common connection connected to a first of said network terminals, said inductor and said capacitor having a common connection separate from said first-named common connection connected to a second of said network terminals, said resistor having a second connection separated from said first-named common connection by at least a portion of the resistance thereof and connected to a third of said network terminals, a second and a third resistor each having a pair of terminals, a first and a second rectifier each having a pair of terminals, conductor means connecting one of said first rectifier terminals to one of said network terminals and connecting the other of said first rectifier terminals to one of said second resistor terminals, conductor means connecting the other of said second resistor terminals to another of said network terminals and to one of said third resistor ter minals, a conductor means connecting still another of said network terminals to one of the terminals of said second rectifier, conductor means connecting the other of the terminals of said second rectifier to the other of said third resistor terminals, an electric valve having a pair of main electrodes and a control electrode, a load device, circuit means including said main electrodes and said load device, and conductor means connecting one of said resistor terminals to said control electrode and another of said resistor terminals to one of said main electrodes, said electric valve actuating said load device if and only if the frequency of said audio-frequency signal is said preselected frequency.

2. In an apparatus of the character described, means responsive to a radio carrier wave modulated by a constant-frequency audio-frequency signal, means for detecting said wave and having an output audio-frequency signal the frequency of which is a function of the modulation frequency and the average amplitude of which is subject to variation, a network, means including audiofrequency coupling means connecting said network to said detecting means, said network comprising a resistor in series with a parallelly arranged capacitor and an inductor, the combination of said capacitor and said inductor resonating at a preselected frequency, said network having three terminals, said resistor and said capacitor and said inductor having a common connection connected to a first of said network terminals, said inductor and said capacitor having a common connection separate from said first-named common connection connected to a second of said network terminals, said resistor having a second connection separated from said first-named common connection by at least a portion of the resistance thereof and connected to a third of said network terminals, a second and a third resistor each having a pair of terminals, a first and a second rectifier each having a pair of terminals, conductor means connecting one of said first rectifier terminals to said network second terminal and connecting the other of said first rectifier terminals to one of said second resistor terminals, conductor means connecting the other of said second resistor terminals to said network first terminal and to one of said third resistor terminals, conductor means connecting said network third terminal to one of the terminals of said second rectifier, conductor means connecting the other of the terminals of said second rectifier to the other of said third resistor terminals, an electric valve having a pair of main electrodes and a control electrode, a load device, circuit means including said main electrodes and said load device, and conductor means connecting one of said resistor terminals to said control electrode and another of said resistor erminals to one of said main electrodes, said electric valve actuating said load device if and only if the frequency of said audio-frequency signal is said preselected frequency.

3. In an apparatus of the character described, means responsive to a radio carrier wave modulated by a constantfrequency audio-frequency signal, means for detecting said wave and having an output audio-frequency signal the frequency of which is a function of the modulation frequency and the average amplitude of which is subject to variation, a network, means including audio-frequency coupling means connecting said network to said detecting means, said network comprising a resistor in series with a parallelly arranged capacitor and an inductor, the combination of said capacitor and said inductor resonating at a preselected frequency, said network having three terminals, said resistor and said capacitor and said inductor having a common connection connected to a first of said network terminals, said inductor and said capacitor having a common connection separate for said firstnamed common connection connected to a second of said network terminals, said resistor having a second connection separated from said first-named common connection by at least a portion of the resistance thereof and connected to a third of said network terminals, a second and a third resistor each having a pair of terminals, a first and a second rectifier each having an anode and a cathode terminal, conductor means connecting said first rectifier anode terminal to said network second terminal and connecting the said first rectifier cathode terminal to one of said second resistor terminals, conductor means connecting the other of said second resistor terminals to said network first terminal and to one of said third resistor terminals, conductor means connecting said network third terminal to said second rectifier anode terminal, conductor means connecting said second rectifier cathode terminal to the other of said third resistor ter minals, an electric valve having a cathode and an anode and a control electrode, a load device, circuit means including said valve anode said valve cathode and said load device, and conductor means connecting said first rectifier cathode terminal to said control electrode and said second rectifier cathode terminal to said valve cathode, said electric valve actuating said load device if and only if the frequency of said audio-frequency signal is said preselected frequency.

4. in a control system, a radio receiver for receiving intermittently transmitted radio-frequency signals generated by modulating a carrier with a modulation signal having a constant frequency, the frequency of said modulation signal being any one of a plurality of audio frequencies including a preselected audio frequency; said receiver comprising detecting means for deriving from the received audio-frequency signal the constant-frequency modulation signal, the average amplitude of the derived signal being subject to variation; an audio-frequency transformer having a primary winding and a secondary winding; means for applying said derived signal across said primary winding whereby an audio-frequency signal of constant frequency but of variable average amplitude is developed across said secondary winding; a first and a second load resistor; reactive means resonant at said preselected frequency; a first rectifier; means including said reactive means, said first rectifier and connecting means connecting said reactive means, said first rectifier and said first load resistor in circuit with one another and with said secondary winding for developing across said first load resistor a first direct voltage the magnitude of which varies in accordance with the variations of the average amplitude of said audio-frequency signal and the magnitude of which attains a limit value when said audio-frequency signal is of said preselected frequency; resistive means; a second rectifier; means including said resistive means, said second rectifier and connecting means for connecting said resistive means, said second rectifier and said second load resistor in circuit with one another and with said secondary winding for developing across said second load resistor a second direct voltage the magnitude of which varies in accordance with the variations of the average amplitude of said audio-frequency signal; an amplifying device having an input circuit; a relay controlled by said amplifying device; a source of fixed biasing potential; and means connecting said first and said second load resistors in series with said source of fixed biasing potential in said input circuit, said rectifying device being connected so that said first and said second direct voltages are in opposition to one another so that said amplifying device is responsive to said fixed biasing potential and to the difference between said first and said second direct voltages, said amplifying device being effective to change the condition of said relay if and only if said first direct voltage has attained its said limit value.

5. The combination of claim 4, in which said reactive means and said resistive means are connected in series with one another directly across said secondary winding and in which there is a direct connection from between said resistive means and said reactive means to between said first and second load resistors.

6. The combination of claim 4, in which there is a first series circuit including said first load resistor and said first rectifier connected directly across said secondary winding and in which there is a second series circuit ineluding said second load resistor and said second rectifier connected directly across said secondary winding.

7. The combination of claim 6, in which said reactive means and said resistive means are connected in series with one another directly across said secondary winding and in which there is a direct connection from between said resistive means and said reactive means to between said first and second load resistors.

8. In a control system, a radio receiver for receiving intermittently transmitted radio-frequency signals generated 'by modulating a carrier with a modulation signal having a constant frequency, the frequency of said modulation signal being any one of a plurality of audio frequencies including a preselected audio frequency; said receiver comprising detecting means for deriving from the received audio-frequency signal the constant-frequency modulation signal, the average amplitude of the derived signal being subject to variation; an audiodrequency transformer having a primary winding and secondary winding; means for applying said derived signal across said primary winding whereby an audio-frequency signal of constant frequency but of variable average amplitude is developed across said secondary Winding; a first and second load resistor; circuit means for developing across said first load resistor a first direct voltage the magnitude of which varies in accordance with the variations of the average amplitude of said audiofrequency signal and the magnitude of which attains a limit value when said audio-frequency signal is of said preselected frequency and for developing across said second load resistor a second direct voltage the magnitude of which varies in accordance with the variations of the average amplitude of said audio-frequency signal; said circuit means comprising reactive means resonant at said preselected frequency, resistive means, a pair of rectifiers, means for connecting one of said load resistors, one of said rectifiers and said resistive means in series with one another directly across said secondary winding, and means for connectingthe other of said load resistors, the other of said rectifiers and said reactive means in series with one another directly across said secondary winding; an amplifying device having an input circuit; a relay controlled by said amplifying device; a source of fixed biasing potential; and means connecting said first and said second load resistors in series with said source of fixed biasing potential in said input circuit, said rectifiers being connected so that said first and said second direct voltages are in opposition to one another so that said amplifying device is responsive to said fixed biasing potential and to the difference between said first and said second direct voltages, said amplifying device being eitective to change the condition of said relay if and only if said first direct voltage has attained its said limit value.

9. The combination of claim 8 in which said reactive means is connected across the series combination of said one rectifier and said first load device, in which said resistive means is connected directly across the combination of said other rectifier and said second load device and in which said reactive means and said resistive means are connected in series with one another directly across said secondary winding.

References Cited in the file of this patent UNITED STATES PATENTS 2,299,581 Korman Oct. 20, 1942 2,323,609 Kihn July 6, 1943 2,333,992 Fox Nov. 9, 1943 2,428,264 Crosby Sept. 30, 1947 2,457,730 Roberts Dec. 28, 1948 2,611,031 Appert Sept. 16, 1952 FOREIGN PATENTS 120,736 Australia Dec. 13, 1945

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