GB2148559A - Power supply for television system - Google Patents

Abstract

A television system for developing a regulated B + scan supply voltage includes a deflection generator (40) with a deflection winding (41); a transformer (22) having a primary winding (22a) coupled to a source (21) of unregulated alternating input voltage for developing across secondary windings (22c, d) an alternating voltage that tends to vary with variations of the unregulated voltage; means (27, 28) responsive to the secondary winding alternating voltage for developing a B + scan supply voltage; and means (32, 39) for applying the B + scan supply voltage to the deflection generator (40) to develop scanning current in the deflection winding. A saturable core (137) with a load coil (37) wound on it is conductively coupled to and magnetically isolated from the transformer secondary windings (22c, d). The load coil (37) is responsive to, and provides regulation of, the transformer secondary winding alternating voltage for developing the regulated B + scan supply voltage. <IMAGE>

Description

1 GB 2 148 559A 1

SPECIFICATION

Power supply for television system This invention relates to power supplies for 70 television systems and can be embodied with particular advantage in ferroresonant televi sion power supplies. It is divided from our application no. 8137753.

Ferroresonant transformers are known which provide regulated ultor voltages and regulated B + scanning voltages for television receivers. A television receiver ferroresonant power supply is described in a U.S. Patent application of F. S. Wendt, Serial No. 007815, filed January 30, 1979, entitled "HIGH FREQUENCY FERRORESONANT POWER SUPPLY FOR A DEFLECTION AND HIGH VOLTAGE CIRCUIT", which corre- sponds to British Patent Application No. 2041668A, published September 10, 1980. When operated at a relatively high input frequency, such as at a horizontal deflection frequency of 15.75 KHz, a ferroresonant transformer is a relatively compact and low weights unit which provides inherent output voltage regulation without the necessity of relatively complex and expensive electronic regulator control circuitry.

To provide high efficiency at 16 KHz, the magnetizable core of a ferroresonant transformer may be formed from a ferrite such as a commercially available manganese-zinc or nickel-zinc ferrite. Such ferrite materials ex- hibit a high resistance to current flow, thereby 100 incurring relatively small eddy current losses, which otherwise would be excessive at the relatively high 16 KHz operating frequency. Hysteresis losses are also relatively low. Even when using a ferrite core, however, 1113 losses 105 in one or more of the windings, eddy current losses, and hysteresis losses may produce a substantial rise in the core temperature.

The saturation flux density, Bsat. of a mag- netizeable material decreases with increasing core temperature. For manganese-zinc ferrites, the saturation flux density may decrease from about 4.5 kilogauss at 20C to 2.5 kilogauss at 1 50C. Since the output voltage of a ferroresonant transformer depends on the Bsat value of the core material under the output windings, a rise in core operating temperature results in an undesirable decrease in the output voltage. If for example, the output voltage is an ultor high voltage, the ultor voltage developed immediately after the television receiver is turned on, while the ferroresonant transformer core is at ambient temperature, is greater than the ultor voltage developed dur- ing subsequent steady-state temperature operation after the core has heated to its normal above-ambient operating temperature.

Heat sinking of the core to reduce temperature rise is relatively difficult in a high fre- quency television receiver ferroresonant trans130 former. The output windings of the ferroreso nant transformer, including the high voltage windings which has a relatively large number of turns, are wound around the saturating core portion of the transformer and tightly coupled magnetically one to another. The multiple output windings and the large num ber of high voltage windings turns restrict access to the core for heat sinking purposes.

According to the present invention, in a television system comprising a deflection gen erator including a deflection winding and a transformer having primary and secondary windings of which the primary windings is coupled to a source of unregulated alternating input voltage for developing across the secondary winding an alternating polarity voltage that tends to vary with variations of said unregulated voltage, a B + scan supply vol- tage is developed from said secondary winding alternating polarity voltage and is applied to said deflection generator to develop scanning current in the deflection windings: a load coil wound on a saturable core and conduc- tively coupled to and magnetically isolated from the transformer secondary winding, is responsive to and provides regulation of the transformer secondary winding alternating polarity voltage for developing a regulated B + scan supply voltage.

In the Drawing:

Figure 1 illustrates a ferroresonant television power supply embodying the invention; and Figures 2 and 3 illustrate waveforms associated with the operation of the circuit of Fig. 1.

In Fig. 1, a ferroresonant television power supply 10 comprises a transformer 22 and a ferroresonant saturable reactor load circuit 20. A primary winding 22a of transformer 22 is coupled to a source 11 of unregulated alternating input voltage Vin comprising an inverter 21 and a DC input terminal 23 coupled to a center tap of primary winding 22a. An unregulated DC voltage V. is applied to termi- nal 23. Inverter 21 is operated at a high frequency of, for example, the 15.75 KHz horizontal deflection frequency. Inverter 21 develops the alternating input voltage Vin as a horizontal rate square-wave voltage across pri- mary winding 22a.

When the voltage Vin is applied to primary winding 22a, horizontal rate alternating polarity output voltages are developed across secondary output windings 22b-22d and a high voltage secondary winding 22e. End leads 49 and 50 of output winding 22b are connected, respectively, to diodes 29 and 30, which act as a full wave rectifier; end leads 48 and 51 of output winding 22c are coupled, respec- tively, to diodes 27 and 28, which act as a full wave rectifier; and end leads 47 and 52 of output winding 22d are coupled, respectively, to diodes 25 and 26, which act as a full wave rectifier. A common center tap lead 53 is coupled to ground.

2 GB2148559A 2 The alternating polarity output voltage developed across winding 22b is full-wave rectified by diodes 29 and 30, and filtered by a capacitor 34 to develop a DC supply voltage at a terminal 31, of illustratively + 25 volts, to energize such television receiver circuits as the vertical deflection circuit and the audio circuit. The alternating polarity output voltage developed across winding 22d is full-wave rectified by diodes 25 and 26 and filtered by a capacitor 36 to develop a DC supply voltage at a terminal 33, of illustratively + 210 volts, to power such circuits as the picture tube driver.

The alternating polarity output voltage developed across winding 22c is full-wave rectified by diodes 27 and 28 and filtered by a capacitor 35 to develop at a terminal 32 a B + scan supply voltage for a horizontal deflection winding 41. To generate horizontal scanning or deflection current in horizontal deflection winding 41, a horizontal deflection generator 40 is coupled to terminal 32 through an input choke 39. Horizontal deflection generator 40 is energized by the B + scan supply voltage and comprises a horizontal oscillator and driver 43, a horizontal output transistor 44, a damper diode 45, a horizontal retrace capacitor 46 and an S-shaping or trace capacitor 42 coupled in series with horizontal deflection winding 41 across horizontal output transistor 44.

The alternating polarity output voltage developed across high voltage secondary winding 22e is coupled to a high voltage circuit 24 to develop a DC ultor high voltage or accelerating potential at a terminal U for the television receiver picture tube, not illustrated. High voltage circuit 24 may comprise a con-_ ventional voltage multiplier circuit of the Cockroft-Walton type, or may comprise a half-wave rectifier arrangement with a plurality of diodes integrally molded as a single unit with a plurality of winding sections, not individually illustrated, of high voltage secondary winding 22e.

Secondary output windings 22b-22d and high voltage secondary winding 22e are closely or tightly coupled magnetically one to another. To achieve the tight coupling, the windings may be wound concentrically around a common portion of the magnetizable core 122 of transformer 22. Because of the tight magnetic coupling among the windings, the alternating polarity output voltages developed across the secondary output windings are all of common waveshape, with little departure being introduced by the relatively small leakage inductances existing between the output windings.

To regulate the secondary output winding voltages against variations in the amplitude of the input voltage Vin and against loading changes by the load circuits coupled to termi nals 31-33 and beam current loading 130 changes on ultor terminal U, the ferroresonant saturable reactor load circuit 20 is coupled across one of the tightly or closely coupled secondary output windings of transformer 22.

In Fig. 1, the saturable reactor circuit 20 is illustratively coupled across secondary output winding 22d.

Ferroresonant saturable reactor load circuit 20 comprises a reactor coil or winding 37 wound around at least a portion of a saturating, magnetizable core 137 and comprises a resonating capacitor 38 coupled across reactor winding 37. Saturable reactor core 137 may be of a conventional toroidal or two- window rectangular core design.

In a ferroresonant circuit such as the ferroresonant saturable reactor load circuit 20 of Fig. 1, the voltage Vout across the saturating coil 37 is regulated. By coupling ferroresonant saturable reactor circuit 20 across the secondary output winding 22d of transformer 22, circuit 20 acts as a regulating load circuit coupled to winding 22d to maintain the voltage across winding 22d at the regulated voltage Vout. With the voltage across secondary winding 22d regulated by the ferroresonant load circuit 20, the output voltages across all the other secondary windings which are tightly coupled with winding 22d are also regulated. Thus, the output voltages across windings 22b and 22c and the high voltage output winding 22e are regulated by the regulating action on the voltage Vout of ferroresonant circuit 20.

Transformer 22 has substantial leakage inductance between primary winding 22a and each of the tightly coupled regulated secondary windings 22b-22e. The loose coupling of the primary winding with the secondary output windings enables the output voltage to be maintained substantially constant by ferroresonant circuit 20 even though the applied voltage across primary winding 22a may change with variations in the alternating input voltage Vin. Leakage inductance between primary winding 22a and each of the secondary windings 22b-22e may be designed into transformer 22 by constructing the magnetizeable core 122 of the transformer as a closed loop core of rectangular shape. Primary winding 22 may be wound on one leg of core 122 and the secondary windings 22b-22e may be concentrically wound on an opposing leg.

When considering the equivalent electrical circuit of transformer 22, the load circuits couple to terminals 31 -33 and to ultor terminal U are reflected to the primary'winding as load impedances in parallel with the reflection of the ferroresonant load circuit 20. Because of the loose magnetic coupling between the primary winding 22a and secondary windings 22b-22e, the reflected ferroresonant load circuit and the other parallel loads see an equivalent impedance in series with the source 11 of alternating input voltage Vin. This equiva- 3 lent impedance produced by the loose magnetic coupling of transformer 22 absorbs the variations in input voltage while enabling the ferroresonant load circuit and output winding voltage amplitude variations to be substantially reduced in comparison to the voltage amplitude variations of the primary winding.

Fig. 2a illustrates the square-wave alternating polarity input voltage Vin developed by source 11 across primary winding 22a of transformer 22. Illustrated in Fig. 2b is the regulated voltage Vout developed across ferroresonant saturable reactor load circuit 20 and secondary output winding 22d of transformer 22. The regulated voltage Vout is an alternating polarity voltage of the same frequency as the input Vin with generally flattened portions 14 alternating in polarity and connected by generally sinusoidal portions 15.

Within the flattened portion intervals of the regulated output voltage Vout, such as between times to-tl of Fig. 2b, the saturable reactor magnetizable core portion associated with coil 37 is being operated in the magneti- cally unsaturated region of the core material B-H loop characteristic. Saturable reactor coil or winding 37 exhibits a relatively large inductance during the flattened portion or unsaturated intervals. Relatively little current i,, flows in the saturable reactor winding 37 as illustrated in the solid-line waveform i,, of Fig. 2b between times to-t, With saturable reactor winding 37 exhibiting a relatively high impedance during the flattened portions or magnetically unsaturated intervals of the output voltage waveform Vout, the resonating capacitor 38 is discharged very little into the saturable reactor coil, and the capacitor maintains a relatively constant out- put voltage Vout applied across the coil terminals, as illustrated by the relatively small capacitor current ic, the dashed-line waveform of Fig. 2b, between times t,,-t,.

The output voltage Vout when applied by capacitor 38 across reactor winding 37 produces a flux buildup in the core 137 until substantial magnetic saturation of the core occurs near time t, When core 137 magnetically saturates near time t, the inductance of reactor coil 37 decreases substantially. The inductance of coil 37 may be, illustratively, 20 to 60 times less than the unsaturated inductance of the coil.

After core 137 becomes magnetically satu- rated, capacitor 38 and reactor coil 37 undergo a half cycle of resonant current oscilla- tion, as indicated in Fig. 2b by the current pulse 12 of the coil current 'sr and as indicated by the current pulse in the capacitor current i., between times tl-t4. The resonant or 125 circulating current in saturable reactor coil 37 and in capacitor 38 reaches maximum magni tude at time t, The output voltage Vout reverses polarity, also at this time.

Near time 14, resonant current pulse 12 has130 GB 2 148 559A 3 decreased sufficiently to enable core 137 to come out of saturation, enabling the reactor coil 37 to reexhibit a high impedance. The voltage across capacitor 38, that is, the regu- lated output voltage Vout, stops its rapid change and assumes the opposite polarity flattened portion values. During the opposite polarity flattened portion interval t4-t., the core 137 is again operated in the magnetically unsaturated region of its B-H loop characteristic. The flux in core 137 reverses direction during this interval and builds up, substantially, to its saturation flux magnitude near time t, when the core again magnetically saturates. The current in the reactor winding 37 then undergoes another half cycle of oscillation between times t,-t6' Ferroresonant saturable reactor load circuit 20 functions as a magnetic voltage regulator to maintain a relatively constant amplitude output voltage Vout under varying input voltage conditions, and under varying loading conditions on the various secondary output windings such as under varying beam current loading of ultor terminal U. With a sufficiently large value for capacitor 38, the AC component of the flattened portions of the output voltage Vout is relatively small. The area under a flattened portion of the voltage waveform Vout equals the time integration of the output voltage Vout over the flattened portion interval, or equivalently represents the maximum change in flux linkage of reactor coil 37.

The maximum flux linkage of coil 37 is proportional to the saturation flux density Bsat of the magnetizable material of reactor core 137. Since the maximum flux linkage of reactor coil 37 is substantially a constant amount independent of input voltage variations, the area under the flattened portion of the output voltage Vout is also a constant independent of input voltage variations. Thus, the amplitude of the output voltage Vout will be regulated and of substantially unchanged value provided that the duration of the flattened portion of output voltage Vout during which the reactor core 137 is unsaturated remains relatively fixed.

The period of the alternating polarity output voltage Vout is that of the input voltage Vin and is of a fixed duration. Also the duration, within this period, of the magnetically saturated intervals tl-t, and t,-t, is fixed by the value of the inductance of coil 37 near or at saturation and by the value of capacitor 38. The duration of the unsaturated portions of the output voltage Vout is therefore also fixed, thereby enabling the output voltage to assume a relatively constant amplitude.

With the secondary output winding 22d of transformer 22 coupled across the ferroresonant saturable reactor load circuit 20, the voltage across output winding 22d is constrained to assume the regulated output voltage Vout even though the input voltage Vin 4 GB 2 148 559A 4 may vary in amplitude. All the other secondary output windings 22b, and high voltage winding 22e are similarly constrained to assume regulated voltages. Varying the input voltage and loading of the output windings varies the shift in phase of the alternating output voltage Vout relative to the phase of the alternating input voltage Vin while maintaining the amplitude of the output voltage Vout relatively unchanged.

As illustrated in Figs. 2a and 2b, at an operating condition of nominal input voltage and of average loading on output windings 22b-22e, e.g., at approximately 1/2 milliampere beam current loading, the output voltage Vout is phase delayed by an amount At relative to the phase of the input voltage. The phase delay At occurs because of the power dissipation in the load circuits coupled to secondary output windings 22b-22e. The phase delay between Vin and Vout enables power to be transferred from source 11 to the secondary output winding load during each cycle of the input or output voltage oscillation.

As illustrated in Figs. 3a and 3b, when the input voltage Vin varies from a high-line input voltage level to a low-line -input voltage level, the phase delay of the output voltage Vout increases from a delay of At, to a delay of At,. The increase in phase delay at the lower input viltage level occurs because a greater phase delay is required at the lower input voltage level to transfer the same average power to the secondary winding loads. Although the phase delay of the output voltage Vout has increased at the lower input voltage level, the amplitude of Vout and the half cycle average voltage has not significantly changed, thereby providing the required regulation against input voltage variations.

As illustrated in Figs. 3c and 3d, when beam current loading of ultor terminal U increases from zero to 1.7 milliamperes, the phase delay of the output voltage Vout in- creases from a phase delay of At. to a delay of At,, at, for example, the same, nominal input voltage level. The increase in phase delay occurs because a greater phase delay is required to transfer more average power at the greater secondary winding loading condition. Although the phase delay of the output voltage Vout has increased, the amplitude of Vout in Fig. 3d and the half cycle average voltage has not significantly changed, thereby providing the required regulation against load- 120 ing variations.

A feature of the invention is to provide regulated output voltages across transformer secondary windings without requiring the core portion of the transformer associated with the secondary windings to magnetically saturate. Thus, the power transformer that is coupled to the AC voltage source, such as transformer 22 of Fig. 1, does not have the design constraints imposed upon it that a ferroresonant transformer has. In contrast to the use of a ferroresonant transformer, the portion of the transformer magnetizable core 122 associated with or under the transformer secondary out- put windings 22b-22d may be operated in the linear region of the core material B-H loop characteristic. The core remains substan tially unsaturated magnetically during the en tire alternating polarity output voltage cycle.

Several advantages accrue by using the inventive arrangement of Fig. 1 wherein a power transformer provides regulated output voltages across secondary output windings but the core of the transformer, nonetheless, is operated in the linear region of its B-H loop characteristic, and wherein the regulation is achieved by a separate ferroresonant saturable reactor circuit coupled as a regulating load across one of the power transformer output winding. For example, in a ferroresonant transformer arrangement, unlike in the arrangement of Fig. 1, a relatively high circulating or resonant current flows in one of the ferroresonant transformer output windings. To reduce IIR losses in that winding, a relatively thick or large cross-section conductor wire is used. Such a thick conductor wire interferes with tight coupling, so that leakage induc tance is higher than desired.

In contrast, no large circulating or resonant current flows in any of the output secondary windings of power transformer 22 of Fig. 1 As illustrated in Fig. 2c, for example, the current iw flowing out of output winding 22d to ferroresonant load circuit 20 is of relatively small amplitude with a peak magnitude, illustratively, ten or more times smaller than the peak magnitude of the resonant current pulse 12 flowing in reactor coil 37. Only enough current iw on average need flow out of transformer 22d to replenish the losses incurred during each cycle of the alternating polarity regulated output voltage Vout. Losses include hysteresis and eddy current heating of the reactor magnetizable core 137, 12 R losses in the reactor coil 37. Losses also include energy losses sustained by capacitor 38 during each cycle of the output voltage Vout that occur due to load current flowing out of terminal 33 and due to load current flowing to the load circuits coupled to terminals 31 and 32 and ultor terminal U as reflected into output winding 22d.

Another advantage of the arrangement of Fig. 1 is the greater design flexibility provided in selecting the parameters of the ferroresonant load circuit 20 of the power supply system 10 without requiring redesign of the power transformer portion 22 of the system.

Because secondary output winding 22d of transformer 22 is magnetically isolated from saturable reactor coil 37 and magnetizable core 137, that is, because the magnetic flux flowing in reactor core 137 does not link the transformer output winding 22d, design GB2148559A 5 changes in the magnetizable core 137 and in the values of the resonant or circulating cur rent provided by capacitor 38 do not require any substantial design changes to transformer 22, provided the changes in ferroresonant load circuit 201 do not significantly degrade the regulation bf output voltage Vout.

The amplitude of the output voltage Vout produced by ferroresonant load circuit 20 is related to the saturation flux density Bsat characteristic of the magnetizable material of reactor core 137. To reduce eddy current losses in core 137 when operating at a rela tively high frequency of 16 KHz or more, a core material with a relatively high resistance to eddy current flow is selected. Commercially available core materials that may be used for saturable reactor core 137 are, for example, manganese-zinc ferrites, nickel-zinc ferrites, or lithium ferrites. Manufacturing process toler ance in the production of the ferrite core material may result in relatively large toler ances in the value of the material Bsat.

To take into account the Bsat tolerance from core unit to core unit, the number of conductor turns of reactor coil 37 wound around core 137 may be varied for each core unit in order to maintain the output voltage Vout unchanged from unit to unit. Since regu lated output voltages for most of the television 95 receiver circuits are obtained across output windings from a separate transformer, the tolerances in Bsat of core 137 and the varia tions in conductor turn number for coil 37 to compensate therefor do not require corre- 100 sponding changes in the number of turns or other parameters of transformer 22.

The value of Bsat of the magnetizable ma terial of reactor core 137 is a function of the operating temperature of the core, with the value of Bsat decreasing as the operating temperature increases. Core 137 heats up after initial television receiver turn-on because of hysteresis and eddy current losses sus tained during operation, and because of the heating due to 12 R losses, of the conductor wire of coil 37 wound around reactor core 137. Prior to energization of power supply 10, the temperature of saturable reactor core 137 is the ambient temperature. After energi- 115 zation of the power supply, the core 137 heats to some steady-state temperature value above ambient. During the time interval when the core is heating, the Bsat of the core decreases.

Thus, the output voltage Vout of the ferroreso nant regulating load circuit 20 decreases from its initial value at turn-on of the television receiver to a lower steady-state value when the final operating temperature of core 137 is 125 reached.

To minimize the temperature change from start-up to steady-state temperature operation, the saturable reactor coil 37 and core 137 may be heat sinked in a conventional manner130 to a cooling plate or to the television receiver metal chassis. Heat sinking of the saturable reactor core 137 of the inventive arrangement of Fig. 1, where only one or a small number of coils are wound around the reactor core 137, is a relatively less difficult pfocedure than heat sinking of the saturating core portion of a ferroresonant transformer which provides multiple output voltages across multiple output windings wound around the saturating core portion of the ferroresonant transformer. Furthermore, it is even more difficult to heat sink a ferroresonant transformer having a high voltage winding because the large number of turns wound around the saturating core portion of the transformer blocks access to the core portion.

In Fig. 1, heat sinking of the core 122 of power transformer 22 is not required since the power transformer core material is operated in the linear region of its B-H loop characteristic and thus incurs relatively little core loss and little operating temperature rise. Furthermore, no circulating or resonant cur- rent flows in any of the output windings of transformer 22. 12 R losses in the transformer output windings and heating of the transformer core 122 therefrom are relatively insignificant.

In an illustrative embodiment of power transformer 22, the primary winding inductance, LP, as measured from the center tap terminal to an end terminal, is 2.03 millihenries; the secondary inductance Ls of secondary winding 22d is 10.3 millihenries; and the mutual inductance, M, between the above two described windings is 3.35 millihenries. The core material may be a manganese-zinc ferrite, and the transformer core geometry may be of any suitable arrangement that will provide the above inductance values while maintaining the core magnetically unsaturated.

In an illustrative embodiment of ferroreso- nant load circuit 20, the valve of capacitor 38 may be 0.033. microfarad; the core material saturation flux density, cross-sectional area and number of turns may then be selected so as to produce a vout waveform similar to that of Fig. 2b, during the unsaturated intervals to-t, and t4- t., with the value of unsaturated inductance of coil 37 being relatively large, on the order of one henry. The number of turns, the core geometry, such as mean mag- netic path length and cross-sectional area, and the core material B-H characteristic are such that when substantial magnetic saturation occurs, near times tj and t, of Fig. 2a, the inductance of coil 37 decreases substantially to around 500 microhenries or even less at peak currents. A suitable core material may be a ferrite such as a lithium-bismuth ferrite which has the added advantage of a relatively small change in Bsat with core operating temperature change when compared to many 6 GB2148559A 6 other ferrites. The core may be constructed as a toroid or as a double Ecore.

Claims (6)

1. CLAIMS 5 1. A television system, comprising: a deflection generator

   including a deflection winding; a tranformer having primary and secondary windings, said primary winding being coupled to a source of unregulated alternating input voltage for developing across said secondary winding an alternating polarity voltage that tends to vary with variations of said unregulated voltage; means responsive to said secondary wind- ing alternating polarity voltage for developing a B + scan supply voltage therefrom; means for applying said B + scan supply voltage to said deflection generator to develop scanning current in said deflection windings; and a saturable core with load coil wound thereon and conductively coupled to and magnetically isolated from said transformer secondary winding, said load coil being responsive to and providing regulation of the transformer secondary winding alternating polarity voltage for developing a regulated B + scan supply voltage.
2. 2. A television system according to Claim 1 including a high voltage winding closely coupled magnetically with said transformer secondary winding voltage and a high voltage circuit coupled top said high voltage winding for developing a picture tube ultor voltage.
3. 3. A television system according to Claim 1 or 2, including a capacitance coupled to said load coil in such a manner as to generate a circulating current that magnetically satu- rates the core portion associated with said coil during each cycle of said load coil alternating voltage, to regulate the voltage developed across said secondary winding without generating said circulating current in said secondary winding.
4. 4. A television system according to Claim 3, wherein the transformer core portion associated with said transformer secondary winding remains substantially unsaturated magneti- cally during the entire cycle of the transformer secondary winding alternating voltage.
5. 5. A television system according to claim 4, wherein said transformer primary and secondary windings are loosely coupled magnetically to enable the secondary winding voltage to be substantially unchanged in amplitude with primary winding voltage variations.
6. 6. A television system according to any one of Claims 1 to 5 wherein said saturable reactor core and load coil are ferroresonant.

   Printed in the United Kingdom for Her Majesty's Stationery Office. Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.