DEVICES AND METHODS OF CONTROLLING ALTERNATING ELECTRIC CURRENT

Abstract: A device for producing alternating electric current of high frequency for power consumers such as fluorescent tubes (Ly1, Ly2) comprises a transformer with a winding (n₃) connected in series with an output terminal ( e ) and active electronic components such as transistors (T1, T2) controlling the output current, the transistors being controlled by electric voltages produced by inductive feedback in feedback windings (n₁₁, n₁₂). Magnetic saturation is utilized to modify the inductive relationship in such a way that the transistors (T1, T2) cyclically change the direction of the output current. The feedback takes place in two magnetic cores (Tr1, Tr2) of the transformer, each core being provided with at least one further electric magnetization winding designated a command winding (n₅, n₆) as electric current is fed through the command windings to control magnetic saturation of the magnetic cores (Tr1, Tr2). As a result, combined control of the frequency and of the active electric power in the fluorescent tubes (Ly1, Ly2) is possible so that the luminous power may be controlled over a wide range while suitably high voltages can be maintained to ignite the tubes properly.

Description: This invention concerns devices for and methods of controlling alternating electric currents for electrically powered devices, in particular but not exclusively discharge lamps such as fluorescent tubes. Fluorescent tubes are nowadays widely used as light sources, although they have not completely replaced the also very popular incandescent lamps on the market. Fluorescent tubes have among their advantages a relatively high luminous output in relation to the electric power consumed, long life and acceptable luminous properties. On the electrical side, fluorescent tubes, though, require more complicated measures than incandescent lamps, since fluorescent tubes, when cold, require a particularly high ignition voltage to ignite the electric discharge, e.g. in the order of 1000 volts peak value, and since the fluorescent discharge has a strongly negative impedance, which furthermore changes during ignition of the electric discharge. Therefore a power supply circuit for fluorescent tubes must be fitted with special equipment for the ignition and special equipment to limit the current. The electrodes of fluorescent tubes are conventionally equipped with means for electric heating, whereby the ignition voltage may be reduced to a magnitude of 800 volts peak value. The impedance, being negative and non-constant, necessitates the use of current limiting equipment and fluorescent tubes to be powered from a conventional voltage source are therefore in practice connected thereto through an induction coil in series. The ignition of a non-burning and therefore cold tube is normally effected by electrical switching, usually by means of an automatic switch, also called a starter, which has the important function of switching off the powered heating of the tube electrodes once the discharge has been ignited. To prevent premature burning of this switch, it is normally also equipped with a capacitor in parallel. All of these components are included in a conventional light unit for fluorescent tubes according to the present art. With the usual mains frequency, whether fifty or sixty Hertz, the series induction coil or inductance must have a considerable size, and it feeds back into the mains line strong reactive currents which are undesirable as they cause electric losses in the supply cabling. They can be reduced by so-called phase compensation by a capacitor, which must also have a considerable size. The induction coil itself consumes a quite substantial amount of electric power, which is fully converted into heat. An ordinary light unit equipped for example with two fluorescent tubes rated at 58 W each, namely a nominal total luminous power of 116 W, thus in reality often takes up power of about 170 W. Other commonly known disadvantages of fluorescent tubes equipped as described is the so-called stroboscopic effect, since the luminous arc is ignited and turned off with a frequency which is double the mains frequency, for instance 100 or 120 Hertz. This stroboscopic effect is usually not visible, but may under adverse circumstances cause inconvenience. Furthermore, acoustic noise is often induced, particularly by the induction coil, and the usual simple ignition device may cause slow ignition requiring several attempts accompanied by an unpleasant flicker. Furthermore, the automatic switch will, in the case that a tube has burnt out and is unable to ignite, still try to ignite it, causing a persistent flicker until the switch has been worn out. It is anticipated that a considerable potential for energy saving can be utilized by the automatic control of illumination, for instance related to daylight variations, as lighting systems of today are often operated on full power over extended periods of time, even though the locations in question may also receive natural daylight so that the artificial illumination is only partly needed or only needed for part of the time. It is today possible to fit automatic systems with light measuring devices and to control the electric power suppled to the lighting systems, for instance to maintain a predetermined illumination level. Control of electric light sources is known in the art, also in relation to fluorescent tubes. With control of fluorescent tubes for the purpose of reducing the luminous power, it must, however, be realised that the voltage cannot be reduced very much before the tubes fail to ignite. Control systems for fluorescent tubes therefore generally utilize a time control system, which is today generally provided by a so-called chopper control, which in essence ignites and turns off the tubes quickly, typically at the frequency of the mains, controlling the light level by reducing the duty cycle, that is the ratio between the ignited time and the dwell time. These control systems which are used today, however, have several disadvantages, among which are the creation of a source of emission and transmission of radio frequency noise, and causing the normally undesirable stroboscopic effect already present in fluorescent tubes to be severely aggravated. Furthermore, full lamp power has to pass the components of these control systems, which must therefore be sized for a similarly large electric power. It is also known in the art to control electric power by utilizing so-called transductors. To explain briefly, transductors are transformers wherein the current transformed is limited by magnetic saturation in the transformer core. The saturation may be controlled by an extra magnetization winding, which influences and controls the power being transformed. In the technology of today, transductor control systems are rarely used, since transductors are rather costly, and since they are unable to control properly when feeding reactive or capacitive loads. The above problems in the control of fluorescent tubes often lead to the practical selection of incandescent lamps for illumination systems which are to have a control facility. Using incandescent lamps, an effective control system may be constructed having, though, two major drawbacks. Firstly, the illumination changes colour by going into the red end of the spectrum when reduced in intensity, and secondly the already-low luminous efficiency of the incandescent lamps is considerably even further reduced. It is understandable that systems with illumination control are not widely used at present since they, as explained, either provide unpleasant lighting or poor economy. It has recently been suggested to feed fluorescent tubes from a high-frequency generator, refer e.g. to Siemens publication "Schaltbeispiele", Ausgabe 82/82, p. 78. Herein a circuit is described for converting a supply voltage at a frequency of e.g. 50 Hertz to AC power at a frequency of approximately 120 kHz. By powering fluorescent tubes with such a circuit, a number of significant advantages are gained, such as: increased light output, as the efficiency of the lamps is higher at this high frequency; longer tube life; no mechanically movable parts in the light unit accessories; no stroboscopic effect, as the electric discharge arc does not turn off during the extremely brief intervals when the current changes to the alternate direction; the circuit is phase compensated; instant ignition of the fluorescent tubes can be achieved; no flicker on burned out tubes; and the conventionally-provided rather costly and energy consuming induction coils are reduced in size, and their power consumption is similarly reduced. Such circuits are still not very common, but it is anticipated that they will soon gain widespread use, as they can be built rather cheaply, and as they have the substantial advantages as explained. It is to be noted that a separate circuit of this type is required in every single light unit since currents at these very high frequencies cannot economically be supplied over any substantial distance, even with special high-frequency cabling. This circuit and similar circuits have, moreover, the disadvantage that they cannot readily be equipped with a control facility. According to one aspect of the invention there is provided a device for the control of alternating electric current to a power consumer, said device comprising an inductance connected in series with an output terminal, active electronic components controlling the output current, said active components being controlled by electric voltages induced in feedback windings by the output current by means of magnetic fields produced in magnetic material, wherein magnetic saturation in the magnetic material is used to modify the inductive relationship in such a way that the active components cyclically change the direction of the output current, characterized by the magnetic material being divided into at least two parts, each part being provided with at least one further electrical magnetization winding designated a command winding, so that electric currents fed through the command windings contribute to magnetization of the magnetic material, whereby saturation will occur with a level of output current different from that current level where saturation would have occurred without command current, one control winding mainly influencing waves of output current in one direction, and the other control winding mainly influencing waves of output current in the opposite direction. According to another aspect of the invention there is provided a method of frequency control of alternating electric current for a power consumer, wherein the alternating current is produced by inductive feedback in magnetic material to active electronic components, amplifying the fed-back voltage using magnetic saturation of the magnetic material to modify the inductive relationships in such a way that the output current cyclically changes direction, and wherein the output current is limited by an inductance connected in series, characterized by the magnetic material being divided into two parts which are influenced by one or more electric windings designated command windings conducting a command current and contributing to the magnetization of the magnetic material, whereby saturation will occur as a result of values of output current differing from those in the absence of command current in order that the periods of time after which the current changes direction can be controlled. A preferred embodiment of the invention, to be described in greater detail hereinafter, provides a device by which a power consumer such as a fluorescent tube can be supplied with electric current at a high frequency, whereby the current is controllable, and whereby output voltages are developed, even when the current is reduced, at such levels that, for example, fluorescent tubes will ignite without difficulties. With such a device, numerous advantages are obtained, among which are the following. A control facility can be provided with a simple command circuit, since the command signal may be a DC signal. The control system does not give rise to the stroboscopic effect present with the control systems of the known art, and neither does it give rise to radio frequency noise. The electric circuitry for the control can operate at low voltages and has no DC coupling to the power supply. The control strategy may be varied over a wide range, and it is possible to control separately the positive and the negative half-periods of the currents, whereby the shape of the curve of current versus time may be influenced, noting though that the circuit shown is not capable of producing a net DC current at the output terminals. The circuitry may further be built in a very compact size in order that it may be fitted inside conventional light units. The command circuitry can be sized to allow for small power demands as a command current of the required magnitude can be generated and maintained stably without difficulties. According to the preferred embodiment, the feedback windings are routed around both of the magnetic cores so that a magnetic signal from either of these cores will induce voltages around both of the magnetic cores, and thus in both feedback windings. However, these windings are sized so that a signal from only one of these cores as a result of the prevailing output currents is not sufficient to effect feedback; this can only be effected by the added signal from both cores, but in opposite directions relative to the feedback loops, a circuit is achieved exhibiting the unexpected and rather surprising behaviour that the maximum power for the power consumer is obtained when the command current is zero, and that the feed-in of a command current will reduce the output regardless of the direction of flow of this command current. Thus the advantage is obtained that the system assembly is facilitated, as an electrician does not have to pay attention to identify the control terminals individually. Furthermore, it is positively guaranteed that the circuit can never produce a larger output current than is acceptable. Furthermore, it is possible even to operate the command circuit with AC, provided that this command current AC has a frequency which is suitably low relative to the output power frequency. This, however, still leaves a wide range, since the output power frequency may be of the order of 100 kHz. This arrangement allows for numerous applications, among which only two examples will be mentioned to illustrate the degree of sophistication possible. A device embodying the invention may as a first example be used to provide a stroboscope operating with fluorescent tubes as a light source, whereby a light output may be provided, exceeding the light power that can normally be provided with a stroboscope. As a second example, illumination can be modulated with an audio signal from a music system, such as one could imagine used in a discotheque or dance restaurant to produce a fancy lighting effect. A further aspect of the invention is providing an illumination system which saves energy by automatically adapting the illumination level in correspondence with the available daylight, ensuring that the illumination level is always sufficient, and ensuring pleasant illumination conditions since frequent switching of the lighting does not take place, and which system can be produced at relatively low cost. This is achieved with an illumination system comprising at least one light unit, an illuminance measuring device for detecting light, and a control device connected thereto, characterized by the light unit being provided by a device for controlling alternating current such as that above-described, and controlled by the control device in such a way that the light measured by the control device is always maintained larger than or equal to a desired minimum reference level while the electric power used is kept to a minimum by the control device being provided with means for switching on power to the light unit in case the light level drops below a predetermined turn-on level, and by the control device being provided with means for switching off power to the light unit, and with a delay device so that switching off occurs once the light level during an uninterrupted interval of time defined by the delay device has exceeded a second predetermined level designated the turn-off level. In the following description, the invention will be explained in more detail with reference to the accompanying drawings, wherein: Figure 1 shows a diagram of an electronic circuit of the known art for producing a high-frequency alternating electric current; Figure 2 shows a circuit according to a first embodiment of the invention; Figure 3 shows a circuit according to a second embodiment of the invention; Figure 4 shows a circuit similar to the circuit of Figure 3, but arranged to feed a vapour lamp instead of fluorescent tubes; Figure 5 shows two arrangements of electric windings on magnetic cores according to alternative embodiments of the invention; Figure 6 is a graph of various illustrative electric signals plotted versus time in a circuit embodying the invention; Figure 7 shows an illumination system with several lighting fixtures controlled automatically according to a further aspect of the invention; Figure 8 shows an electronic control circuit for providing command signals for control devices in the lighting fixtures of Figure 7; and Figure 9 shows examples of illumination levels that can be produced by an illumination system according to Figures 7 and 8, illustrating also the influence of various external factors, and plotted versus time. To understand the invention better, a high-frequency circuit according to the known art will first be explained, referring to Figure 1. This circuit is suppled through a resistor R1 with electric power from a mains circuit, which power is rectified in a bridge rectifier D1, D2, D3 and D4 and smoothed by a capacitor C1 to produce a direct current. By using two active electronic devices such as transistors T1 and T2 in a push-pull coupling, the voltage at a terminal e in Figure 1 may be controlled within the range defined by the DC voltage. From the terminal e a current is drawn, which is fed through a winding n₃ of a transformer Tr1 to two parallel inductances L1 and L2, each connected to a respective fluorescent tube Ly1, Ly2 in series. The current power loop is completed by a capacitor C5. With this circuit, it is possible to feed the fluorescent tubes with alternating current with a frequency determined by the values of the components. The active electronic devices T1 and T2 are metal-oxide power transistors, such as those commercially available under trade names such as Mosfet, Sipmos, and Hexfet. A component of this type has three terminals marked S for "source", D for "drain", and G for "gate". The components are commercially available with various polarities, and the type explained in the following is the so-called N-channel type where the D terminal in a practical application is connected to a positive voltage and the S terminal to a negative voltage, whereafter the current flowing from the terminals D to S can be controlled by the voltage applied to the terminal G. It is one of the characteristic features of these types of transistors that the G terminal exhibits an extremely high impedance, and that the current flowing from the terminals D to S may be controlled with a very high current gain factor. When the voltage on G is negative relative to S, the transistor is completely switched off. With a positive voltage on G, not exceeding a characteristic threshold value typically of the magnitude of 4 volts, this transistor is still switched off. Only when the voltage on G exceeds this threshold value is a current allowed to flow from the terminals D to S. Because of the extremely high impedance of the G terminal in such transistors, external components must be provided to protect the transistor against overvoltages. Therefore the transistor T1 in Figure 1 has been provided with a resistor R4 and a zenerdiode D7 in the gate circuit, and the transistor T2 has similarly been provided with a similar resistor R5 and a zenerdiode D8, which components ensure that the voltages fed to the G terminals can never rise to a level which could cause damage to the transistors. The explanation of the start-up of this circuit will be postponed, until the function of the circuit during regular oscillations has been explained. During the regular oscillations, the transistors T1 and T2 are arranged to switch on and off alternately as they, of course, may never be switched on simultaneously. In the moment that e.g. the transistor T2 switches on, the voltage at the terminal D of this transistor and thereby at the terminal e assumes a value, which, apart from a negligible voltage drop from the terminal D to the terminal S on the transistor T2, will be equal to that of the negative pole of the supply voltage. The circuit will therefore attempt to conduct current through the small transformer winding n₃ from the components around the fluorescent tubes. As can be seen from Figure 1, there is parallel to each fluorescent tube connected a capacitor C6 or C7, and there is in series with each fluorescent tube connected an inductance L1 or L2. As the inductances L1 and L2 are connected in series with the respective fluorescent tubes and have a considerable inductance, they will limit the current allowed through so that the current will only gradually increase. As long as the fluorescent tubes are not ignited, the current may pass through each of the parallel capacitors C6 and C7, and also be drawn through the capacitor C5, completing the power loop. Once the luminous arc in the tubes has ignited, current is drawn through the tubes and also through the parallel capacitors C6 and C7. In Figure 6, the curve a in solid lines indicates the voltage at the terminal e and the curve b the current through the winding n₃ versus time, and it can be seen from the curve a that this voltage for a certain interval of time is generally constant at a negative value. Curve b of the same figure shows how the current changes, the sign of the figure being selected so that the current by the start of the time interval, where e has a negative voltage, is at a high level and shifting towards a lower level. This change of current through the winding n₃, however, induces a magnetic field in the magnetic core of the transformer Tr1. This changing magnetic field induces voltages in two feedback windings n₁ and n₂ thereof, n₁ being connected to the G terminal of the transistor T1, and n₂ being connected to the G terminal of the transistor T2. The directions of these windings are selected so that a current being drawn through the transistor T2 induces such a voltage in the winding n₁ that the voltage on the transistor T1 terminal G stays negative relative to the T1 terminal S, so that the transistor T1 remains completely switched off. The feedback loop n₂ is connected so that the same magnetic field simultaneously induces a voltage on the transistor T2 terminal G, which is positive relative to the transistor T2 terminal S, and this positive voltage keeps the connection through the transistor T2 from D to S switched on. However, the current through the winding n₃ will with suitable setting of the values of the components in the circuit after some time have risen to such a level that the magnetic core in the transformer Tr1 is magnetically saturated, whereafter it is no longer possible through this core to induce voltages in the windings n₁ and n₂. Therefore the voltage in the winding n₁ drops to zero, but since the transistor T1 at this time was already switched off, the state of the transistor T1 is not changed. Simultaneously, the voltage in the winding n₂ drops to zero, but this causes the transistor T2 to switch off and stops the current from D to S of the transistor T2. The current through the winding n₃ does not drop instantly, even when both transistors T1 and T2 are blocked, as the inductances L1 and L2 can maintain some current through the winding n₃, which is possible because of the connection to the resistor R3 and the capacitor C4; therefore the current will not instantaneously disappear, but will instantaneously initiate a decrease. This starting decrease of the current through the winding n₃ will immediately induce current in the feedback loops including the windings n₁ and n₂, having opposite directions to those described in the previous period. Thus a voltage is induced in the winding n₂, making the transistor T2 terminal G negative relative to the T2 terminal S, whereby the transistor T2 will be switched off. Simultaneously, however, a voltage is induced in the winding n₁, making the T2 terminal G positive relative to the T1 terminal S, and thus the transistor T1 will be switched on for current from the terminal D to the terminal S. The voltage at the terminal e will therefore, apart from a negligible voltage drop over the transistor T1, essentially equate the positive supply voltage pole, as can be seen from the curve a in Figure 6 at a later interval of time. Because of the series inductances L1 and L2, the current changes gradually so that continued voltages are induced in the windings n₁ and n₂, which maintain this process, since the induction in a transformer, as is well-known for those skilled in the art, is proportionate to the rate of current change rather than to the magnitude of the current. It is to be understood that the capacitance of the capacitor C5 is sufficiently large to ensure that the voltage on that terminal of C5 which is connected to the lamps remains essentially constant at a value at the midpoint between the positive and the negative supply voltage, and it is therefore possible to feed a current through the lamps when the transistor T1 is on and the transistor T2 is off;. The current through the winding n₃ follows the pattern shown at a later stage of the curve b in Figure 6, and it can be seen that the pattern is similar to the pattern of the first time interval, only with a change of sign. The current through the winding n₃ continues to increase in the new direction, until the Tr1 core is again saturated, this time in the direction opposite the one previously, whereupon the voltages in the windings n₁ and n₂ drop to zero, and the transistor T1 (as previously with T2) switches off, whereby the transistor T2, because of a newly induced voltage in the winding n₂, is switched on and the whole cycle is repeated. It is to be understood that the circuit can thus maintain cyclic oscillations, the circuit being designed so that the frequency of these oscillations is essentially governed by the inductances L1 and L2, the capacitances C6 and C7, and by the lamps. The capacitor C4 ensures, during the switching-over interval, when both transistors T1 and T2 are switched off, that the voltages on the terminal S and the thereto connected T2 terminal D will not rise to such high levels that they could be harmful to the transistors. The voltage and the current at the fluorescent tube Ly1 are respectively shown with solid lines in curves c and d in Figure 6. It is to be noted that the impedance of a fluorescent tube at frequencies of the order of 100 kHz, as in the present case, exhibits a more stable value than is normally observed when powering the tubes at 50 Hz or 60 Hz. Now the start up of the oscillations will be explained. Initially all voltages of the circuit are zero, and no currents are flowing. When the mains supply is connected to the mains terminals in Figure 1, the parts of the circuit mentioned so far will in fact be unable to initiate oscillations. This may be surprising as electronic oscillators are generally self-starting, since small random noise signals, which are in practice always present, are generally amplified and fed back, and therefore generally will provide the starting signal for a feedback generator. However, a field effect transistor, as used here, does not respond until the voltage on the G terminal exceeds the voltage on the S terminal by a substantial amount, e.g. 4 volts. The circuit has therefore been provided with a number of dedicated components such as a resistor R2, a capacitor C3, and diodes D5 and D6, which have been included in the circuit with the sole purpose of starting the oscillations. At the point in time where the power is switched on the circuit, the capacitor C3 will slowly be charged through the resistor R2. The electronic component D6 is, however, a so-called DIAC, which exhibits the particular behaviour that it is completely blocked for current until the voltage exceeds a predetermined level, the so-called breakdown voltage, e.g. 32 volts, whereupon it suddenly allows flow of current, remaining on even with decreasing voltages as long as any current continues to flow through it. When the voltage on the capacitor C3 thus exceeds the DIAC breakdown voltage, the DIAC D6 will switch on, and the T2 terminal G will be fed with a positive voltage, which is sufficiently high to open up for current from the T2 terminal D to the T2 terminal S, whereby oscillations will be started. During cyclic oscillations, the capacitor C3 will have only very brief intervals, namely the intervals when the transistor T1 is open, to be charged through the resistor R2, whereafter the capacitor C3 upon switching on of the transistor T2 will be immediately and fully discharged through the diode D5. By suitable arrangement of values of the resistor R2 and the capacitor C3, it can therefore be ensured that the voltage on the capacitor C3 during cyclic oscillations will never reach such a level that the DIAC D6 will open. The tubes may be provided with conventional series-connected fuses (not shown in the drawings). EXAMPLE 1 A circuit similar to the one in Figure 1 is constructed with the following component values: R1 = 3.3 Ω, R2 = 270 kΩ, R3 = 330 kΩ, R4 = 100 Ω, R5 = 100 Ω, C1 = 47 µF, C3 = 0.1 µF, C4 = 1nF, C5 = 100 nF, C6 = 3.3 nF, C7 = 3.3 nF, L1= L2 = 420 µH, and the lamps being 50 W fluorescent tubes. The transistors T1, T2 are Sipmos BUZ 41A, the zenerdiodes D7 and D8 are BZY 97 C8V2, and the transformer Tr1 is wound around a ferrite ring core, Siemens R12.5, the windings n₁ incorporating three turns, n₂ three turns, and n₃ one turn. With these component values, the above-mentioned Siemens publication states the idle frequency, when the lamps are not ignited, to be around 150 kHz, and the duty frequency, when the lamps are lit, to be around 120 kHz. The idle frequency essentially equates the resonance frequency of the oscillation pair L1, C6, which is equal to the resonance frequency of the other pair L2, C7, whereby the voltages across the lamps will rise to very high values, e.g. of the magnitude of 1000 volts, causing immediate ignition of the lamps. Now the circuit of the first embodiment of the invention will be explained by reference to Figure 2. As may be seen in this figure, it is distinguished from the previously-proposed circuit shown in Figure 1 by the feedback transformer which has been divided into two parts. Furthermore, the circuit is equipped with terminals for the feed-in of a command current. The remaining parts of the circuit are quite similar to the circuit of Figure 1, and similar components have been indicated with the same references; regarding the general operation, reference may be made to the above-given explanation in connection with Figure 1. The circuit of Figure 2 is distinctively featured by the feedback transformer being split into two parts, Tr1 and Tr2. The transformer part Tr1 has a feedback winding n₁₁ connected to the T1 terminal G, a winding n₁₃ conducting the lamp output current, and a further winding n₅ to be connected to a command current circuit (not shown). The transformer part Tr2 has a feedback winding n₁₂ connected to the T2 terminal G, a winding n₁₄ conducting the lamp output current, and a winding n₆ to be connected to a further command current circuit (not shown). As may be understood from the figure, the output current from the terminal e to the lamps passes windings on both transformer parts Tr1 and Tr2. The orientation of the windings has been marked with dots on the figure according to a standard conventionally used. Considering initially the case where no current flows in the command circuits, it may be understood that the lamp output current is capable of inducing voltages in the feedback windings n₁₁ and n₁₂, since the output current passes a winding on the transformer part Tr1 and thereafter a winding on the transformer part Tr2. The function of the circuit is thus exactly similar to the function of the circuit of Figure 1. It is now assumed that by means of an external current generator (not shown), the winding n₅ is fed with a direct current called here a command current. This current produces a contribution to the magnetization of the transformer part Tr1. The circuit is assumed to oscillate largely as previously, and it can be understood that the current fed through the winding n₅ does not affect the winding n₁₂ connected to the transistor T2, thus the transistor T2 will switch on exactly as previously. Once the transistor T2 has switched on, current will be drawn from the lamps, in the direction from the terminal f to the terminal e . This causes a magnetization of the core of the part Tr1 in a direction opposite that of the magnetization caused by the current in the winding n₅, and under the assumption that the magnetization generated by means of the winding n₅ has a limited magnitude and specifically is smaller than the magnetization produced by the winding n₁₃, a voltage will be induced by the part Tr1 in the winding n₁₁ developing a negative voltage on the T1 terminal G relative to the T1 terminal S. This part of the operation is thus quite similar to the function described with reference to Figure 1. During that interval when the transistor T2 is switched off and the transistor T1 is switched on, a current will flow through the lamp circuit in a direction opposite to the one previously, namely from the terminal e to the terminal f . This produces a magnetization inducing a voltage in the winding n₁₁, developing a positive voltage on the T1 terminal G, to maintain the current through the T1 terminals D and S as previously. However, the contribution to the magnetization by means of the winding n₅ will now cause the transformer part Tr1 core to be magnetically saturated at a lower value of current in the winding n₁₃ than was the case when the winding n₅ did not contribute. Once saturation of the transformer part Tr1 core takes place, the transistor T1 switches off as explained earlier and this causes, as previously explained, the transistor T2 to switch on. It is to be understood that the control system makes use of a transductor principle, but that it is the command current to the transistors that is controlled by the transductor system rather than the full lamp current, such as is the case with the previously-proposed transductor control systems. It may be seen that the current feed through the winding n₅ has the effect of shortening the time interval during which the transistor T1 is switched on. Since the lamps are connected in series with the capacitor C5, it is apparent that no net direct current can pass through the lamps, but that the curve shape of the current passing through the lamps is modified by the control of the current waves passing through the transistor T1. Similarly, it can be understood that a current fed through the winding n₅ in a direction opposite to the one described above will have the effect that a correspondingly larger current through the winding n₁₃ will be required to saturate the magnetic core in the transformer part Tr1, thus the time interval during which the transistor T1 is switched on will therefore be lengthened. It is to be understood that the command winding n₆ is quite similar to the winding n₅, and that by feeding currents through the winding n₆ in one direction or the other, the time intervals during which the transistor T2 allows current through may be shortened or lengthened. By feeding in symmetrical currents through the windings n₅ and n₆, i.e. currents of equal magnitude and in directions such that the periods during which the transistors T1 and T2 are switched on can both be shortened or lengthened, it is to be understood that a frequency control facility of the oscillating circuits is provided, wherein a change of frequency relative to the idle frequency is variable being related to the command currents fed in, although the relationship is not necessarily linear. An example of the curves of voltages and currents that may be produced by symmetrical shortening of the switch-on intervals for the transistors T1 and T2 is shown in Figure 6 in dotted lines. As the usual frequency of the oscillating circuit, that is the frequency when the command current is zero and the lamps are lit, is somewhat lower than the resonance frequencies of the pairs C6 and L1 and C7 and L2 respectively, an increase in the frequency will feed a larger current through the capacitors C6 and C7, this current being reactive current and therefore not representing any loss of power as the current effectively oscillates to and fro between the capacitors and the inductances. This, however, reduces the power supplied to the lamps, but maintains peak voltages of almost unchanged magnitude so that the luminous power of the lamps is reduced while the lamp voltage, even with a substantial reduction, is sufficient to ensure proper ignition of the lamps. A further preferred embodiment of the invention will now be described by reference to the circuit diagram in Figure 3 and to the arrangements of the transformer windings according to Figure 5. As may be seen in Figure 5a or in Figure 5b, two ring cores or annular cores are used, and the winding for the lamp current is in either of the Figure 5 embodiments a simple straight passage of a conductor from the terminal e to the terminal f . The feedback winding for the transistor T1, i.e. n₁₁, connected from the terminal a to the terminal b in Figure 5a or Figure 5b, is wound around both ring cores in the same direction. In the embodiment of Figure 5a, each winding in the circuit from a to b is firstly trained around the first ring core transformer and then the second ring core transformer. In the embodiment of Figure 5b, the conductor passes all the windings around the first ring core and thereafter makes all windings around the second ring core in the same direction. It is to be appreciated by those skilled in the art that these two embodiments, though physically different, are electrically equivalent and perform in exactly the same manner. The feedback winding for the transistor T2, i.e. the conductor from the terminal c to the terminal d is similarly trained around both ring cores, and the figure indicates that the direction of rotation is opposite that of the feedback winding between the terminals a and b . Each ring core is provided with a command winding, and the two command windings are connected in series so that a command current, e.g. from the terminal g , flows in a first direction around the first ring core and in the opposite direction around the second ring core before exiting at the terminal h . It is to be appreciated that Figure 5 illustrates the concept of the arrangement and the directions of the windings, but that the number of turns in each of the windings shown may differ from that indicated. It is, though, preferred to make the arrangement symmetrical, so that the winding ratios among the various windings on one core should be exactly identical to the winding ratios on the other core. It is to be appreciated that, by interconnection of the two command windings as shown, there is achieved the advantageous effect that any voltage induced in one command winding by current in the output power winding e - f will always be balanced by an oppositely directed voltage of equal magnitude induced in the second command winding. On the command winding output terminals g - h , no net voltage is therefore induced. In reality there may, because of manufacturing tolerances, be minor differences between the two command windings so that moderate voltages may be induced that are not completely balanced. Furthermore, when a core saturates magnetically, a net voltage will be induced at the command winding terminals. Such voltages, however, are dampened by a capacitor C8 (Figure 3) arranged in parallel across the terminals g - h . The electric circuit to produce the command current can therefore be sized moderately as it will not be subjected to induced voltages of any considerable magnitude. Besides the capacitor C1, a further and smaller capacitor C2 is arranged parallel thereto with the purpose of dampening out possible high frequency noise signals to prevent them from being propagated to the mains circuit. The operation of the circuit of Figure 3 will initially be explained for the situation without command currents. It may be seen that it is then exactly equivalent to the circuit according to Figure 1. It is now presumed that a direct current is fed through the command windings from the terminal g to the terminal h . This current will produce some magnetization of both transformer cores, it being here presumed that this magnetization is of limited scale and in particular smaller than the maximum magnetization that can be produced by the output current from the winding e - f . The oscillator circuit will largely oscillate as explained earlier, the transistors T1 and T2 alternately conducting current. During the time intervals when the transistor T2 is switched on, current passes the output winding from f to e , causing magnetization of both transformer cores. It may be seen that these two magnetization effects in the transformer Tr1 will be mutually opposed while in the transformer Tr2 they will be summed. Therefore saturation of the core in the transformer Tr2 will occur at a lower output current than was the case when no command current was present. The voltages induced in the feedback windings will therefore be reduced as the core of the transformer Tr2 no longer contributes hereto. In the transformer Tr2, on the other hand, saturation will not occur until an increased output current level relative to the level of current is achieved that would have produced saturation, if no command current was present. With current levels in the output circuit f - e of the magnitude that the transformer Tr2 is saturated, thus no longer contributing to the induction in the feedback windings, the core of the transformer Tr1 may therefore still contribute to this feedback induction. The net voltage induced in either of the feedback windings n₁₁ or n₁₂ thus will not completely disappear with the saturation of one transformer core, but will drop generally to about half of the immediately preceding value. As earlier explained, the transistors used, however, have the peculiar property of being completely switched off in the forward direction (D to S) when the voltage on the terminal G does not exceed a predetermined threshold value, e.g. around 4 volts. By suitable sizing of the winding ratios on the transformer cores, it is therefore possible to design a circuit where the voltage induced in the feedback winding for the switched on transistor, in this case T2, upon saturation of one transformer core will drop to below this threshold value so that the transistor essentially blocks the current between its terminals D to S completely, even though the other transformer still induces some voltage. It is here to be noted by reference to the curve b of Figure 6 that the output current at the moment of switching on of one transistor is changing steeply initially and thereafter at a decreasing rate, because of the inductances connected in series with the lamps. Therefore, in the feedback windings, a relatively large voltage is induced initially during the interval of switching on one transistor, while this voltage thereafter is gradually reduced. It can therefore easily be accomplished to design the windings so that the feedback voltage upon saturation of one of the transformer cores, which is likely to occur at the latter part of this interval, drops below the threshold value for the transistor in question. As the transistor T2 is now switched off, the circuit performs, as earlier explained, so that the output current, at this time flowing from f to e , starts decreasing from the maximum value, thereby inducing a magnetic field in both transformer cores directed opposite to that earlier, and causing the contributions to magnetizations from the output current and from the command current to be summed in the transformer Tr1 while they are mutually opposing each other in the transformer Tr2. In the feedback windings, voltages are therefore induced, keeping the transistor T2 blocked and switching on the transistor T1. The output current, initially flowing in the direction from f to e , will drop to zero and start increasing in the opposite direction, i.e. from e to f . Once the output current in the circuit from e to f has started to increase, it will after some time reach such a magnitude that the transformer core Tr1 will be saturated, whereby the voltage induced in the feedback windings drops to such a level that the voltage on the T1 terminal G drops below the threshold value, and the transistor T1 blocks. This, however, as earlier explained, causes the switching on of the transistor T2, and it can be understood that the circuit will continue oscillating, but with shorter time intervals than in the case without command currents. Thus a frequency control facility is provided. Now the case where a direct current is fed through the command circuit in a direction from the terminal h to the terminal g will be explained. As earlier explained, this will cause magnetization of both cores Tr1 and Tr2. As above, the moment of switching on the transistor T2 for current running from the terminal f through the transformers to the terminal e will be explained. It is to be appreciated that the contributions to magnetization from the lamp current and from the command winding current are added in the transformer core Tr1 while mutually opposing each other in the transformer core Tr2. As the lamp circuit current increases, saturation of the core in the transformer Tr1 will occur at some point in time while the transformer Tr2 core at the same time will not yet be saturated. The saturation of the transformer Tr1 core, however, causes the voltage induced in the feedback winding c to d to drop, and the transistor T2 blocks. As above, the blocking of the transistor T2 causes the transistor T1 to switch on and the lamp current, flowing at this time in the direction from f to e , will start to decrease. After some time, the lamp current will change direction and now flow from e to f , and increase since the contributions to magnetization from the lamp current and from the command current will be mutually opposed in the transformer Tr1 and will be summed in the transformer Tr2. At some level of lamp current, saturation in the transformer Tr2 core will therefore occur, whereby the voltage induced in the feedback winding n₁₁ will drop so that the transistor T1 blocks. It is to be appreciated that the oscillations will continue in this way exactly as explained above. It is hereby to be understood that the circuit exhibits the rather particular behaviour that the command current has a similar effect regardless of the direction thereof. The frequency of the output terminal voltage fed to the lamps is at a minimum when the command current is zero, whereby the lamps are supplied with maximum power, and the frequency is increased by feeding in a command current, regardless of the direction of the command current, whereby the lamp power is reduced. A number of very important advantages are accordingly gained, as follows. The power fed to the lamps can never exceed a predetermined value depending upon the circuit, it being understood that the circuit is suitably designed so that this maximum value is equal to the nominal power rating for the lamps. Accordingly there is complete safety against damage to the lamps even in the case of malfunctions or errors in the command circuit or errors in the connections. This also facilitates installation, since the electrician installing the circuit does not have to keep track of a specific order of connection. Furthermore, the command signal does not necessarily have to be a direct current signal; as a matter of fact, it may be alternating signal, provided that the frequency does not rise to such magnitude as to produce interference from interaction between the command current and the power circuit. Since the power circuit is operating at frequencies of the order of 100 kHz, problems of mutual interference should practically not arise as long as the command frequencies do not exceed e.g. 20 kHz. Therefore the command circuit could for instance be connected to an audio output terminal of a music system, so that the audio signal could modulate the light in such a way as one could imagine used for special effect lighting in a discotheque. The command current could for instance instead follow the common mains frequency, whereby the circuit to produce the command currents could be extremely simple; it could as a matter of fact simply be a transformer connected to the mains. The circuit diagram of Figure 4 shows a further preferred embodiment. This embodiment is used for vapour lamps without electrode heating facilities, such as mercury lamps, sodium lamps, and xenon lamps. The circuit will, as a matter of fact, operate perfectly with fluorescent tubes, although the electrodes in this case are not heated. The circuit is similar to that of Figure 3, although with the differences that only one lamp La is shown and that the capacitor C6 is here not connected to heating resistors in the lamp electrodes, but rather connected directly to the lamp electrodes, being thereby connected to the inductance L1 and the capacitor C5. It is to be understood that the circuit, apart from that explained above, operates exactly as the circuit of Figure 3; thus reference may be made to the above-given explanation. EXAMPLE 2 For the transformers Tr1, Tr2, two ferrite cores are used of the Siemens R12.5 type. The winding e to f is a simple straight conductor. The winding a to b makes three turns around each ring core, and the winding c to d also makes three turns around each ring core. The command windings n₅, n₆ comprise thirty windings around each core. The capacitor C2 has a magnitude of 1nF and C8 of 0.1 µF. The resistor R1 has a value of 1.5 Ω. The remaining components are equivalent to those listed under Example 1, noting though that the inductance of the windings L1 and L2 is approximately 580 µH each, although they may, because of manufacturing tolerances, deviate from the said design values. The fluorescent tubes are two tubes with a nominal rating of 36 W each. Without command current, the oscillation frequency with the fluorescent tubes lit was 80 kHz. When a current of 20 mA was fed through the command circuit, the oscillation frequency was 140 kHz and the power consumed by the lamps was about 20 W each. When the command circuit current was increased to 40 mA, the lamps were turned off. The power consumption of the electronic circuit is in the order of 4 W and it varies with the lamp power so that the total system at maximum luminous output consumes power in the order of 80 W, at a command current of 20 mA consumes around 38 W, and at 40 mA command current consumes about 1 W. EXAMPLE 3 Components are as in Example 2 with the following exceptions. The fluorescent tubes are rated at 58 W each, and the feedback windings are made so that the winding a to b makes six turns around each transformer core, and the winding c to d correspondingly makes six turns around each transformer core. The inductances of L1 and L2 are around 500 µH each. Without command current, and thus full luminous power, the oscillation frequency was 70 KHz, and the power consumption 2 x 58 W for the fluorescent tubes and about 5 W for the remaining components, giving a total of 121 W. With a command current of 20 mA, the oscillation frequency was 125 kHz and the lamp power 2 x 30 W. The resistance in the command circuit windings is about 0.8 ohms so that the voltage drop over the command circuit at 20 mA is about 16 mV. As mentioned above, the relationship between command current and luminous power is not necessarily linear, but approximately follows a square function. It is within the state of the art to design a control circuit which can compensate for this relationship. In reality, this problem does not cause extra complications as the non-linear relationship between the lamp power and the luminous output makes special precautions necessary in any case. Figure 7 shows an example of a possible application of a device embodying the invention. In a room with a floor 24 and a ceiling 25, a number of light units 21 are arranged, each being equipped with a device embodying the invention. Each light unit 21 is supplied with mains power, which may have an on/off-switch facility, but has no intensity control facility. A control current circuit is also routed through the lamps, connecting all the light units in series so that the current from a single command current source passes all the light units. At a conveniently accessible place, a command unit 23 is arranged with operation buttons or keys to turn the light on and off and with a tuning facility, whereon a desired luminance reference value may be dialled. In the room, an illuminance meter 22 is also arranged. From the illuminance meter 22, the command unit 23 receives a signal indicating the illuminance level actually present. The command unit 23 is equipped with a control circuit that produces a command signal depending upon the illuminance level measured, the command signal being routed to the light units 21 to control their light output. Figure 8 shows an example of a control circuit that may be incorporated in the command unit 23. As the function of this circuit may be appreciated from the figure by those skilled in the art, it will only be briefly explained. The circuit has input connections for supply voltages 5V DC, 12V DC, and 220V AC; input terminals for the illuminance meter 22, output terminals for the command current circuit, and output terminals for supplying the power to the light units. The illuminance meter 22 is in this case a so-called photoresistor, having the property that the resistance decreases when the illuminance increases. An operational amplifier Op1 on the basis thereof produces a voltage, which is related to the illuminance level being measured. By selection and respective tuning of the components around the amplifier Op1, the required minimum illuminance level, designated N2 (refer to Figure 9) is defined. The signal from the amplifier Op1 branches along two paths. The first path routes the signal through another operational amplifier Op2, serving together with its associated components the purpose of limiting the signal in order that a voltage is produced, having a predetermined maximum value (e.g. 2V) by illuminance levels above a certain limit, whereas the voltage below this limiting level varies proportional to the illuminance level. The limiting level defined by the components around the amplifier Op2 defines the minimum illuminance level designated N1 (to be explained further below with reference to Figure 9). This limited signal is passed on to a further operational amplifier Op3, which amplifier together with its associated components, among which is a transistor T₁₁, converts the voltage signal to a current signal for use as the command current for the light units. The signal from the amplifier Op1 is, as mentioned above, also routed along another branch, feeding it to a further operational amplifier Op4. This operational amplifier Op4 performs along with its associated circuitry as a so-called Schmidt-trigger with hysteresis, so that with an increasing input signal, the output signal is set until the input signal exceeds a predetermined first level called the turn-off level (N4 in Figure 9), and upon decreasing input signal the output signal will only be set after the input signal has dropped below a predetermined second and lower level. This second level is designated the turn-on level (N3 in Figure 9). The output signal from the amplifier Op4 is passed to a delay unit Tim, which with its associated components serves the purpose of passing on the trigger signal after a delay designated the turn-off delay with increasing illuminance level, whereas the trigger signal will be passed through without delay with decreasing illuminance level. This output signal controls a relay serving to turn the power supply on or off for the light units. The operational amplifiers Op1 to Op4 may be provided in a single integrated component commercially available under the type identification LM 324, containing just four operational amplifiers in a common casing. The delay unit Tim may be realized by a component designated CD 4060. The operation of the illuminance system with the circuitry shown in Figure 8 will now be explained by reference to Figure 9. In Figure 9, Figure 9a shows an extended span of time, for example in the order of 14 hours, whereas Figures 9b and 9c illustrate shorter intervals of time such as 20 minutes each. The artificial illuminance system in the room is capable of providing an illuminance level N2, which is equivalent to the desired and for operational reasons required minimum reference level, e.g. an illuminance level of 300 lux. However, a room which is equipped with translucent portions or windows 26 in the ceiling 25 and possibly other windows and other openings, also receives external light such as daylight. Figure 9a illustrates how the contribution from the daylight to the total illumination in the room may vary from nothing very early in the morning, rising gradually to a maximum at noon, and thereafter decreasing again to nothing at night. In the figure there is also shown how the lighting contribution from the artificial illuminance system varies. Initially only the artificial lighting is active and operating on full power, whereby the illuminance level is maintained at N2. Once daylight starts coming in, the artificial lighting is immediately turned down in equal proportion, thus keeping the total illuminance level constant. By increasing the illuminance level, at some point in time the level is reached where the circuitry around the amplifier Op2 will limit the control signal as explained above, whereafter the artificial lighting will not be turned down further, but will keep contributing a fixed minimum level N1, e.g. 100 lux. The room now receives a fixed illuminance contribution from the artificial lighting and a possibly increasing illuminance contribution from daylight. With further increasing daylight, at some time the turn-off level N4, e.g. 750 lux, may be reached, and the artificial lighting is switched off after expiry of the turn-off delay defined by the delay unit Tim, e.g. after 10 minutes. The room is now exclusively illuminated by daylight, which is increasing and decreasing. If daylight should later drop below the turn-on level N3, e.g. 450 lux as shown further to the right in the figure, the artificial lighting will immediately be switched on, operating on the low level N1. Only when daylight contributes less than the amount N2 minus N1, the artificial lighting will be turned up in order that the required minimum level N2 will just be maintained. When the daylight contribution has completely vanished, the artificial lighting operates on full power. As is commonly known, daylight may fluctuate rapidly and irregularly due to various weather circumstances, such as passage of clouds. The examples shown in Figures 9b and 9c server to illustrate the performance of the control system during rapid fluctuations. Figure 9b illustrates a situation which could prevail at the middle of the day when daylight is strong and the artificial lighting is turned off. Suddenly a very dark cloud passes, and the daylight contribution drops to a very low level. The artificial lighting is immediately switched on and immediately turned up to a level where the requested minimum illumination level is just maintained, taking full advantage of the remaining low daylight contribution. At a later point in time, the cloud disappears. The artificial lighting is immediately turned down to the level N1, but will only be turned off after the expiry of the turn-off delay defined by the delay unit Tim. Figure 9c illustrates a different situation conceivable on a day with heavy clouding. Daylight gives but a small contribution, and the artificial lighting is turned on and turned up to provide a suitable contribution. Suddenly the cloud cover opens up and strong daylight comes in. The artificial lighting is immediately turned down to the minimum level N1, but will not even with plenty of daylight be turned off until the turn-off delay has expired. Before this can take place, cloud, however, is assumed to cover the sky again, and the artificial lighting is immediately turned up to a suitable level. It is to be understood from the above given explanation that the system described operates well under practical circumstances as the lighting of the interior is always adequate, since frequent turning on and turning off, which might shorten the life of the light sources and which might be psychologically unattractive, is avoided, and since the energy used for illumination is kept at a minimum. Although the invention has been described with particular reference to the application of fluorescent tubes, it is clearly applicable to the controlled powering of any consumer of electric power. As already mentioned, it is very well applicable to other discharge lamps such as mercury lamps, sodium lamps, or xenon lamps. The control facility using a command signal of direct current or alternating current of small magnitude also makes the invention applicable for control or modulation in numerous ways, for instance application as a stroboscope or similar.

Claims: A device for the control of alternating electric current to a power consumer, said device comprising an input terminal for receiving an input power supply, an output terminal (f) for feeding an output current to the power consumer (Ly1), a control terminal for receiving a command input current, a transformer means comprising magnetically saturable material (Tr1,Tr2), at least one power winding (n3) and two feedback windings (n11,n12), said power winding (n3) being connected to said output terminal (f), and active electronic components (T1,T2) for controlling said output current, said active components (T1,T2) being controlled by electric signals produced in said feedback windings (n11,n12) by inductive feedback in said transformer means induced by said output current, wherein the fed back signals are produced as a result of magnetic saturation in said magnetic material (Tr1,Tr2) modifying a relationship of induction in such a way that said output current cyclically changes direction, characterized by said magnetic material being divided into at least two parts (Tr1,Tr2), each part being provided with at least one further electrical winding positioned about the part of magnetic material and designated a command winding (n5,n6), both of said command windings (n5,n6) being connected to said control terminal in order that electric current fed through one of said command windings (n5,n6) will contribute to the magnetization of the respective part (Tr1,Tr2) of said magnetic material, whereby saturation will occur at a current level of said output current different from that current level at which saturation would have occurred in the absence of said command current. A device according to claim 1, characterized by said magnetic material (Tr1,Tr2) comprising two separate cores of magnetic material, each core supporting one of two mutually similar power windings (n13,n14), respectively, interconnected in series, and one of two mutually similar command windings (n5,n6), respectively, said command windings (n5,n6) being interconnected in series and connected between a pair of control terminals, and being arranged with such winding directions that changes in said output current will induce voltages in said two command windings (n5,n6) of substantially equal magnitude but of opposite polarity so that substantially no net voltages are induced at said control terminals. A device according to claim 2, characterized by said power windings (n13,n14) being routed around said two magnetic cores in a first direction, by a first one (n11) of said feedback windings being routed around both magnetic cores in the same first direction, by a second one (n12) of said feedback windings being routed around both magnetic cores and around each of them in a second direction opposite to said first direction, and by said command windings (n5,n6) being routed so that a first one (n5) of said command windings surrounds a first one of said two magnetic cores in said first direction and a second one (n6) of said command windings surrounds a second one of said two magnetic cores in said second direction. A device according to claim 1, claim 2 or claim 3, characterized by inductance means (L1) connected between said power winding (n3) and the power consumer (Ly1) in order that said output current passes through said inductance means (L1). A light unit (21) for a gas discharge lamp (Ly1), the light unit (21) comprising a device according to any one of claims 1 to 4 and connected to drive said gas discharge lamp (Ly1) at a power level controlled by said command input current. A light unit (21) according to claim 5, characterized by a command unit (23) connected to the control terminal of the device for producing controlled currents for said command windings (n5,n6). A system comprising at least two light units (21) according to claim 5 or claim 6, characterized by said control terminals of said respective light units (21) being connected in series so that a common command current passes through both or all of said light units (21) controlling each of said devices. An illumination system comprising at least one light unit (21), an illuminance measuring device (22) for detecting light, and a command unit (23) connected thereto, characterized by the or each light unit (21) being provided with a device according to any one of claims 1 to 4 and controlled by said command unit (23) in such a way that the illuminance level measured by said illuminance measuring device (22) is always maintained greater than or equal to a desired minimum reference level at a minimum consumption of electric power, by said command unit (23) being provided with means for switching on power to said light unit (21) in case said measured illuminance level drops below a predetermined turn-on level, by said command unit (23) being provided with means for switching off power to said light unit (21), and by said command unit (23) being provided with a delay device (Tim) which causes the power switching off to occur only after said illuminance level has uninterruptedly exceeded a second predetermined illuminance level for a predetermined time. Use of a device, light unit or system according to any one of claims 1 to 7, characterized in that means (22) is provided to measure a physical parameter produced by one or more of the devices and by a control loop, wherein the devices may be controlled automatically by a comparison between the measured parametric value with a reference parametric value. A method of controlling a frequency of an alternating electric current supplied to a power consumer, wherein said alternating current is controlled by active electronic components (T1,T2), wherein said active electronic components (T1,T2) are controlled by signals produced by inductive feedback in magnetic material (Tr1,Tr2) induced by said alternating electric current, wherein said fed back signals are produced as a result of magnetic saturation in said magnetic material (Tr1,Tr2) modifying a relationship of induction in such a way that output current supplied to said power consumer cyclically changes direction, and wherein said output current is limited by being fed through inductance means (L1), characterized by said magnetic material being divided into two parts (Tr1,Tr2) which are influenced by one or more electric windings designated command windings (n5,n6) conducting a current designated a command current contributing to magnetization of said magnetic material (Tr1,Tr2), whereby saturation will occur at values of output current differing from those in the absence of a command current in order that the periods of time after which said output current changes direction can be controlled.

Inventors: HALBERG, KURT, HERBSLEB, KJELL, HERBSLEB, PEER, JENSEN, KARL AGE
Applicants: HALBERG & THOMSEN ELEKTRONIK I/S, JORCK & LARSEN A/S
Application year: 1987
Main classification: H05B41
IPC codes: H05B41, H05B37