BACKGROUND OF THE INVENTION 
   For interrupting and making electrical circuits, use is made both of mechanical contacts, sliding contacts such as in the case of current collectors on rail vehicles or commutators in electric motors, fusible links, and of semiconductor switches such as transistors, thyristors and semiconductor relays. 
   During interruption of the electrical circuits, all these switching elements are exposed to a high self-induced voltage as a result of the rapid reduction of the energy stored inductively in the entire electrical circuit. 
   Said self-induced voltage heats and destroys semiconductor switching elements and protective circuits, causes material migration and welding at contact areas and can prevent the breaking of the electrical circuit as a result of arcing between contact areas. 
   During making of an electrical circuit, the capacitance present in the circuit has to be charged rapidly, which momentarily leads to a large switch-on current. 
   Said switch-on current brings about material migrations at contact areas that have not yet completely closed, and can destroy semiconductor switches through local thermal overloading. 
   During the transition of the switching elements from the conducting to the blocking state and from the blocking to the conducting state, a power loss is produced at the switching elements due to the simultaneous presence of current and voltage, said power loss being referred to as switching power. 
   In the case of frequent switching operations, this switching power leads to the heating of the switching elements and of adjacent components, and thereby jeopardizes the reliable operation of entire apparatuses and installation. 
   In order to protect the switching elements from the harmful effects of the self-induced voltage, use is made of RC circuits, but the latter are heated greatly in the event of high switching frequency. 
   Diode circuits, also known as freewheeling diodes, protect the switching elements from self-induced voltage only after a response time, but cannot be used with AC voltage, and cause a power loss during each switching operation, which limits the efficiency of frequently switching circuit arrangements such as voltage converters or switched-mode power supplies and leads to the heating and damage thereof. 
   Furthermore, varistor circuits are known, which protect the switch from particularly high self-induced voltages. However, said varistors are rapidly heated and are therefore unsuitable in the event of high switching frequency and high voltage and also for precise limiting of the overvoltages to low values, for the protection of semiconductor components. 
   It is also known that the self-induced voltage and heating of the switching element during interruption of the electrical circuit can be effectively limited by means of capacitor connected in parallel with the load or else in parallel with the switching element. However, this circuit has the disadvantage that, during the closing of the switching element, the capacitor would have to be short-circuited or abruptly charged, which causes very high switch-on currents, high switching losses and severe wear of the switching elements, so that the capacitance of the capacitor remains limited to a very small value and the effectiveness thereof is thus greatly restricted. 
   Taking this as a departure point, it is an object of the invention to specify a circuit arrangement which enables the reliable switching of electrical circuits. 
   SUMMARY OF THE INVENTION 
   The circuit arrangement according to the invention prevents the occurrence of high self-induced voltage by means of a capacitor which momentarily accepts the current from the electrical load circuit to be broken and, by means of its discharge operation, prevents a rapid rise of the voltage across the windings of the transformer and the opening switching elements connected in series therewith. 
   The—according to the invention—reliable switch-off operation of current-carrying switching elements is in this case achieved by avoiding voltage spikes, power loss and heating. 
   Avoiding power loss during the switch-off operation according to the invention also prevents the production of arcs in the case of electromechanical switching elements and fusible links and thus enables the latter to be reliably switched off according to the invention. 
   If large quantities of energy are present in the electrical load circuit, the circuit arrangement according to the invention may be configured such that the load is short-circuited after disconnection from the voltage source and the energy is held in the electrical load circuit. 
   During making of an electrical circuit, the load is connected to the voltage source by the circuit arrangement according to the invention via a transformer winding acting as an inductance, which brings about a slow controlled current rise and a slow controlled charging of the capacitance in the electrical load circuit. The slow controlled current rise and the small power loss in the switching element when the load current is switched on via an inductance enables an—according to the invention—reliable closing operation of the switching elements. 
   The voltage at the transformer winding is transformed to a second winding, which, when the capacitance in the electrical load circuit is charged to the voltage of the voltage source, brings a second switching element into a voltageless state in which it can be reliably closed according to the invention, with little power loss. 
   Since the circuit arrangement according to the invention enables the reliable switching operations substantially by avoiding power loss, the invention can make a significant contribution to miniaturization and reduction of costs for frequently switching apparatuses such as DC voltage converters, switched-mode power supplies, motor drives, since it permits significantly higher switching frequencies. 
   Reducing the power loss during switching operations also makes an important contribution to environmental protection. 
   Since the circuit arrangement according to the invention uses only one transformer and a usually very small capacitor for limiting damaging voltages and damaging rapid current rises, the circuit arrangement can be used for DC voltages and sinusoidal or rectangular AC voltages. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated in the drawing and are described in more detail below. 
     In the figures: 
       FIG. 1  shows a first exemplary embodiment of the circuit arrangement according to the invention, 
       FIG. 2  shows an example of alternating current direction in the load, 
       FIG. 3  shows an example with transistors and control device  33 , 
       FIG. 4  shows an example of the operation of the circuit arrangement according to the invention from two voltage sources, 
       FIG. 5  shows an example of the operation of the circuit arrangement according to the invention from one AC voltage source formed from a transformer winding with a center tap, and 
       FIG. 6  shows an exemplary application of the circuit arrangement according to the invention. 
   


   DETAILED DESCRIPTION 
   The circuit arrangement according to the invention has two series paths  1  and  2 , which are located in the lead to the load  3  and contain two connecting points  4  and  5  for the connection of the voltage source, and also two connecting points  6  and  7  for the connection of the load  3 . 
   The series path  1  is subdivided into two further series paths containing in each case a winding  8  and  9  of the transformer  10  and also in each case a switching element  11  and  12 , respectively. 
   Downstream of the transformer  10 , the two windings  8  and  9  are connected to one another and to the connecting point  6 . 
   A capacitor  13  is connected in parallel with the load between the series paths  1  and  2 . The capacitor  13  may be chosen to be very large in the case of slowly switching circuit arrangements or high load currents, and, in the case of rapidly switching circuit arrangements, be so small that the line capacitance between 1 and 2 suffices for obtaining the desired protective effect. 
   The load current  14  from series path  1  is divided into the component currents  15  and  16  such that the magnetic field strengths  17  and  18  thereof in the core of the transformer  10  act oppositely to one another and mutually compensate for one another. 
   If the two component currents  15  and  16  in the windings  8  and  9  are of the same magnitude, the load current  14  cannot store energy in the transformer core. 
   If the component current  15  is interrupted by the switching element  11 , then its compensating field strength  17  is omitted, as a result of which the remaining current-carrying winding  9  takes effect as an inductance and momentarily interrupts the remaining component current  16  since the transformer core is not yet magnetized. Therefore, immediately after the interruption of the component current  15 , the load current  14  is drawn completely from the capacitor  13 , so that the remaining, second switching element  12 , according to the invention, can be reliably opened in a virtually currentless and voltageless state with little power loss. 
   The capacitor  13  is discharged by the load current  14  after the opening of switching element  11 , as a result of which the capacitor voltage falls, and a voltage is produced across the transformer winding  9 , which is transformed to the transformer winding  8 . The voltage in transformer winding  8 , together with the voltage across capacitor  13 , has the effect that the switching element  11 , according to the invention, can open reliably in a virtually voltageless state, with very little power loss. 
   In the simplest case, the switching elements  11  and  12  may comprise a switching contact, a fusible link or a transistor, which undergo transition to the nonconducting state simultaneously, or with a short delay, when the electrical circuit is interrupted. The switching elements  11  and  12  may also be formed as changeover contacts, push-pull or CMOS transistor stages which, after the disconnection of the series path  1  from the connecting point of the voltage source  4 , establish a connection to the series path  2  in order to conduct away the load current  14  and to hold the energy stored in the load  3  in the electrical load circuit. 
   In order to conduct away the energy stored in the electrical circuit, the switching elements  11  and  12  may also be provided with diodes which conduct away the load current  14  after the disconnection from the voltage source to the series path  2 . 
   A further possibility is to measure the voltage across the switching elements by means of a voltage measuring device and to establish the connection to the series path  2  with a controllable switching element when the measured voltage has become zero, thereby achieving an—according to the invention—reliable, low-loss closing operation in a voltageless state. 
   The load can be connected to the voltage source again by only one or both switching elements  11  and  12  interrupting the connection to the series path  2 , and then only one switching element connecting the corresponding transformer winding to the series path  1 , so that the capacitor  13  is charged via the winding inductance of the transformer  10 . In this case, according to the invention, the winding inductance prevents a rapid rise of the charging current and thus enables, according to the invention, the reliable, low-loss closing of the switching element in a virtually currentless state. If the capacitor  13  is completely charged, the second switching element may likewise establish the connection to the voltage source in low-loss fashion in a virtually voltageless state reliably, according to the invention, whereupon the component currents  15  and  16  in the windings of the transformer match one another. 
   A further embodiment of the circuit according to the invention consists in the fact that the load  3  is connected to the series path  1  or  2  via a capacitor  19  connected in series. This results in an alternating current direction in the load, which permits the capacitor  13  to be charged to the voltage of the voltage source by the load current in the event of switch-on. The two switching elements  11  and  12  can then establish the connection to the voltage source in a currentless and voltageless state without energy being fed into the transformer  10 . 
   The circuit according to the invention may likewise be used on voltage sources of alternating polarity for rectifying the current and for regulating the power drawn from the voltage source. In this case, at the beginning of the positive half-cycle, the transformer  10  is connected to the voltage source by the switching elements  11  and  12  via the series path  1  and, during the positive half-cycle, after a time period determined by a suitable regulating device, is again connected to the neutral conductor of the voltage source, series path  2 , in order to generate a positive rectified current for the load. At the beginning of the negative half-cycle, the transformer  10  is connected to the voltage source by the switching elements  11  and  12  via the series path  1  and, during the negative half-cycle, after a time period determined by a suitable regulating device, is again connected to the neutral conductor of the voltage source, series path  2 , in order to generate a negative rectified current for the load. 
   The circuit according to the invention may furthermore be used on voltage sources of alternating polarity with a neutral conductor, such as, for example, the secondary winding of a transformer with a center tap, for rectifying the current and for regulating the power drawn from the voltage source. In this case, at the beginning of the positive half-cycle, the transformer  10  remains connected by the switching elements  11  and  12  to the—at this point in time—positive terminal of the voltage source, and, during the positive half-cycle, at a switching instant determined by a suitable regulating device, is connected via the switching elements  11  and  12  to the—at this point in time—negative terminal of the voltage source. This connection persists to the end of this half-cycle, and through the polarity reversal of the voltage source right into the next half-cycle. In this way, by defining the switching instant, it is possible for the load to be fed a positive current if the switching instant lies in the second half of the half-cycle, a negative current if the switching instant lies in the first half of the half-cycle, and no current if the switching instant lies in the center of the half-cycle. 
     FIG. 6  shows the possibility of how the circuit described in the application can be used as an active impedance. It can thereby be used very simply as a replacement for an ohmic resistor for current limiting. In this case, the circuit is completely encapsulated and, like a simple impedance, provided only with two terminals. One path is thus not connected. 
   The circuit may acquire an ohmic characteristic, be embodied as a voltage source or current source, or operate with an additional control input as a potentiometer or power controller, an extremely low power loss occurring in each case. 
   In the case of the circuit illustrated in  FIG. 6   b , the power loss amounts to only approximately 0.05*U*I, that is to say approximately 5 percent of the power loss which would occur at the ohmic resistor of  FIG. 6   a . The value of the impedance is dependent on the value of the inductance L  10  and the capacitance C  13 . 
   Subclaims relate to further refinements of the invention. In this case, such feature combinations for which no express example has been specified are also to be regarded as claimed.
A circuit arrangement for the reliable switching of electrical circuits contains two series paths, two switching elements being arranged in parallel with one another in one of the series paths, the switching inputs of said switching elements being connected to the input point of the series path and the switching outputs of said switching elements being connected to the input side of a respective winding of a transformer.