GB2087113A - Device for voltage regulation in direct current electrical systems - Google Patents

Abstract

A direct current regulator for motor vehicles and the like gives a constant output voltage in the case of a greatly variable input voltage, even when the throughput power is high. It comprises a blocking converter circuit in which a by-pass connection (1), optionally controllable, leads to the secondary winding of the converter transformer (20) from the input (23). In the step-up regulation mode, the transistor (2) is repeatedly switched on and off in the manner of a blocking converter but a proportion of the power to be transmitted is fed to the secondary side by way of this by-pass connection (transistor 1), whilst, in the step-down regulation mode, the blocking transistor (2) is kept off while the by-pass connection (transistor 1) is repeatedly interrupted, the decay current through the secondary winding (5) flowing through a free-running diode (4). <IMAGE>

Description

SPECIFICATION Device for voltage regulation in direct current electrical systems The invention relates to a device for regulation in direct current electrical systems on mobile units, such as motor vehicles and railway rolling stock.

Despite the high demands made on them, it is known to influence current generators, such as three-phase generators with rectified output in motor vehicles or the like, by regulating the excitation current fed thereto such that the generator output voltage is maintained at a substantially constant, desired level. The excitation current, and thus the excitation field in the rotor of the generator, are thus controlled in dependence upon the voltage generated in the generator such that the generatortermi- nal voltage remains constant up to the maximum current despite a rotational speed which fluctuates considerably between idling speed and full load, and despite considerable fluctuations in the load on the generator. Mechanical single-contact regulators or multi-contact regulators are known.Regulators used nowadays are chiefly electronic transistorised regulators which regulate the generator voltage by periodic attenuation of the excitation current, normally by periodically switching it on and off, since the voltage generated in the generator is substantially proportional to the product of the rotational speed and the excitation current.

Problems in this connection arise, for example, in motor vehicle electrical systems in winter or in town traffic, since the internal resistance of the battery increases to a considerable extent at low temperatures, so that there is a considerable reduction in the cold starting performance of the battery. Furthermore, battery charging problems arise in the case of low rotational speeds. Although, with corresponding design of the generator and thus a correspondingly adequate generator output, an adequate charging current could be produced even at low temperature by at least intermittently increasing the charging voltage, the voltage of the electrical system cannot be optionally increased in view of the voltage-sensitive loads, such as the incandescent lamps which are connected to the distribution system.

Thus, future increasing demands on the quality of the supply voltage in vehicle electrical systems will require a central, efficient voltage regulator which, with a variable input voltage, and despite a considerable power flowing through the voltage regulator, is able to make available an output voltage which is as constant as possible.

The present invention resides in a device for voltage regulation in a direct current electrical distribution system which receives its electrical power from a rotary-driven generator connected to a battery for storing electrical energy, said device comprising a blocking converter circuit having a transformer primary winding and a transformer secondary winding with a given winding ratio and at least one active semiconductor switching element in series with the primary winding, the input of the blocking converter circuit being connected to the variable, rectified generator output voltage and a constant regulated output voltage of the converter circuit being connected to the electrical distribution system, said device further comprising at least one connection lead for connecting the input region to the region of the secondary winding to by pass the primary winding of the blocking converter circuit.

This has the advantage that, in electrical distribution systems, it is in the first instance generally possible to generate a substantially constant output voltage when the input voltage fluctuates between considerable extreme values, even during high consumption of power. Thus, in a 12 volt system which, in the present instance, is specified by way of example, the input voltage could lie between the values of from 10 to 18 volts while the output voltage is maintained constant at 13.8 volts.

It is thereby possible for the usual voltage regulator fitted to the generator to adjust the generator voltage for example, in conformity with the requirements of the battery and to avoid the conventional direct electrical connection of the battery and the electrical distribution system, this connection then being undertaken by the device in accordance with the invention (optionally acting as a second regulator) which maintains the voltage of the electrical distribution system at its desired value in the case of a varying input voltage and a varying load.It will be appreciated that it is then possible to decouple uncritical loads, such as rear window heaters and the like based on pure resistance heating, from the electrical distribution system maintained art a constant output voltage and to associate the electrical distribution system with the loads which require a constant voltage of the electrical system or which react thereto with a particularly long working life, such as lamps or electronic loads.

It is advantageous that the very accurate voltage regulator device in accordance with the invention can be manufactured in a simple and inexpensive manner and can also be installed existing systems.

Thus, it is possible to free oneself from the rigid constraints of an electrical system voltage which is as constant as possible for only a few voltagedependant loads and to conform with, for example, the greatly varying charging power requirements of, for example, the battery. Modern electrical distribution systems fundamentally constitute a compromise between, for example, the charging power requirement of the battery on the one hand and, on the other hand, the requirement ofthe incandescent lamp used for a voltage which is as constant as possible.

Most embodiments of the invention are operable in two modes, viz., step-down regulation in which the by pass connection is intermittently interrupted to regulate the output voltage and step-up regulation in which the by pass connection is kept closed and the switching element is intermittently closed to regulate the output voltage. In advantageous embodiments an economy circuit or auto-circuit is achieved such that it is additionally possible to reduce the semiconductor switching load and thus the power loss.

The invention is further described, by way of example, with reference to the drawings, in which: Fig. 1 is a circuit diagram of a blocking converter, Figs. la and lbare graphs ofthe input and output current of the blocking converter illustrated in Fig. 1, Fig. 2 is a circuit diagram of a first embodiment of a voltage regulating circuit, in accordance with the invention, in the form of a blocking converter economy circuit, Figs. 3 to 6 are circuit diagrams of further embodiments of blocking converter economy circuits, and Figs. 7 and 8 are block circuit diagrams of two embodiments of trigger for triggering the at least one semiconductor provided in each voltage regulator circuit in accordance with the invention.

In order to facilitate comprehension of the embodiments, in accordance with the invention, illustrated in Fig. 2, Fig. 3, Figs. 4, 5 and Fig. 6 the basic circuit diagram of a known blocking converter (illustrated in Fig. 1) will first be briefly discussed and its use with reference to maintaining an output voltage of an electrical distribution system constant will be explained. If the regulating task already described above by way of example is based on converting an input Ue laying between 10 volts and 18 volts constank to an output voltage Ua of, for example, 13.8 volts, the switching transistor Tr of the blocking converter illustrated in Fig. 1 must be rated or dimensioned for at least a voltage of uT=uemax+ua/u (1 )t (1), and a current of T=2 . (u+ua/uemin).ia . . .. (2), where Li is the trans ratio of the transformer Tf.

These symbols are also given in the current curves of Figs. la and 1 b, T,+T2=T being the operating period of the blocking converter.

The blocking converter illustrated in Fig. 1 comprises the actual converter or transformer Tr having a primary winding W1 and a secondary winding W2 with the step-up ratio ü=w2/w1. A semiconductor switch, preferably a transistor Tr, is connected in series with the primary winding W1. Alternatively, however, other suitable semiconductor switches can be used, such as thyristors of suitable construction and which are triggered in a suitable manner. The construction of the blocking converter illustrated in Fig. 1 is completed by an input capacitor Ce which is connected in parallel with the series combination comprising the primary winding and the semiconductor switching element, and to which the input voltage ue is applied.On the output side, the secondary winding of the converterTr is connected in series with a diode D, and an output capacitor Ca, across which the output voltage ua appears, is connected in parallel with the secondary winding and the diode D. The mode of operation of a blocking converter of this kind is shown in Figs. 1a and 1 b.

During the "on" period T1 of the semiconductor switch, the current i, flowing through the primary winding W1 increases to the peak current 1T. After the transistorTr becomes non-conductive, a voltage is induced in the secondary winding of the converter and leads to a corresponding output current ia hav ing the peak value D. The input current ie and the output current 1a are mean arithmetical values of the peak current characteristics.

The switching load or load handling capacity of the transistor Tr is obtained by combining equations (1)and (2) to form:

The equations ua=const. and uemin --ue -u,rnaxthen also apply in accordance with supposition.

Forgiven voltages and currents, Li is chosen such that the switching load PT is minimised:

If the product ua. .ia is designated throughput power PD and the term c=ueaxIuemin is introduced as an abbreviation, there is obtained: 2opt = 2 Ua iå (1 + c + 2#c) = 2'(1 + ' -D (5) when the equation (4) is introduced into the equation (3).

With a constant input voltage, that is to say, when c=1.0, the required switching load is then eight times the throughput power; with the numerical values of the example of a 12 volt system specified above, c=1.4 and the switching capacity is increased to eleven times. This gives rise to considerable problems in the case of large throughput powers in the case of an electrical distribution system of a mobile unit in which approximately 300 watts are required as the average value of the output power of the voltage regulator, and to considerable power losses it should be possible to make available semiconductor switching elements having such high switching loads.

The embodiment illustrated in Fig. 2 drastically reduces the power required and nevertheless ensures a highly accurate voltage regulator which can be economically used in any optional electrical distribution systems, that is to say, in a distribution system which receives its electrical power from a generator driven at greatly fluctuating speeds and itself fitted with a voltage regulator and used to charge a battery. Electrical isolation between the input and the output is not provided, since it is unnecessary. The transformer of the blocking converter is designated 20 in Figs. 2 and has a primary winding 6 and a secondary winding 5. The primary winding 6 is connected in series with a first semiconductor switch 2 and, together with the input capacitor 21, thus forms the input circuit.The rectifier diode 3, already provided in the basic circuit diagram of the blocking converter of Fig. 1, is connected in series with the secondary winding 5. However, a second "free-running" diode 4 is also provided for conducting transients and connects the other terminal of the secondary winding 5 to earth or to the second pole of the circuit and is biassed in the same forward direction as the rectifying diode 3. In a development of the present invention, again referring to Fig. 2, the junction between the secondary winding 5 and the free-running diode 4 is connected via a by-pass connection comprising a second semiconductor switching element 1 to the input terminal 23 of the voltage regulator of Fig. 2 to which, for example, a rectified and regulated alternator output voltage is fed when used for the electrical distribution system. The lead 24 is then the negative pole.

The first embodiment of a high performance voltage regulator in accordance with the invention then functions as follows. If the input voltage is greater than the output voltage, the circuit operates on the principal of step-down regulation and the switching transistor2 is permanently non-conductive. In this mode, the switching transistor 1 is rendered alternately conductive and non-conductive. The current flowing through the transistor 1, the secondary winding 5 of the transformer 20 and the diode 3 increases during "on" period te of the switching transistor 1, as is also illustrated in Fig. 1a.During the non-conductive period ts of the switching transis tor 1, the further flow of current induced by the indutance of the secondary winding 5 can pass through the free-running diode 4 instead of the transistor 1, although no further energy is fed from the input side.

The value of the output voltage plotted against the duty edge of relative duty ratio is then:

When in this mode, the circuit operates on the principle of a so-called step-down adjuster. However, if the input voltage is lower than the output voltage, normally only for short periods, the basic principle of step-up regulation ensues and the switching transistor 1 is in the first instance switched to a permanently conductive state so as to maintain open the circuit by-passing the primary winding 6. In this second mode, the switching transistor 2 is rendered conductive and non-conductive alternately, transformed power then being transmitted to the output side, although this power is only a fraction of the throughput power PD which a blocking converter would have to transmit in the full circuit of Fig. 1. The transformed power transmitted by the circuit of Fig.

2 during step-up regulation is then onlyP=(ua-ue) .ia. Thus voltage regulator (not illustrated) fitted to the generator can be designed or adjusted to provide a generator output voltage ue which varies to conform with optimum requirements for charging the battery whilst the converter output voltage ua is maintained substantially constant to suit the voltage-sensative loads connected to the distribution system.

The following data give a few more details with reference to the switching loads to be bourne by the individual switching transistors during step-down and step-up regulation.

The switching transistor 1 is subjected to the following switching load during step-down regulation: PT1 = UemaX-ia ' ks (7) wherein the constantk can be reduced from its maximum value k=2 to the vicinity of its minimum value k=1 by increasing the switching frequency.The magnitude of the switching load of the switching transistor 2 during step-up regulation is as follows:

If the following abbreviations are introduced

the total switching load amounts to

With given voltages and currents, U is again chosen such that the switching load PT is reduced to a minimum tiopt = 1 - 1 (10) Using the throughput power PD already defined, and introducing the equation (10) into the equation (9), the following data are obtained for the ratio of the throughput powers to be switched Popt = 8 Cl - b+ k8.a) . PD (11) Thus, when an auto-circuit, in accordance with the invention, based on the blocking converter principle is used in a 12 volt system for the numerical example given above, the transistor switching load required is reduced to at least 4.8 times the throughput power, and, in the most advantageous case, to 3.5 times the throughput power, compared with the 11 times amount calculated further above.

A further embodiment of a voltage regulator, in accordance with the invention, for electrical distribution systems, is illustrated in Fig. 3 in which the transistor switching load can be additionally reduced provided that the transition between the two modes i.e. between step-up regulation and step-down regulation only occurs infrequently and then relatively slowly. However, it must first be pointed our that the input or control electrodes of the switching transistors used in each case must be supplied with corresponding control or trigger pulse trains whose duty ratio, and, optionally frequency are adjusted and related to the desired ratio of the input voltages to the output voltages such that the voltage can be maintained constant in the manner desired.An embodiment of a circuit of this kind produces the trains of trigger pulses is illustrated in Fig. 7 and will be described in greater detail further below.

Only one switching transistor 1' is required in the embodiment of Fig. 3, change over-contacts additionally being provided which, advantageously, are controlled by a relay 7 having three, optionally four, contacts 8, 9 10 and 12.

In the mode of step-down regulation, the relay 7 connects the output terminal of switching transistor 1' directly to the secondary winding 5 of the transformer 20 by way of the contact 8 which is then closed, the accurate regulating ratio between input voltage and constant output voltage being determined by the duty ratio of the train of control pulses then fed to the switching transistor 1'.

In the mode of step-up regulation, the output terminal of the same transistor is connected to the primary winding 6 of the transformer 20 by way of the switch contact 9 which is then closed, a then closed switch contact 10 of the relay 7 at the same time connecting the input voltage at the input 23 to the junction between the secondary winding 5 and the free-running diode 4. The relay 7 is triggered by way of a trigger circuit 11, which is responsive to the value of the input voltage, and is pulled in for stepdown regulation when the input voltage exceeds a predetermined value at which the transition from step-up regulation to step-down regulation has to be effected.The trigger circuit 11 thus has a reference voltage circuit which is internally compared with the input voltage fed to the circuit: the relay 7 is then actuated in conformity with the output voltage of a comparator used, for example, for this comparison.

In this circuit of Fig. 3, the switching load required for single resistor 1' still provided is reduced to the larger of the values PT1 and PT2 which are calculated in accordance with the equations (7) and (8) already given above: P = a'PD .k (lea) or PU2 = 8,(1-b).pg (13) The switching load PTa is thereby relevant to the more applicable mode in which 8.(1-b) > a.k. In this numerical example given above, the transistor load required is reduced to 2.2 times the throughput power in the most favourable case, and a saving of 80% of the transistor switching load can be achieved compared with the initially described blocking converter in full circuit.

A further advantage of this voltage regulator, illustrated in Fig. 3, having relay switching resides in the fact that, in the step-up regulation mode, the total power loss of the transistor 1 is saved (compared with Fig. 2), since this transistor is replaced by the contact 10 in Fig. 3. The total power loss can thereby be reduced to approximately half.

Furthermore, the diode 3 no longer required during step-down regulation can be by-passed without great expense by means of a further switch contact 12 associated with the relay 7, so that the power loss can also be reduced in this mode.

In conformity with a further development of the present invention, it is otherwise possible, in a modification as shown in Fig. 4, to use a blocking converter voltage regulator in a particularly advantageous manner when step-up regulation is exclusively envisaged and is to be taken into account. The freerunning diode 4 and the switching transistor 1 shown in Fig. 2 are omitted in the modification of a blocking converter economy circuit illustrated in Fig.

4,so that, with permanent electrical connection of the voltage at the input to the secondary winding 5 by way of the connection lead 25, only the switching transistor 26 is provided which is connected to the primary winding 6 of the transformer 20. The transistor load required by this modified circuit is obtained from the above equation (13), and, in this case also, shows a distinct reduction.

In the embodiment of Fig. 5, it is possible, by a further modification, to switch between full circuit and auto-circuit of the blocking converter voltage regulator according as to whether the output voltage is lower (step-down regulation mode) or higher (step-up regulation mode) than the input voltage.

In this embodiment, the secondary winding 5 of the transformer 20 is connected by way of a change-over contact 14 either to the bottom earth lead orto a connection lead 25' which connects the secondary winding 5 directly to the input voltage of the circuit, as is also shown in Fig. 4. The changeover contact is actuated by means of a relay 13 which can be actuated by a trigger circuit 11 which is identical to that in the embodiment of Fig. 3 and which is able to distinguish between step-down reg ulation and step-up regulation. Thus, one achieves the same end as in the circuit of Fig. 4, but step-down regulation is also possible. In the present case, the transistor switching load required increases to 8 times the throughput power in conformity with equ ation (5) in which c=1.However, in return for this, the logic circuit required for triggering the switching transistor 26 is simpler in this case, since regulation is always effected in accordance with the same scheme.

In the embodiment of Fig. 6, the previously described possibility of switching between the full circuit and the auto circuit of the blocking converter voltage regulator is achieved by a thyristor 30 and the diode 4 which are combined to replace the change-over contact 14 of the relay 11 of Fig. 5. This modification is advantageous when there is frequent and/or rapid alteration between the step-up regulation mode and step-down regulation mode.

In this embodiment, the secondary winding 5 of the transformer 20 is connected on the one hand to the bottom earth lead by way of the diode 4 and, on the other hand, to the input lead 23 carrying the positive potential by way of the thyristor 30, as is shown in Fig. 6. In the step-up regulation mode, the thyristor is fixed continuously by a trigger circuit 11'.

It will be appreciated that the economic optimum in the circuits illustrated in Figs. 3,4, 5 and 6 is dependent upon the required throughput power and the prescribed characteristic of the input voltage, and thus has to be correspondingly ascertained in each case.

The possible construction, and mode of operation, of a logic circuit for triggering the switching transistors will be briefly explained hereinafter with reference to Figs. 7 and 8.

The logic circuit of Fig. 7 required for triggering the at least one switching transistor 1, 2, 1', 26 chiefly comprises a proportionally and integrally acting control circuit 28, a generator module 27 for generating a square-wave voltage characteristic whose duty ratio or duty cycle is varied in a modulator 31 connected on the output side in conformity with the set or reference signal of the control circuit 28, a trigger circuit 32 for triggering the switching transistors of the embodiments specified above, and a comparator 11 for controlling the switching relay 13 (see Fig. 5).

The output voltage ua to be regulated to a constant value is fed to the input 28a of the control circuit 28 by way of an adjustable voltage divider 33a, 33b, and the control circuit compares the said output voltage with the constant desired voltage which is present at its input 28b and which, by using at least one switching element, such a Zener diode 29, supplying a constant voltage value, is obtained as a reference voltage from the input voltage ue variable between uemjn and uemax.

The function is such that any differential voltage at the inputs 28a and 28b of the control circuit 28 is proportionally amplified and/or integrated and is fed as a continuously variable adjusting quantity to the modulator module 31 which varies the duty ratio of the square-wave voltage, supplied to its input 31 b from the generator 27, in conformity with the adjusting variable at its other input 31 a. Generators which generate, at their output, a control pulse train having a varying duty cycle in dependence upon a varying analog input voltage, are known. In the case of a square-wave pulse train, the term "duty cycle" relates to, for example, the "on" time relative to the overall period, so that the duty cycle has the value 1/2 in the case of equal "on" and "off" times.

The square-wave voltage having a variable duty cycle is subsequently fed to a trigger circuit 32, which can be in the form of a conventional amplifier, and is converted to suitable voltages and currents for triggering the respective control transistor 1,2, 1', and 26 such that the voltage difference or error signal present in the control circuit 28 is reduced to zero. The circuit block 32' shown in Figs. 7 and 8 constitutes the entirety of the voltage regulating circuit in accordance with the invention and which has already been comprehensively explained with reference to Figs. 1 to 6.

The input voltage ue variable between uemin and Uemax is also fed to the input 11 a of the comparator 11 by way of an adjustable voltage divider 34a, 34b, and the comparator compares this voltage with the reference voltage of the switching element 29 present at its other input 11 b. It will be appreciated that, alternatively, the reference voltage can be produced in an individual reference circuit associated only with the comparator 11. The switching threshold of the comparator is set to, for example, ue -Au = ua by the adjustable voltage divider 34a, 34b, Au beside others designating the unavoidable voltage drops across the switching transistors 1', 26 and in the diode 3.An absolute signal then appears at the output of the comparator 11 and merely indicates whether regulation has to be step-down in the case ue- u ua or step-up in the case ue -A u < ua, and which can be used directly, or after suitable amplification, to actuate the relay 7 or 13. The comparator 11 can be dispensed within the case of exclusive step-up regulation in accordance with Fig. 4.

If the change-over contact 14 of the relay 13 is replaced by, for example, the thyristor 30 of Fig. 6, a comparator 11' having an inverted output signal must be used instead of the comparator 11 for the purpose of triggering the gate terminal of the thyristor.

In the case of control by way of two transistors 1 and 2, as is shown in Fig. 2, the outputs of the comparator 11 and of the modulator 31 can be connected to the trigger circuits 32a and 32b of the transistors 1 and 2 by way of, for example, three logic gates 35,36 and 37, as is shown in Fig. 8. Thus, as already described above, in the step-up regulation mode when the comparator 11 is at logic level "0", the transistor 2 is switched on and off cyclically in the manner predetermined by the modulator 31, the transistor 1 at the same time being continuously conductive, whilst, in the step-down regulation mode when the comparator 11 is at logic level "1", the transistor 1 is switched on and off cyclically with the transistor 2 at the same time permanently nonconductive. For this purpose, a respective AND gate 35 or 36 is connected to the outputs of the modulator 31 and of the comparator 11 which can be identical to the corresponding circuit elements of Fig. 7.

Alternatively, the AND gates 35, 36 can be triggered in a cross-over manner by the modulator and the comparator. The AND gate 35 acts upon the switching transistor 2 directly by way of a trigger circuit 32a, while a function block 37 is connected to the output of the AND gate 36 and, in the illustrated embodiment, only allows, for example, positive signals to pass. The switching transistor 1 is then triggered by way of a further trigger circuit 32b. The AND gate 35 and the function block 37 are otherwise controlled by the negated output signal of the comparator 11.

Claims (14)

1. A device for voltage regulation in a direct current electrical distribution system which receives its electrical power from a rotary-driven generator connected to a battery for storing electrical energy, said device comprising a blocking converter circuit having a transformer primary winding and a transformer secondary winding with a given winding ratio, and at least one active semiconductor switching element in series with the primary winding, the input of the blocking converter circuit being connected to the variable, rectified generator output voltage and a constant regulated output voltage of the converter circuit being connected to the electrical distribution system, said device further comprising at least one connection lead for connecting the input region to the region of the secondary winding to by pass the primary winding of the blocking converter circuit.

2. A device as claimed in claim 1, in which a rectifier diode is connected in series with the secondary winding.

3. A device as claimed in claim 1 or 2, in which a free-running diode is connected in series with the secondary winding, the by pass circuit being connected to the junction between the secondary winding and the free-running diode.

4. A device as claimed in claims 2 and 3, in which the rectifier diode is biassed in the same direction as the free-running diode.

5. A device as claimed in any of claims 1 to 4, in which the by-pass circuit comprises a first semiconductor switch which is connected between one input terminal of the blocking converter circuit and one end of the secondary winding whose other end is connected to one output terminal of the converter circuit, and a free-running diode is connected between the said one end of the secondary winding and the other output terminal of the converter circuit which other output terminal is common with the other input terminal to the converter circuit, and in which said active semi-conductor switching element comprises a second semiconductor switch which is connected between said one input terminal and one end of the primary winding whose other end is con nectedto said common other terminals, the arrangement being such that the device operates in two modes, one for step-down regulation and the other for step-up regulation, and such that, in each mode, one of the two semiconductor switches is alternately switched between its conductive and non-conductive states, and the other semiconductor switch either remains switched off or remains switched on.

6. A device as claimed in claim 5, in which a control logic circuit is provided for feeding to the respec tive, alternately switched semiconductor switch a train of trigger pulses whose duty cycle is such that the desired constant output voltage is obtained with avarying input voltage.

7. A device as claimed in any of claims 1 to 4, in which the by-pass circuit comprises a first contact of a relay actuable in dependence upon the ratio of the input voltage to the output voltage and is con nected between one input terminal of the converter circuit and the secondary winding and in which the active semiconductor switching element comprises a switching transistor connected between said one input terminal and second and third contacts of the relay, which contacts lead respectively to the prim ary winding and to the secondary winding, the arrangement being such that the device operates in two modes, one for step-down regulation and one for step-up regulation and such that in one mode the first and third relay contacts are closed and the sec ond relay contact is open and vice versa in the other mode, the switching transistor being alternately switched between its conductive and non conductive states in both the modes.

8. A device as claimed in claims 2 and 7, in which the relay has a fourth contact which shunts the rectifier diode when the device is operating in the step-down mode.

9. A device as claimed in claim 1 or 2, which is operable in only one mode, viz. step-up regulation and in which the end of the secondary winding, remote from the end thereof connected to one out put terminal is connected directly to one inputter minal and the other output terminal is common with the other input terminal.

10. A device as claimed in claim 1 or 2, in which the semiconductor switching element comprises a switching transistor connected to the primary wind ing and in which the by-pass connection comprises a changeover contact of a changeover relay which changeover contact in its other state connects the secondary winding between the two output termi nals of the device, the arrangement being such that the device operates in two modes, one for step down regulation and the other for step-up regulation according to the state of the changeover contact, the switching transistor being alternately switched between its conductive and non-conductive states in both modes.

11. A device as claimed in claim 3 or4, in which the by-pass connection comprises a thyristor and the arrangement is such that the device operates in two modes, one for step-down regulation and one for step-up regulation and such that the thyristor is non-conductive in the step-down regulation mode in which the free-running diode is effective and remains conductive in the step-up regulation mode and connects one input terminal to the secondary winding, the free-running diode thereby being rendered non-conductive.

12. A device as claimed in any preceding claim, which further comprises a logic circuit for providing a control pulse train having a variable duty cycle to control the switching element the logic circuit including a frequency generator and being fed with a reference value for the output voltage to be kept constant and with the actual output voltage.

13. A device as claimed in claim 12, in which the logic circuit includes a control circuit to which are fed the reference value obtained fromthe variable input voltage on a switching element supplying a constant voltage value, and the actual output voltage, a modulator connected to the output of the control circuit for influencing the duty cycle of the control pulse train in accordance with the output voltage of the control circuit, and a trigger circuit by which the modulator is connected to the respective switching element.

14. Adeviceforvoltage regulation constructed and adapted to operate substantially as herein described with reference to and as illustrated in Figs.

2 to 8 of the drawing.