SHUTDOWN CONTROL MEANS FOR AN ELECTRIC POWER SUPPLY

Abstract: There is described control means for shutting down a constant current supply (21) suitable to a double-end power feeding system for a long-haul wire telecommunication sys­ tem. The sutdown control means comprises a voltage detect­ ing circuit (26), a current detecting circuit (25) and an AND gate (27). The voltage and current detecting circuits (26, 25) pro­ vide the respective detecting signals when the constant cur­ rent supply (21) provides an output voltage larger than a predetermined voltage value and the current at the output voltage is smaller a predetermined current value. The prede­ termined voltage and current values are determined corres­ ponding to a input resistance of a load circuit to which the feeding of a current by the constant current supply (21) is stopped. Thus, the shutdown condition of the constant cur­ rent supply (21) can be determined free from the drooping characteristic of the constant current supply (21) and hence the withstand voltages required for the circuit components and the assemblage thereof for the constant current supply (21) and the electronic equipment fed by the current supply (21) can be reduced. There are also described embodiments in which the basic concept of the invention is applied to constant voltage supplies connected in parallel to each other to a load circuit.

Description: BACKGROUND OF THE INVENTION The present invention relates to an electric power feeding system, more particularly to an improvement in the control means for shutting down a constant current supply for a wire telecommunications system operated in a double-end feeding mode. In a long-haul wire telecommunications system, a submarine cable transmission system, for example, a number of repeaters between the terminal stations are fed with an electric power from constant current supplies installed at both or one of the terminal stations. These two modes of a power feeding are referred to as double-end feeding and single-end feeding, respectively. General discussions on the configuration and operation of the constant current supplies in a double-end feeding system for a wire telecommunication system are given in the following with reference to FIGs.1 and 2. FIG.1 is a conceptual circuit block diagram of a double-end power feeding system. The system comprises a number of repeaters from 1 to n and constant current supplies 11 and 12 positioned at both terminal stations. The repeaters are connected in series with a transmission line 50, a submarine cable, for example, between the respective output terminals 111 and 121 of the current supplies 11 and 12. Each of the constant current supplies 11 and 12 includes respective ones of constant current generating sources 13 and 17, resistors 14 and 18, over-voltage protecting elements 15 and 19, and bypassing diodes 16 and 20. The resistors 14 and 18, which are referred to as slope resistors, are for providing the current-voltage characteristic curve of the sources 13 and 17 with a desired tilt in the constant current region, as explained later. The over-voltage protecting elements 15 and 19 of such as voltage limiters are for protecting the repeaters from a transient over-voltage caused when the power feeding system is open-circuited, for example, and the bypassing diodes 16 and 20 are for keeping the wire telecommunications system operable even when one of the current supplies 11 or 12 becomes inoperable. In a submarine cable transmission system using fiber optics, hundreds of repeaters are connected in series with a span of approximately 30km between each adjacent ones, and a voltage E of 7000volts, for example, is fed from each of the constant current supplies 11 and 12. As shown in FIG.1, the constant current supplies 11 and 12 are connected in series in the circuitry of the power feeding system. That is, the polarities of the output voltages E of the constant current supplies 11 and 12 are opposite to each other with respect to ground. Accordingly, the potential at the mid-point of the power feeding line 50 is at the ground level. Thus, the output voltage E in the double-end feeding system is approximately one half as compared with the voltage necessary when the repeaters are assumed to be fed by a single-end feeding system. This implies that the withstand voltages required for the circuit components and the assemblages thereof for the power feeding system including the constant current supplies, repeaters and the power feeding line can be reduced one half of those required in a single-end power feeding system and, hence, the design and cost reduction of a power feeding system can be facilitated by employing the double-end feeding configuration. As the distance between the terminal stations increases, therefore, the number of repeaters increases, the feeding voltage E becomes higher, and such double-end feeding is an indispensable technology to a long-haul wire telecommunication system. However, it is important in the double-end feeding that each of the constant current supplies therefor should not have so large power as sufficient for a single-end feeding system. In other words, the constant current supplies is designed to be merely applicable to the double-end feeding. The inconvenience of the constant current supplies having such large power for a double-end feeding is described in the following. FIG.2 is the current-voltage characteristic diagram of a constant current supply which is capable of providing the system as shown in FIG.1 with a necessary power even in a single-end feeding mode. Referring to FIG.2, the current and voltage supplied by the constant current supply are regulated along the solid line in accordance with the change in the load resistance connected thereto. The dotted line connecting the origin 0 and the point "a" on the current-voltage curve corresponds to a load characteristic curve when the constant current supply is operated in a double-end feeding mode, and the dotted line connecting the origin 0 and the point "b" on the current-voltage curve corresponds to a load characteristic curve when the constant current supply is operated in a single-end feeding mode. Therefore, the points "a" and "b" are the respective stable points for the operations in the two-modes. That is, the output voltage of the constant current supply becomes steady at the voltage Ea in the double-end feeding mode and Eb in the single-end feeding mode. The current-voltage curve exhibits a slope having a negative tilt in the voltage range between 0 and Ec. The negative tilt is provided by the aforesaid resistor 14 or 18 so as to reduce the voltage fluctuation during the current regulating operation. The constant current supply reveals a so-called drooping characteristic in a voltage higher than Ec. That is, as the voltage increases, the current begins to decrease at the point "c" corresponding to the voltage Ec and becomes zero at the voltage Ee. Accordingly, in a normal operation, the repeaters are prevented from the application of a voltage larger than Ee. However, there may be a fast transient over-voltage higher than Ee, which is caused when the power feeding system line is abruptly cut off, for example. The aforesaid over-voltage protecting elements 15 and 19 in FIG.1, having a threshold voltage Ef, are provided to protect the repeaters from such transient over-voltage higher than Ef. Further, the constant current supply is generally equipped with a facility for shutting down thereof from supplying a current to the repeaters when the current supply loses its regulation function or when the load resistance on the power feeding line becomes larger than a predetermined value. The facility is activated when the output current or voltage of the current supply become larger than a predetermined current Ig or voltage Ed, both indicated in FIG.2. The predetermined voltage Ed is usually selected to be in the voltage range in which the current supply reveals the aforesaid drooping characteristic. The current Ig and the voltage Ed and the corresponding point "d" on the drooping characteristic curve are referred to as the shutdown current, shutdown voltage and shutdown point, respectively. It is obvious from FIG.2 that the above voltages are generally in the relationships, 2Ea ≃ Eb and Eb < Ec < Ed < Ee < Ef. It should be noted that there is an inevitable timing difference between the starts of the operations of the constant current supplies in a double-end power feeding system. Accordingly, in the beginning of the operation, the double-end power feeding system is in a pseudo single-end feeding mode, and the output current and voltage from the current supply starting earlier change along the dotted line connecting the origin 0 and the point "b" in FIG.2. In the constant current supply, the drooping characteristic does not appear even when the output voltage reaches Eb corresponding to the point "b". Thus, if the double-end power feeding system as shown in FIG.4 is fed by the constant current supplies having characteristic as shown in FIG.2, the voltage E or E may occasionally rise up to Eb because of the starting timing difference in the constant current supplies. When the both constant current supplies come into their operations, the voltage become stable at Ea. The above over-voltage during the pseudo single-end feeding mode requires the repeaters and the power feeding line to withstand the voltage Eb, at least, which is approximately double the operating voltage Ea in the double-end feeding system. Therefore, such excess power is not only useless but undesirable in view of the aforesaid withstand voltage. Accordingly, the constant current supplies for a double-end power feeding system must be those which have power merely enough for the operation in the double-end feeding mode. The current-voltage characteristics of a constant current supply which is designed to be exclusively used for a double-end feeding is explained together with a conventional shutting down controlling therefor with reference to FIG.3, wherein like references designate like or corresponding parts in FIG.2. Referring to FIG.3, the dotted line connecting the origin 0 and the point "a" corresponds to the load characteristic curve of the constant current supply operating in a double-end feeding mode, and the dotted line connecting the origin 0 and the point "b1" corresponds to the load characteristic curve in the aforesaid pseudo single-end feeding mode in the beginning of the operation of the double-end feeding system. The constant current supply is provided with a drooping characteristic as represented by the solid line connecting the point "c" and the point Ee1 on the voltage axis as shown in FIG.3. The drooping point "c" is set adjacent to the stable point "a" in the double-end feeding operation, corresponding to a drooping voltage Ec which is shifted lower than the voltage Eb as different from that in FIG.2. During the period of the pseudo single-end feeding mode, the current and voltage increase along the dotted line connecting the origin 0 and the point "b1" on the drooping characteristic curve. The current and voltage reach a stable point "a" through the points "b1" and "c" when the remainder constant current supply becomes in its operation. Thus, the voltage E in the double-end feeding system can be reduced lower by using constant current supplies having the characteristics as shown in FIG.3. As a result, the threshold voltage Ef of the aforesaid over-voltage protecting elements 15 and 19 in FIG.1 can be lower and the withstand voltages for the circuit components and the assemblages thereof of the current supplies and repeaters can also be decreased. The constant current supply having current-voltage characteristics as shown in FIG.3 is provided with a facility for shutting down thereof when the current or voltage exceeds the respective predetermined values Ig and Ed. The predetermined voltage Ed is selected to correspond to a point "d" on the drooping characteristic curve. Obviously, the voltage Ed must be between a not=shown voltages Eb1 corresponding to the point "b1" and the voltage Ee1, therefore, a higher accuracy is required for the detection of Ed compared with that in the current-voltage characteristics of FIG.2. FIG.4 is a circuit block diagram of conventional shutdown controlling means for a constant current supply. The shutdown means comprises a current detecting circuit 22, voltage detecting circuit 23 and OR gate 24. The current detecting circuit 22 detects the current fed by the constant current supply 21 to a load resistance R, the serially connected resistors in FIG.1, for example, and outputs a signal when the current exceeds the value Ig, the shutdown current. The voltage detecting circuit 23 detects the voltage fed by the constant current supply 21 and outputs a signal when the voltage exceeds the shutdown voltage Ed. The OR gate 24 outputs a control signal to the constant current supply 21 to be shut down therewith upon receiving either output signal of the current detecting circuit 22 or the voltage detecting circuit 23. Referring again to FIG.3, the withstand voltage requirements can be eased further by providing the constant current supplies with a steeper or inverted drooping characteristic as shown by the dotted lines including one connecting the drooping point "c" and the point Ee2 on the voltage axis and the other connecting the points "c" and Ee3 on the voltage axis. However, the respective shutdown points (not shown) on these drooping characteristic curves are at an equi- or lower voltage compared with the point "b2" or "b3" which are the respective intersections of the steeper and inverted drooping characteristic curves and the load characteristic curve for the aforesaid pseudo single-end feeding mode. Accordingly, the current supply is shut down as soon as the output voltage thereof reaches the voltage corresponding to the point " b2" or before the output voltage reaches the voltage corresponding to the point "b3". This means that the withstand voltage reduction by the steeper or inverted drooping characteristic can not be achieved as far as the conventional shutdown means as shown in FIG.4 is employed. Therefore, it is desired to provide novel controlling means for shutting down an electric power supply having a steep or aforesaid inverted drooping characteristic. The above discussion also applies to a constant voltage supply if the relationship between the current and voltage in the current-voltage characteristic diagram of a constant current supply is substituted for each other, however, the details will be discussed in the description of preferred embodiments. US-A-3 509 448 describes a circuit used to control an unregulated DC supply to produce a constant voltage output with an inverted drooping characteristic. The circuit includes a constant current source and three current sinks. One of the current sinks is arranged to limit load current at normal regulated load voltages and to cut back load current when the load resistance drops below a predetermined value thus producing the inverted drooping characteristic. The circuit does not however calculate or measure the load resistance as such. FR-A-2 152 395 describes a device suitable for connecting to a DC source to supply a load with a constant voltage. The device includes a voltage sensing circuit, a circuit breaking means and a current sensing circuit. The circuit breaking means interrupts the supply when the voltage determined by the voltage sensing means falls below a predetermined value and when the current sensing means detects an extreme current overload. SUMMARY OF THE INVENTION It is an object of the present invention to provide controlling means for shutting down a constant current or voltage supply having a steep or aforesaid inverted drooping characteristic. It is another object of the present invention to provide controlling means capable of shutting down a constant current or voltage supply regardless of the drooping characteristics of the current or voltage supply. It is still another object of the present invention to provide shutdown controlling means for constant current supplies in a double-end power feeding system for a long-haul wire telecommunication system, wherein the shutdown controlling means allows to reduce the withstand voltage required for the circuit components and the assemblages thereof for the constant current supplies and repeaters. Above objects can be solved by a shutdown controlling means for a constant current supply having an output terminal connected to a load circuit having an input resistance which may occasionally vary to above a first allowable input resistance, and detecting means for detecting the voltage at the output terminal and the current flowing therethrough, characterized by said detecting means providing a signal for shutting down the constant current supply in accordance with a condition where the current is smaller than a predetermined value when the output voltage is above a predetermined value, wherein said predetermined voltage and current are determined so that the ratio thereof corresponds to a second maximum allowable input resistance of the load circuit to which feeding of the current from the constant current supply is permitted, said second maximum allowable input resistance being higher than said first allowable input resistance. The invention also provides a shutdown control means for a constant current supply as defined in claim 3, and a shutdown control means for a constant voltage supply as defined on claim 6 or claim 8. The detecting means comprises a voltage detecting circuit, a current detecting circuit and an AND gate. The AND gate provides a control signal for shutting down the constant current or voltage supply upon receiving output signals from both of the voltage and current detecting circuits. The AND gate can be replaced by an arithmetic means which processes the output signals from the voltage detecting circuit and the current detecting circuit so as to provide a ratio of the voltage to the current and outputs a shutdown control signal when the resistance corresponding to the ratio is respectively larger or smaller than the second allowable input load resistance to which feeding of the power from the constant current or voltage supply is permitted. The foregoing and other objects and features of the present invention will be apparent from a reading of the following description and claims taken in connection with the accompanying drawings in which: FIG.1 is a circuit block diagram of a double-end power feeding system; FIG.2 is the current-voltage characteristic diagram of a conventional current supply applicable to both of a single-end feeding and a double-end feeding; FIG.3 is the current-voltage characteristic diagram of a conventional constant current supply which is applicable only to a double-end feeding; FIG.4 is a circuit block diagram of a conventional shutdown control circuit for a constant current supply; FIG.5 is a current-voltage characteristic diagram of a constant current supply, illustrating the operation principle of the shutdown control means for a constant current supply in accordance with the present invention; FIG.6 is a circuit block diagram of a shutdown control circuit for a constant current supply in accordance with a first embodiment of the present invention; FIG.7 is a circuit block diagram of a shutdown control circuit for a constant current supply in accordance with a second embodiment of the present invention; FIG.8 is a circuit block diagram showing an exemplary detailed configuration of the shutdown control means for a constant current supply in accordance with the present invention; FIG.9 is a block diagram of a circuit in which the power is supplied from a pair of constant voltage supplies connected in parallel to each other; FIG.10 is a voltage-current characteristic diagram of one of a conventional constant voltage supplies connected in parallel to each other to a load circuit, each constant voltage supply having a so large power as sufficient for operating the load circuit 1 by itself; FIG.11 is a voltage-current characteristic diagram of a constant voltage supply, illustrating the operation principle of the shutdown control means for a constant voltage supply in accordance with the present invention is applied; FIG.12 is a circuit block diagram of a shutdown control circuit for a constant voltage supply in accordance with a third embodiment of the present invention; FIG.13 is a circuit block diagram showing an exemplary detailed configuration of the shutdown control means for a constant voltage supply in accordance with the present invention; and FIG.14 is a circuit block diagram of a shutdown control circuit for a constant voltage supply in accordance with a fourth embodiment of the present invention. Throughout the drawings, like reference numerals designate like or corresponding parts. DESCRIPTION OF PREFERRED EMBODIMENTS FIG.5 is a current-voltage characteristic diagram of a constant current supply to which shutdown control means of the present invention is applied, and FIG.6 is a circuit block diagram of a shutdown control circuit in accordance with a first embodiment of the present invention. The operation principle of the shutdown control will be explained with reference FIGs.5 and 6. The constant current supply is designed to be exclusively applicable to a double-end feeding and the current-voltage characteristic is quite similar to that shown in FIG.3, hence, having a drooping characteristic represented by a line connecting the drooping point "c" and the point Ee1 on the voltage axis, wherein the drooping voltage Ec (not shown) and the voltages Ee1 and Ef are equal to the corresponding voltages in FIG.3. Further, a point "d1" which corresponds to the shutdown point "d" in FIG.3 is defined on the drooping characteristic curve. Operation of the shutdown controlling means of this embodiment is carried out based on the load characteristic line connecting the origin 0 and the point "d1". The load characteristic line corresponds to an input resistance of a load circuit to which feeding of a current from the constant current supply is stopped. The input resistance is referred to as a maximum allowable load resistance, hereinafter. The shutdown control circuit shown in FIG.4 provides a signal for shutting down the constant current supply 21 as soon as it detects an output voltage larger than the shutdown voltage Ed. On the other hand, the shutdown control circuit of this embodiment does not provides a shutdown control signal for the current supply, even detecting an output voltage larger than the voltage Ed1, unless the corresponding output current is smaller than Id1 which is the current corresponding to the point "d1". That is, the shutdown signal is generated when the detected output voltage and current correspond to a point below the load characteristic curve. Referring to FIG.6, the shutdown control circuit of this embodiment comprises an AND gate 27 having respective inputs connected to a current detecting circuit (C.D.C) 25 and a voltage detecting circuit (V.D.C) 26 and an output connected to an OR gate 24. The current detecting circuit 25 provides a signal for the OR gate 24 when the output current from the constant current supply (C.C.S) 21 is larger than a predetermined shutdown current Ig, and with which signal, the constant current supply 21 is shut down. The current detecting circuit 25 continues to provide another output signal for the AND gate 27 during when the output current is smaller than Id1. At the same time, the voltage detecting circuit 26 detects the output voltage from the constant current supply 21 and provides a signal for the AND gate 27 when the output voltage is larger than Ed1. Accordingly, the AND gate 27 outputs a signal for the OR gate 24 for shutting down the constant current supply 21 only when the output current is smaller than Id1 at the output voltage having reached Ed1. Referring back to FIG.5, in the normal operation of the constant current supply, the current and voltage fed therefrom increase along the load characteristic line connecting the origin 0 and the point "b1" on the drooping characteristic curve due to the aforesaid timing difference between the current supplies operated in the double-feeding mode and reach the stable point "a" for the double-end feeding after passing through the point "b1" and the drooping point "c". The current on the load characteristic line connecting 0 and the point "b1" becomes larger than Id1 before the corresponding voltage reaches Eb1, therefore, the current supply 21 can be prevented from an erroneous shutdown even when there is only a small difference between the voltage Ed1 and the voltage Eb1 (not shown) corresponding to the point "b1". Accordingly, the present invention shutdown control means permits a current-voltage characteristic to have a steep drooping characteristic as shown by the line connecting the point "c" and Ed2 on the voltage axis or to have an aforesaid inverted drooping characteristic as shown by the line connecting the point "c" and Ed3 on the voltage axis. For the former and the latter drooping characteristics, the current detecting circuit 25 in FIG.6 is designed so as to continue to provide a signal for the AND gate 27 during the output current from the constant current supply 21 is smaller than Id2 and Id3, respectively, and the voltage detecting circuit 26 detects the output voltage form the current supply 21 and provides a signal for the AND gate 27 when the output voltage is larger than Ed2 and Ed3, respectively. Although the voltages Eb2 (not shown) corresponding to the point "b2" on the former drooping characteristic curve is almost equal to Ed2 for the former drooping characteristic, the output current increasing along the line connecting the origin 0 and the point "b2" becomes larger than Id2 before the output voltage reaches the voltage Ed2, therefore, any shutdown signal for the current supply 21 is not output from the AND gate 27. Similaly, the voltage Ed3 is lower than the voltage Eb3 (not shown) corresponding to the point "b3" in the latter drooping characteristic curve, however, the output current increasing along the line connecting the origin 0 and the point "b3" becomes larger than Id3 before the output voltage reaches the voltage Ed3 and, therefore, any shutdown control signal for the current supply 21 is not provided by the AND gate 27. Again, if the input resistance R of a load circuit is so large as corresponding to the load characteristic line passing through the points d2 and d3, the output current becomes Id2 and Id3 at the respective corresponding output voltages Ed2 and Ed3, and the AND gate 27 provides a shutdown control signal for the current supply 21. Thus, according to the shutdown control means this embodiment, the constant current supply can be designed to have a drooping characteristic steeper than that allowed for conventional constant current supplies, even having the aforesaid inverted drooping characteristic which can not conventionally be employed. The above shutdown control condition is achieved when the input resistance of the load circuit connected to a constant current supply is larger than the aforesaid maximum allowable load resistance defined by each line connecting the origin 0 and the point d1, d2 or d3 on the respective drooping characteristic curves. This means that the shutdown control means is operable before the output voltage reaches the voltage Ed1, Ed2 or Ed3. Since the input resistance can easily be obtained as the ratio of the output voltage to the output current, an arithmetic operation means is provided for this purpose in the second embodiment of the present invention shutdown control means. FIG.7 is a circuit block diagram of a shutdown control circuit in accordance with a second embodiment of the present invention. In the circuit of FIG.7, a divider circuit (DIV) 28 is provided to substitute the AND gate 27 in FIG.6, to which respective detecting signals are always sent form the current detecting circuit (C.D.C) 25 and voltage detecting circuit (V.D.C) 26, corresponding to the output current and voltage of the constant current supply (C.C.S) 21. The divider circuit 28 outputs a shutdown control signal when the result of the division, i.e. the ratio of the output voltage to the output current is larger than the maximum allowable load resistance, and hence the constant current supply 21 is shut down. FIG.8 is a circuit block diagram showing an exemplary detailed configuration of the shutdown control means for a constant current supply in accordance with the present invention. Referring to FIG.8, the constant current supply 21 includes a circuit breaker (CB) 210 which is operatively connected to a direct current (DC) source 30. DC voltage supplied from the DC source 30 is input to a DC-to-AC (alternating current) converting circuit 211, which is referred to as an inverter, and converted to an alternating current (AC), and then, input to an AC-to-DC converting circuit 213, which is referred to as a rectifier, after being made stepup by a transformer 212. Thus, the rectifier 213 outputs a DC power, from which AC components included therein are removed by a low-pass filter circuit 214. The resistors 215 and 216 and control amplifier 217 are those for providing the constant current supply 21 with constant-current and drooping characteristics. That is, if the output current of the rectifier 213 increases, the voltage drop across the resistor 215 increases, therefore, the control amplifier 217 instructs the inverter 211 to decrease the output current. On the other hand, if the reaching of the output voltage up to the voltage corresponding to the drooping point "c" in FIG.5 is detected by means of the resistor 216, the control amplifier 217 instructs the inverter 211 to decrease the current output therefrom. The output current from the inverter 211 is controlled by changing the chopping frequency of the inverter 211. The current detecting circuit 25 comprises a resistor 251 connected in series to the current supplying line 50 and operational amplifiers 252 and 253. The operational amplifier 252 detects the voltage drop across the resistor 251 and compares it with a reference voltage 254. The reference voltage 254 is set equal to the voltage drop caused by the aforesaid over-current Ig flowing through the line 50. Thus, an over-current detection signal can be provided for the OR gate 24. The operational amplifier 253 also detects the voltage drop across the resistor 251 and compares it with another reference voltage 255. The reference voltage 255 is set equal to the voltage drop caused by the current Idi corresponding to the shutdown point di, where i denotes 1, 2 or 3, in FIG.5. Thus, a detection signal for the shutdown control operation is provided for the AND gate 27. The voltage detecting circuit 26 comprises a resistor 261 and an operational amplifier 262. The operational amplifier 262 receives an input voltage relative to the output voltage of the current supply 21 from the resistor 261 and compares the input voltage with a reference voltage 263. The reference voltage 263 is set equal to the input voltage corresponding to the Edi, where i denotes 1, 2 or 3, in FIG.5. Thus, a detection signal indicating the output voltage equal to or larger than the voltage Edi is provided for the AND gate 27. The AND gate 27 provides a control signal for the OR gate 24 when receiving the detection signals from both of the operational amplifiers 253 and 262. The OR gate 24 outputs a shutdown control signal for the circuit breaker 211 in the constant current supply 21 when receiving a control signal from either of the operational amplifier 252 or AND gate 27, and thus, the constant current supply 21 is separated from the DC supply 30 and is shut down from supplying current to the load resistance R. The shutdown controlling mean of the present invention can also be applied to a constant voltage supply which is connected in parallel to another equivalent constant voltage supply. The more the electronic equipment is provided with versatile and complicated functions, the current supplied thereto increases. Accordingly, plural constant voltage supplies are sometimes provided for the equipment, being connected in parallel to each other as shown in FIG.9. Referring to FIG.9, a load circuit 1 is supplied with electric power from a pair of constant voltage supplies 2 and 3. In the configuration as shown in FIG.9, there is an inevitable starting timing difference between the constant voltage supplies as described in the above double-end feeding system including constant current supplies. FIG.10 is a voltage-current characteristic curve of a constant voltage supply which is assumed to have a power sufficient for operating a load circuit in a configuration as shown in FIG.9. Referring to FIGs.9 and 10, if the constant voltage supply 2, for example, starts the operation thereof earlier, the voltage and current fed by the constant voltage supply 2 increase along the dotted line connecting the origin 0 and a point "b" on the voltage-current characteristic curve and becomes stable at Ib corresponding to the point "b". When the remainder constant voltage supply 3 goes into its operation, the voltage and current move to a point "a" along the voltage-current curve and the current becomes stable at Ia corresponding to the point "a". Ia is the current fed from each of the constant voltage supplies 2 and 3 in the concurrent operation. Diodes 4 and 5 in FIG.9 are provided for preventing the current output from the earlier starting one of the constant voltage supplies 2 and 3 from flowing into the later starting one. Each of the constant voltage supplies 2 and 3 is provided with a drooping characteristic represented by a line connecting a point "c" on the voltage-current characteristic curve and Ie on the current axis. That is, as the current increases larger than the current Ic corresponding to the point "c", the output voltage decreases toward Ie on the current axis. The point "c" is referred to as the drooping point. Further, each of the constant voltage supplies 2 and 3 is shutdown when the output voltage thereof becomes larger than the voltage Eg or the output current is larger than the current Id corresponding to a shutdown point "d" defined on the drooping characteristic curve. In FIG.10, the above mentioned characteristic currents are in a relationships of 2Ia ≃ Ib and Ib < Ic < Id < Ie. In views of reducing current capacities required for the circuit elements and assemblage thereof for the constant voltage supplies 2 and 3, and of decreasing an over-current when the input of the load circuit 1 is short-circuited, it is desired for the constant voltage supplies 2 and 3 to have the point "c" shifted to lower current side and a steeper drooping characteristic, wherein the characteristic currents are in a relationships of Ib < 2Ia and Ia < Ic < Ib < Id < Ie. That is, the point "b" is located on the drooping characteristic curve. Accordingly, there are arised problems involved in the detection of Id and shutdown controlling based on Id, similar to the problems relating to Ed in the double-end power feeding system including constant current supplies as described with reference to FIGs.3 and 4. However, the present invention shutdown control means operating based on the output current and voltage corresponding to a shutdown point can be applied to a constant voltage supply having any drooping characteristic. FIG.11 is a voltage-current characteristic diagram of a constant voltage supply, illustrating the operation principle of the shutdown control means for a constant voltage supply in accordance with the present invention. Referring to FIG.11, the dotted lines connecting the origin 0 and the point "a" and the the origin 0 and "b1" are load characteristic curves corresponding to the respective conditions where both constant voltage supplies 2 and 3 in FIG.9 are in operations and only one of the constant voltage supplies 2 and 3 is in operation. The dotted line connecting the origin 0 and the point "d1" corresponds to a load resistance, i.e. an input resistance of the load circuit 1 in FIG.9 to which the feeding of the voltage is to be stopped. Since the slope of the dotted lines in FIG.11 represent respective resistance values, therefore, the dotted line passing through the origin 0 to the point "d1" corresponds to a minimum allowable load resistance. FIG.12 is a circuit block diagram of a shutdown control circuit in accordance with a third embodiment of the present invention. Referring to FIG.12, the shutdown control circuit comprises OR gate 24, current detecting circuit (C.D.C) 25, voltage detecting circuit (V.D.C) 26 and AND gate 27. The voltage detecting circuit 26 provides the OR gates with a shutdown control signal for the constant voltage supply (C.V.S) 29 when the output voltage of the constant voltage supply 29 is larger than Eg, and also provides the AND gate 27 with a detection signal when the output voltage is lower than the voltage Ed1. The current detecting circuit 25 provides the AND gate 27 with a detection signal when the current from the constant voltage supply 29 is larger than the current Id1. Accordingly, the AND gate 27 outputs a shutdown control signal for the constant voltage supply 29 only when the both conditions of the output voltage lower than Ed1 and the current larger than Id1, respectively, are achieved. As described above, the present invention shutdown control circuit of the third embodiment, which is based on the detection of the output voltage and current, can operate independent from the drooping characteristic of the constant voltage supply. For instance, the drooping characteristic can be steep as represented by a line connecting the points "c" and "b2" or can be inverted as represented by a line connecting the points "c" and "b3", as shown in FIG.11. For these drooping characteristics, the shutdown points are "d2" and "d3", respectively. Accordingly, corresponding to the drooping characteristic curves, the current detecting circuit 25 detects Id2 or Id3, while the voltage detecting circuit 26 detects Ed2 or Ed3. FIG.13 is a circuit block diagram showing an exemplary detailed configuration of the shutdown control means for a constant voltage supply in accordance with the present invention. Referring to FIG.13, the constant voltage supply 29 includes a circuit breaker (CB) 211 which is operatively connected to a direct current (DC) source 30. DC voltage supplied from the DC source 30 is input to an inverter 211 and converted to an alternating current (AC), and then, input to a rectifier 213, after being made stepup by a transformer 212. Thus, the rectifier 213 outputs a DC power, from which AC components included therein are removed by a low-pass filter circuit 214. The resistors 218 and 219 and control amplifier 217 are those for providing the constant voltage supply 29 with constant-voltage and drooping characteristics. That is, if the output voltage of the rectifier 213 increases, the voltage drop across the resistor 218 increases, therefore, the control amplifier 217 instructs the inverter 211 to decrease the output voltage. On the other hand, if the reaching of the output voltage up to the voltage corresponding to the drooping point "c" in FIG.11 is detected by the resistor 219, the control amplifier 217 instructs the inverter 211 to decrease the voltage output therefrom. The output voltage of the inverter 211 is controlled by changing the chopping frequency of the inverter 211. The current detecting circuit 25 comprises a resistor 251 connected in series to the current supplying line 50 and operational amplifier 253. The operational amplifier 253 detects the voltage drop across the resistor 251 and compares it with reference voltage 255. The reference voltage 255 is set equal to the voltage drop caused by the current Idi corresponding to the shutdown point di, where i denotes 1, 2 or 3, in FIG.11. Thus, a detection signal for the shutdown control operation is provided for the AND gate 27. The voltage detecting circuit 26 comprises a resistor 261 and operational amplifiers 262 and 264. The operational amplifier 264 detects the voltage drop across the resistor 261 and compares it with a reference voltage 265. The reference voltage 265 is set equal to the voltage drop-corresponding to the aforesaid over-voltage Eg on the line 50. Thus, a control signal for the shutdown operation based on the over-voltage Eg is provided for the OR gate 24. The operational amplifier 262 receives an input voltage relative to the output voltage of the constant voltage supply 29 from the resistor 261 and compares the input voltage with a reference voltage 263. The reference voltage 263 is set equal to the input voltage corresponding to the voltage Edi, where i denotes 1, 2 or 3, in FIG.11. Thus, a detection signal indicating the output voltage equal to or lower than the voltage Edi is provided for the AND gate 27. The AND gate 27 provides a control signal for the OR gate 24 when receiving the detection signals from both of the operational amplifiers 253 and 262. The OR gate 24 outputs a shutdown control signal for the circuit breaker 211 in the constant voltage supply 29 when receiving a control signal from either of the operational amplifier 264 or the AND gate 27, and thus, the constant voltage supply 29 is separated from the DC supply 30 and is shut down from supplying voltage to the load resistance R. FIG.14 is a circuit block diagram of a shutdown control circuit in accordance with a fourth embodiment of the present invention. In the circuit of FIG.14, a divider circuit 28 is provided to substitute the AND gate 27 in FIG.12, to which the respective detecting signals are always sent form the current detecting circuit 25 and the voltage detecting circuit 26, corresponding to the output current and voltage of the constant voltage supply 29. The divider circuit 28 outputs a shutdown control signal when the result of the division, i.e. the ratio of the output voltage to the output current is equal to or smaller than the minimum allowable load resistance, and hence, the constant voltage supply 29 is shut down.

Claims: Shutdown control means for a constant current supply (21) having an output terminal connected to a load circuit having an input resistance which may occasionally vary to above a first, allowable input resistance and detecting means (25,26,27) for detecting the voltage at the output terminal and the current flowing therethrough, characterized by said detecting means providing a signal for shutting down the constant current supply (21) in accordance with the condition where the current is smaller than a predetermined value when the output voltage is above a predetermined value, wherein said predetermined voltage and current are determined so that the ratio thereof corresponds to a second, maximum allowable input resistance of the load circuit to which feeding of the current from the constant current supply (21) is permitted, said second, maximum allowable input resistance being greater than said first, allowable input resistance. Shutdown control means of claim 1, wherein said detecting means comprises: a voltage detecting circuit (26) for detecting the voltage at the output terminal, said voltage detecting circuit having an output and providing a high level output signal when the voltage is larger than said predetermined voltage; a current detecting circuit (25) for detecting the current flowing through the output terminal, said current detecting circuit having an output and providing a high level output signal when the current is smaller than said predetermined current; and an AND gate (27) having two inputs connected to respective said outputs of said voltage and current detecting circuits (26, 25) and providing said shutdown signal for the constant current supply (21) upon receiving respective said high level output signals from said voltage and current detecting circuits (26, 25). Shutdown control means for a constant current supply (21) having an output terminal connected to a load circuit having an input resistance which may occasionally vary to above a first, allowable input resistance including: a voltage detecting circuit (26) for detecting the voltage at the output terminal, said voltage detecting circuit having an output and providing an output signal relative to the voltage; and a current detecting circuit (25) for detecting the current flowing through the output terminal, said current detecting circuit having an output and providing an output signal relative to the current; characterized by arithmetic operation means (28) which processes the output signals of said voltage detecting circuit (26) and said current detecting circuit (25) so as to provide a ratio of the voltage to the current, said arithmetic operation means providing said shutdown signal when said ratio of the voltage to the current is larger than a second, maximum allowable input resistance to which feeding of the power from the constant current supply (21) is permitted, said second, maximum allowable input impedence being greater than said first, allowable input resistance. Shutdown control means of claim 3, wherein said voltage and current detecting circuits (26, 25) include means for providing respective digital values corresponding to the voltage and the current, and said arithmetic operation means (28) includes means for processing said digital values so as to provide a digital value corresponding to the ratio of the voltage to the current. Shutdown control means of claim 3, wherein said arithmetic operation means (28) has first means for providing digital values corresponding to the respective output signals of said voltage and current detecting circuits (26, 25) and second means for processing said digital values so as to provide the ratio of the voltage to the current. Shutdown control means for a constant voltage supply (29) having an output terminal connected to a load circuit having an input resistance which may occasionally vary to below a first, allowable input resistance, and detecting means (25,26,27) for detecting the voltage at the output terminal and the current flowing therethrough, characterized by said detecting means providing a signal for shutting down the constant voltage supply (29) in accordance with a condition where the voltage is smaller than a predetermined value when the current is above a predetermined value wherein said predetermined voltage and current are determined so that the ratio thereof corresponds to a second, minimum allowable input resistance of the load circuit to which feeding of the voltage from the constant voltage supply (29) is permitted, said second, minimum allowable input resistance being lower than said first, allowable input resistance. Shutdown control means according to claim 6, characterized by said detecting means including: a voltage detecting circuit (26) for detecting the voltage at the output terminal, said voltage detecting circuit having an output and providing a high level output signal when the voltage is smaller than said predetermined voltage; a current detecting circuit (25) for detecting the current flowing through the output terminal said current detecting circuit having an output and providing a high level output signal when the current is larger than said predetermined current; and an AND gate (27) having two inputs connected to respective said outputs of said voltage and current detecting circuits (26, 25) and providing said shutdown signal for the constant voltage supply (29) upon receiving respective said high level output signals form said voltage and current detecting circuits (26, 25). Shutdown control means for a constant voltage supply (29) having an output terminal connected to a load circuit having an input resistance, which may occasionally vary to a value below a first, allowable input resistance, including: a voltage detecting circuit (26) for detecting the voltage at the output terminal, said voltage detecting circuit having an output and providing an output signal relative to the voltage; and a current detecting circuit (25) for detecting the current flowing through the output terminal, said current detecting circuit having an output and providing an output signal relative to the current; characterized by arithmetic operation means (28) which processes the output signals of said voltage detecting circuit (26) and said current detecting circuit (25) so as to provide a ratio of the voltage to the current, said arithmetic operation means providing said shutdown signal when said ratio of the voltage to the current is smaller than a second, minimum allowable input resistance to which feeding of the power from the constant voltage supply (29) is permitted, said second, minimum allowable input resistance being lower than said first, allowable input resistance. Shutdown control means of claim 8, wherein said voltage and current detecting circuits (26, 25) include means for providing respective digital values corresponding to the voltage and the current and said arithmetic operation means (28) includes means for processing the digital values to provide the ratio of the voltage to the current. Shutdown control means of claim 8, wherein said arithmetic operation means (28) includes first means for providing digital values corresponding to the respective output signals from said voltage and current detecting circuits (26, 25) and includes second means for processing the digital values to provide the ratio of the voltage to the current.

Inventors: HANYUDA, KEN, OHARA, MICHIMASA
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: G05F1
IPC codes: H02J1, G05F1, H02H3