Abstract:
In order to provide as short a switching delay as possible in an undervoltage release which contains an electromagnet for tripping a switching device, and a driver circuit feeding the winding of the electromagnet, with the driver circuit being used to produce a pulsed holding current for the electromagnet, an electronic switch is fitted in the self-holding circuit that is produced via a freewheeling diode. This electronic switch can be driven as a function of the output signal from an undervoltage detector which is arranged within the driver circuit.

Description:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/DE00/03435 which has an International filing date of Sep. 27, 2000, which designated the United States of America, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to the field of protection technology for single-phase or polyphase low-voltage power supply systems. In one embodiment, it can be applicable to the circuitry configuration of an undervoltage release. Such a release can include, for example, an electromagnet for tripping a switching device, when the voltage of the power e supply system falls below a predetermined magnitude, and a driver circuit feeding the electromagnet, with the driver circuit having a rectifier circuit, an auxiliary voltage source, a pulse transmitter for producing a pulsed holding current for the electromagnet, and a threshold value circuit, which is provided with an undervoltage detector, for interrupting the holding current. 
     BACKGROUND OF THE INVENTION 
     In one known undervoltage release, the rectifier circuit is used to obtain a direct current, the auxiliary voltage source which is arranged downstream from the rectifier circuit is used for supplying voltage to the electronics, and the pulse transmitter is used for producing a holding current which, provided the power supply system voltage is sufficiently high, is essentially independent of this power supply system voltage. In this case, the pulse transmitter has an operational amplifier and an electronic switch which is controlled by the operational amplifier and is connected in series with a resistor in the holding circuit of the electromagnet, which is connected to a freewheeling diode. Furthermore, the pulse transmitter contains a feedback path to allow the voltage which is dropped across the resistor to act on the input of the operational amplifier (U.S. Pat. No. 4,788,621 A, DE 195 07 936 A1). It is also known for an undervoltage release such as this to be combined with an overcurrent release, with the overcurrent detector acting optoelectronically on a further electronic switch which is arranged in parallel with the electromagnet and the freewheeling diode and can short-circuit them (U.S. Pat. No. 4,890,184). 
     In the known undervoltage releases, the freewheeling diode which is arranged in parallel with the electromagnet is used itself to hold the electromagnet when the voltage briefly falls below a minimum level or in the event of brief interruptions. The tripping delay that occurs in this case is more than 100 ms. It is known for the tripping delay to be increased by using a capacitor and by a parallel arrangement comprising a series circuit of a resistor and a further electronic switch for the resistor that is connected in series with the electromagnet (DE 195 07 936 A1). 
     However, sometimes it is also desirable to ensure that the electromagnet trips in as short a time as possible (less than 100 ms) when the voltage briefly falls below a minimum voltage or in the event of brief voltage interruptions. 
     SUMMARY OF THE INVENTION 
     An object of an embodiment of the invention is to provide a circuit arrangement which is suitable. 
     In order to achieve an object, an embodiment of the invention provides for a further electronic switch which can be driven as a function of the output signal from the undervoltage detector and which is arranged between the electromagnet and the first-mentioned electronic switch, to be arranged in the self-holding circuit which is formed from the electromagnet and the freewheeling diode. Further, a timer may be provided for driving the further electronic switch, one pole of which timer is coupled to the auxiliary voltage source via a diode circuit. 
     The use of a timer for driving the second electronic switch is advantageous in comparison to the possible use of an optocoupler to drive it. The further electronic switch can expediently also be short-circuited via a link in order to make it possible to operate the undervoltage release with the “normal” tripping delay, as well. 
     The refinement of the circuit arrangement provided according to an embodiment of the invention allows an additional electronic switch to be activated by mechanically releasing a short-circuiting link, with this additional electronic switch being located in the same circuit as the freewheeling diode and thus being able to interrupt the self-holding circuit of the electromagnet with a delay of less than 100 ms. 
     It is known per se from EP 0 203 366 A1 for two switching elements which are optionally in the form of electronic switches, to be provided for a control apparatus for electromagnetic switching devices, which switching elements are located in the circuit of an operating coil. One of these switching elements allows a freewheeling diode to be rendered ineffective in order, when required, to cause the current in the operating coil to decay more quickly. In this case, and in contrast to the invention, the primary factor is not measured monitoring of a power supply system voltage, but the protection of the switching device by influencing its own magnet drive. The known control apparatus therefore cannot be used as an undervoltage release. In addition to this, there are no details, corresponding to the invention, relating to the connection of a timer for controlling the switching element located in the self-holding circuit of the operating coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE illustrates one exemplary embodiment of the circuit arrangement designed according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The voltage which is to be monitored by the undervoltage release is applied to the terminals K 1  and K 2 . The terminals are followed by current-limiting resistors R 1  and R 2 , a varistor V 1  and a bridge circuit BS for rectifying the applied AC voltage and for providing a direct current for the winding (which can be connected to the terminals K 5 /K 6 ) of an electromagnet (EM) for tripping a low-voltage circuit breaker, which is not shown in any more detail. The rectifier circuit BS is followed by an auxiliary voltage source HQ with a voltage divider comprising a resistor R 3  and a voltage limiter Z 1 , as well as a smoothing A capacitor C 1  and an energy storage capacitor C 2 . An integrated circuit containing the operational amplifiers OV 1 , OV 2 , OV 3  and OV 4  is supplied with power from the connecting point K 4 . 
     A resistor R 4  and an electronic switch ES 1  are connected in series with the winding of the electromagnet EM; a diode D 1 , which is referred to as a freewheeling diode, is arranged electrically in parallel with the winding of the electromagnet EM and a self-holding circuit for the electromagnet EM is closed via this diode D 1 , to be precise irrespective of whether or not any short-circuiting link KB which can be plugged onto the terminals K 7 /K 8 , is inserted. The electronic switch ES 1  is part of a pulse transmitter for pulsing the holding current for the electromagnet, and can be controlled by the operational amplifier OV 1 , whose inputs  12  and  13  are supplied with reference and measurement voltages via the further operational amplifiers OV 1 , OV 3  and OV 4 . The operational amplifier OV 2 , which is located in a feedback path, in this case receives its reference voltage at the input  10  from a reference voltage diode Z 2  via the resistors R 10 /R 11  and, possibly, in a pulsed manner from the output  2  of the operational amplifier OV 3  via the capacitor C 4 ; it receives its measurement voltage from the voltage drop across the resistor R 4 . The operational amplifier OV 2  is followed by a diode D 2  and a capacitor C 3 . The capacitor C 3  provides a variable drive voltage for the input  12  of the operational amplifier OV 1 . 
     The operational amplifier OV 3  acts as an undervoltage detector and is part of a threshold value circuit for interrupting the holding current, which holds the electromagnet in the pulled-in state. For this purpose, the input  5  of the operational amplifier OV 3  is driven via a voltage divider R 5 /R 6 . A small reference voltage is defined at the input  4 , via of the reference voltage diode Z 2 . The output of the operational amplifier OV 3  is connected to the input  6  of the operational amplifier OV 4 ; since the reference input  7  of the operational amplifier OV 4  receives the same reference voltage as the reference input  4  of the operational amplifier OV 3 , the operational amplifier OV 4  acts as an inverter for the operational amplifier OV 3 . The output  1  of the operational amplifier OV 4  is coupled to the capacitor C 3 , and hence to the input  12  of the operational amplifier OV 1 . 
     A further electronic switch ES 2  is arranged in series with the electronic switch ES 1  in the holding circuit of the electromagnet EM and at the same time in the self-holding circuit that contains the freewheeling diode D 1 . This further electronic switch ES 2  is driven by a timer comprising a resistor R 7  and a capacitor C 5  in conjunction with a zener diode Z 3 , with one pole of this timer being coupled to the auxiliary voltage source HQ via a diode circuit D 3 , D 4 , D 5 . 
     The further electronic switch ES 2  is short-circuited when the contact link KB is inserted so that, if a brief undervoltage occurs at the terminals K 1 /K 2 , the self-holding circuit is activated with a tripping delay of more than 100 ms, via the freewheeling diode D 1 . When the contact link KB is withdrawn, the electronic switch ES 2  is also included in the self-holding circuit and, when it is disconnected, immediately interrupts the current flow through the electromagnet EM. 
     Provided the input voltage between the terminals K 1 /K 2  is sufficient, the output  2  of the operational amplifier OV 3  changes to “high”, as a result of which the output  1  of the inverter OV 4  changes to “low”. This allows the capacitor C 3 , which is connected to the output  12  of the operational amplifier OV 1 , to discharge itself; on reaching the voltage which is applied to the reference input  13  of the operational amplifier OV 1 , the output  15  of the operational amplifier OV 1  jumps to “high”, thus switching the electronic switch ES 1  on. Since the input of the electronic switch ES 2  is also “high” and, in consequence ES 2 , is switched on, a rising current starts to flow through the winding of the electromagnet EM. 
     The rising current produces an increasing voltage drop across the resistor R 4 . When this voltage drop becomes greater than the comparison voltage which is applied to the reference input  10  of the operational amplifier OV 2  and which is input from the reference voltage diode Z 2 , the output  16  of the operational amplifier OV 2  jumps to “high” so that the operational amplifier OV 1  is reversed, and changes to “low” at the output  15 . The electronic switch ES 1  is thus switched off, and the electromagnet EM is held temporarily via the self-holding circuit. 
     Since the output  16  of the operational amplifier OV 2  is “high”, the capacitor C 3  can also be charged. Once the electronic switch ES 1  has switched off, the voltage drop across the resistor R 4  changes to zero, and the output  16  of the operational amplifier OV 2  changes to “low”. In consequence, the capacitor C 3  now discharges itself; as soon as the capacitor voltage falls below the reference voltage at the input  13  of the operational amplifier OV 1 , the output  15  of the operational amplifier OV 1  jumps to “high”, and the electronic switch ES 1  is closed. The clock frequency of this switching process is approximately 1 kHz. 
     When, with the coupling link KB inserted, the voltage between the input terminals K 1 /K 2  falls below the lower undervoltage value, the output of the operational amplifier OV 3  switches to “low”, the output of the inverter OV 4  changes to “high”, the output of OV 1  At changes to “low”, and the electronic switch ES 1  is switched off. The winding of the electromagnet EM forces a decreasing current to flow via the diode D 1  and, after approximately 100 to 200 μs, this decreasing current is no longer sufficient to hold the electromagnet. 
     When, with the coupling link KB withdrawn, the voltage between the input terminals K 1 /K 2  falls below the lower undervoltage value, the electronic switch ES 1  is likewise switched off. As long as ES 1  was switched on, the capacitor C 5  was being charged via the resistor R 9  and the diodes D 5 , D 3  and thus switched the electronic switch ES 2  on. If the electronic switch ES 1  is now switched off, the capacitor C 5  discharges itself via the resistor R 7 . As soon as the voltage across the capacitor C 5  falls below the value that is required to drive the electronic switch ES 2 , the electronic switch ES 2  switches off. The self-holding circuit via the diode D 1  is thus interrupted, and the electromagnet trips. The switching delay is in this case approximately 50 to 60 ms. 
     Alternatively, the capacitor C 5  can also be charged via the output  15  of the operational amplifier OV 1 . The electronic switch ES 2  can also be driven optoelectronically, directly from the output of the operational amplifier OV 3 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.