CROWBAR CIRCUIT, ESPECIALLY FOR A VOLTAGE CONVERTER, IN EXPLOSION-PROOF APPLICATIONS

Abstract

Crowbar circuit (1) for use in a voltage converter, comprising: - a triggering device (P1) designed to trigger or cause a short circuit across input terminals (E1, E2) depending on a control voltage; - a comparator (IC1) which is designed to control the triggering device (P1) to trigger the short circuit depending on a comparator voltage at a comparator input of the comparator (IC1) and depending on a predetermined reference voltage threshold value, - a Zener diode (NZ1) with a predetermined breakdown voltage in series with a fifth resistor (R5) connected between one of the input terminals and the comparator input, so that when the breakdown voltage is exceeded by the applied input voltage, the reference voltage threshold is exceeded or undershot by the comparator voltage and the short circuit is thereby triggered or caused; - a series circuit of a seventh resistor (R7), an eighth resistor (R8), the fifth resistor (R5) and a sixth resistor (R6), wherein a load current at an output of the crowbar circuit (1) flows through the seventh and the sixth resistor (R7, R6) and the comparator input is electrically connected to a node between the fifth and eighth resistors (R5, R8) and the series circuit is connected between the input terminals, so that when a predetermined threshold current is exceeded, the reference voltage threshold is exceeded or undershot by the comparator voltage and the short circuit is thereby triggered.

Description

Technical FieldThe invention relates to crownbar circuits for limiting over-voltages and over-currents in applications in explosion-proof environments. The invention relates in particular to measures for reducing the power design of Zener diodes and resistors of the crowbar circuit.Background ArtIn order to reliably prevent overvoltage and overcurrent events of a device / operating medium which provides electrical energy in the explosion-protected area, crownbar circuits are frequently used. The principle of crowbar circuits is to short-circuit the input voltage source via a semiconductor switch (MOSFET, thyristor) when an overvoltage and / or an overcurrent is detected, and thus to quickly trigger a fuse coupled in series to the input voltage source.Since the response time of the circuit as a whole (including due to the triggering time of the semiconductors and due to EMC or functional requirements. Comparators, filtering, etc.) typically range from several microseconds to milliseconds to respond, Crowbar circuits are often combined with zener diodes in parallel with the output to further limit the voltage transient of the output voltage of any overvoltage.For this purpose, the document U.S. Pat. No. 5,144,517 A discloses, for example, a protective barrier device comprising a DC voltage source at a first and a second input terminal and a first and a second output terminal, wherein the first output terminal is in a first low-impedance series path with the first input terminal and the second output terminal is in a second low-impedance series path with the second input terminal. Current sensing means are provided for sensing current in at least one of the first and second series paths. Further, by means of an overcurrent level signal generating means, an overcurrent level signal is generated in accordance with a detected overcurrent level consisting of a current exceeding a preselected level. By means of an amplifier device which is at least partly fed by the excess current in at least one of the first and second series paths in order to amplify the excess current level signal provided by the excess current level signal generating means in order to form a trigger signal. A crowbar circuit for switching a low resistance current path between the first and second series paths in accordance with the trigger signal is further provided.EP 1 388 192 B1 discloses a circuit arrangement for limiting current in a feed line to a current consumer, having a first series branch which galvanically connects a first input point to a first output point, having a second series branch which galvanically connects a second input point to a second output point, having a current measuring device which detects the current in at least one of the series branches, having a controllable shunt device which is connected on the side of the current measuring device adjacent to the input point and which has a control input which is connected to the current measuring device, and having a nonlinear element which is situated between one of the input points and the shunt device and which responds at a low impedance below a current limit value and becomes high impedance when the current limit value is exceeded. In the activated state, the shunt device generates a shunt current for a short time, which brings the nonlinear element into response, at least a third and a fourth series branch branching off from the first and the second series branch, the branching point of the third and fourth series branch being situated on that side of the shunt device which is adjacent to the starting points. Furthermore, a further impedance is connected in series with the Zener diode between the output terminals, so that a higher output voltage is present at the output terminals than provided until the semiconductor switching element is closed.US 2019 / 0 058 326 A1 discloses an electronic blocking device comprising a voltage limiting device for voltage limiting in a circuit during a fault condition and a crowbar device arranged to switch depending on the voltage limiting device to reduce the power dissipation in the voltage limiting device in the fault condition of the circuit, wherein the crowbar device is arranged to switch in response to a change in a current detected in the voltage limiting device.When Zener diodes are used for voltage limiting and resistors for current limiting, high power losses arise in the event of a fault, which leads to increased outlay in design by the provision of additional cooling elements, limits the possible packing density and places high requirements on the thermal load capacity of these components.When using a crowbar, in which an overvoltage or an overcurrent is switched off with the aid of a power semiconductor switch, voltage and current transients can occur within the crowbar circuit and at the output terminals of the device, which transients can be critical for use in explosion-protected fields of application. Therefore, a complicated test of the device must be carried out in order to ensure explosion safety. Likewise, a distance between the functionally available output voltage and the available output current and the maximum output voltages and output currents relevant for the test must be maintained in the event of a fault.It is therefore an object of the present invention to provide an improved crowbar circuit which suppresses a passage of high voltage and current transients to the output side of the device or prevents the occurrence thereof.Disclosure of the InventionThis object is achieved by the crownbar circuit for a device according to claim 1, in particular for use in explosion-proof applications.Further embodiments are given in the dependent claims.According to a first aspect, a crownbar circuit is provided in particular for use in an e.g. electronic device, comprising:a triggering device for triggering a short circuit via input terminals depending on a control voltagea comparator which is designed to actuate the triggering device for triggering the short circuit as a function of a comparator voltage at a comparator input of the comparator and as a function of a predefined reference voltage threshold value,a zener diode having a predetermined breakdown voltage in series with a fifth resistor, which are connected between one of the input terminals and the comparator input, so that when the breakdown voltage is exceeded by the applied input voltage, the reference voltage threshold value is exceeded or undershot by the comparator voltage and the short circuit is thereby triggered;a series circuit of a seventh resistor, an eighth resistor of the fifth resistor and a sixth resistor, wherein a load current at an output of the Crowbar circuit flows through the seventh and the sixth resistor and the comparator input is connected in particular directly between the fifth and eighth resistors, wherein the series circuit is connected between the input terminals, so that when a predetermined threshold current is exceeded, the reference voltage threshold value is exceeded or undershot by the comparator voltage and the short circuit is thereby triggered.Crowbar circuits generally have Zener diodes, which limit the voltage until, when an overvoltage is detected, a controllable semiconductor switch is closed in order to short-circuit the voltage input. This leads to a fuse connected there in series triggering. In the event of too high a current, the semiconductor switch should likewise trip and thus protect the downstream applications from thermal overloading.The power design of a Zener diode used in a crowbar circuit provides that it is usually designed for 1.7 times the rated fuse current of the preceding fuse. This is disadvantageous because a high power loss in the application circuit beyond the desired level can thereby occur.The above crowbar circuit has a short-circuit shutdown which short-circuits the input side when an overload in the form of an overvoltage or an overcurrent is detected and triggers a fuse located there. When the semiconductor switch is closed, the input side is short-circuited, which leads to the fuse being opened. At the same time, any capacitive load present is discharged on the output side of the crowbar circuit.The crowbar circuit comprises a triggering device, in particular with a semiconductor switch, e.g. in the form of a MOSFET or thyristor, which is triggered directly or via a preamplifier circuit in order to short-circuit the input side. The triggering device or the semiconductor switch is controlled depending on a state of a comparator. Depending on the level of a comparator voltage at a comparator input with respect to a predefined reference voltage threshold value, a control signal for the triggering device / the semiconductor switch results, depending on which the short circuit is brought about, in that the semiconductor switch is closed, i.e. switched to low impedance.In principle, the above-mentioned method can be used. The circuit can be constructed in two variants that are complementary to one another, so that the short circuit is triggered when it is determined that the reference voltage threshold value is exceeded or that the reference voltage threshold value is not reached. Complementary construction herein means that the polarities are inverted and in each case inverse conductivity types of the semiconductor components used are used, so that in the case of the comparator the short circuit is triggered when the reference voltage threshold value is exceeded or-in the case of a correspondingly complementary construction-when the reference voltage threshold value is undershot.The triggering of the crowbar circuit thus takes place as a function of a comparator voltage, which is set as a function of an input voltage present on the input side and as a function of a flowing load current. For this purpose, in the load path between the input side and the output side of the crowbar circuit, a seventh resistor can be located between a first input terminal and a first output terminal and a sixth resistor can be located between a second input terminal and a second output terminal. The values of the sixth and seventh resistors are dimensioned such that a predefined triggering current or a current exceeding it leads to a change in the comparator voltage at the comparator input, which leads to a switching over of the control signal on the output side of the comparator and thus to the triggering of the triggering device.Furthermore, the zener diode and the sixth resistor can be connected in series, in particular directly between the input connections.Furthermore, the comparator input of the comparator may be connected to a series connection of a reverse zener diode and a fifth resistor. Depending on the design, the resulting comparator voltage is oriented at one of the supply voltage potentials, in particular at the higher of the supply voltage potentials. The comparator voltage then corresponds to a voltage value at the input of the comparator which is dependent on the breakdown voltage of the zener diode and on a voltage drop at the fifth resistor. The breakdown voltage of the zener diode is defined such that, at a load current of 0 and when a predetermined triggering voltage is exceeded, the comparator voltage exceeds / falls below the reference voltage threshold and thus triggers the triggering device.To set the thresholds for a given trigger current and the maximum zener diode current without inserting an additional impedance in series with the zener diode, an eighth resistor is used that connects a first output terminal to the comparator input of the comparator. Due to the seventh resistor in the load path, the output voltage potential is high at low output currents and the comparator input of the comparator is biased with an offset voltage. As a result, even a smaller additional current through the Zener diode is sufficient to trigger the crowbar circuit. As a result, the thermal load on the Zener diode is lower in the event of a fault, and a smaller or lower-power design of the Zener diode can be used. Furthermore, a smaller cooling surface can be provided.At higher output currents, the output voltage across the output terminals is low and the comparator input of the comparator is biased to a lesser extent via the resistor. Thus, a higher current through the output terminals and a higher triggering current of the crowbar are made possible.It can be provided that the comparator input is connected, in particular electrically directly, via the fifth resistor to one of the first or the second output terminal at which a high and a low output potential is provided.Furthermore, the fifth resistor can be directly electrically connected to the output terminal for the low output potential, in particular, wherein the resistance value of the seventh resistor is greater than the resistance value of the sixth resistor.In an alternative complementary construction, the fifth resistor can be connected in particular electrically directly to the output terminal for the high output potential, wherein the resistance value of the seventh resistor is lower than the resistance value of the sixth resistor.It can be provided that the resistance value of the seventh resistor is greater than the resistance value of the sixth resistor. Thus, as the output current increases, the voltage at the first output terminal decreases more rapidly than it increases at a second output terminal. The input voltage at the comparator input thus also decreases with increasing output current.It can be provided that a further zener diode is connected in series with the eighth resistor in the reverse direction.In the transition region between open circuit and short circuit at the output connections, the voltage offset at the comparator input continuously decreases. By inserting the optional further zener diode in series with the eighth resistor between the output terminal for the higher voltage potential and the comparator input of the comparator, the voltage offset from a specific threshold value of the output current is completely omitted, and the triggering threshold of the output current in the medium and high load range can be increased further.It can be provided that the comparator comprises a shunt reference with a predetermined reference voltage threshold value or a bipolar transistor, wherein the reference voltage threshold value is determined by its base-emitter voltage.If a higher variation of the triggering threshold with respect to the load current is acceptable, an NPN bipolar transistor or an N-channel MOSFET, for example, can be used instead of the comparator. This implementation involves less outlay on components, since the comparator can be dispensed with. In addition, the power consumption of the circuit decreases, since the supply of the comparator is dispensed with. Possible analog signals are therefore not distorted by an undesired cross current.Furthermore, a low-pass filter can be provided between the triggering device and the comparator, so that a time delay is provided when triggering the short circuit if it is determined that the comparator voltage exceeds or falls below the reference voltage threshold value.Consequently, the triggering circuit can be driven from the semiconductor switch and the preamplifier circuit can be carried out via a low-pass filter in order to be able to set a time delay for driving the triggering circuit or for closing the semiconductor switch. This allows the crowbar circuit to be inertiad in order not to let very short voltage peaks lead immediately to the fuse being triggered.Furthermore, current limiting can be realized within a line carrying load current, which limits the flowing load current when the comparator voltage exceeds or falls below the reference voltage threshold value.It can be provided accordingly to provide a limiting transistor in the current path, which limiting transistor can be designed as an NPN bipolar transistor or as a self-blocking N-channel MOSFET. As long as the load current is less than the trip current of the crowbar circuit, no appreciable current flows through the comparator, so that the limiting transistor conducts.As soon as the predetermined triggering current of the load current is exceeded, the comparator switches and a current flows which reduces the voltage at the control terminal of the limiting transistor and thus limits the load current. In this way, the current rise can be limited until the semiconductor switch is triggered, for example due to an output-side short circuit.Furthermore, one of the input connections can be provided with a fuse which interrupts the current flow when the short circuit is triggered.Brief Description of the DrawingsEmbodiments are explained in more detail below with reference to the attached drawings. The following are shown: FIG. 1 is a schematic illustration of a crownbar circuit according to a first embodiment of the invention; FIG. 2 shows a schematic illustration of a crowbar circuit according to a further embodiment; FIG. 3 shows a schematic illustration of a crowbar circuit according to a further embodiment.DESCRIPTION OF EMBODIMENTSFIG. 1 shows a crowbar circuit 1 for exemplary use in a power supply unit having a voltage converter for explosion-proof applications.The crowbar circuit 1 is arranged between an input side E with a first and a second input terminal E 1, E 2 for applying an input voltage U e and an output side A with a first and a second output terminal A 1, A 2 for tapping an output voltage U a between a high and a low output potential. A capacitance C1 between the input connections E1, E2 serves to smooth the input voltage against voltage disturbing influences.The purpose of the crowbar circuit 1 is to trigger a fuse F1 by a semiconductor switch P1 shorting the input terminals E1, E2 via which the input voltage U e is applied. The fuse F 1 is in series with the input voltage U e so that it is permanently opened by the short-circuit current.In the present embodiment, the semiconductor switch is designed as a thyristor P 1, which can be closed by means of a preamplifier circuit 3. Alternatively, the semiconductor switch can be designed as a MOSFET or as another semiconductor switch.An anode terminal of the thyristor P1 is connected to the first input terminal E1 via the fuse F1, and a cathode terminal of the thyristor P1 is directly connected to the second input terminal E2, so that when the input terminals are short-circuited by the thyristor P1, a high short-circuit current flows and interrupts the fuse. A transformer can optionally be located between the fuse F 1 and the thyristor P 1, in order to realize, for example, a galvanic isolation.A direct connection is understood herein to mean the conductive, very low-ohmic electrical connection as is created by a conductor track or a conductor connection, i.e. without an electrical or electronic component being interposed therebetween.The preamplifier circuit 3 may include a PNP transistor P2 in series with a first resistor R1 and a second resistor R2. Instead of a PNP transistor, an FET or the like may be provided. The node between the first resistor R 1 and the second resistor R 2 may be connected to the gate terminal of the thyristor P 1. A first terminal of the first resistor R 1 may be connected to the second input terminal directly or via another device. A second terminal of the first resistor R 1 may be connected to the gate terminal of the thyristor P 1 directly or via another device.A first terminal of the second resistor R 2 may be connected to an emitter terminal of the PNP transistor P 2 directly or via another device. A second terminal of the second resistor R 2 may be connected to the gate terminal of the thyristor P 1 directly or via another device. The collector terminal of the PNP transistor P 2 may be connected to the fuse F 1 directly or via another device or may be connected directly to the input terminal.The base input of the PNP transistor P 2 may be connected via a common node between third and fourth resistors R 3, R 4, which may be connected in series with a shunt reference IC 1 between the voltage potential of the first input terminal E 1 and the voltage potential of the second input terminal E 2.A first terminal of the fourth resistor R 4 may be connected to the base terminal of the PNP transistor P 2 directly or via another device. A second terminal of the fourth resistor R 4 may be connected to the voltage potential of the first input terminal E 1 directly or via a further component. A first terminal of the third resistor R 4 may be connected to the base terminal of the PNP transistor P 2 directly or via another device. A second terminal of the third resistor R 4 may be connected to the output of the shunt reference directly or via another device.The shunt reference IC 1 can be configured as a comparator, the output of which controls a semiconductor switch. If a comparator voltage is present at a comparator input of the shunt reference IC 1 that is higher than a predefined reference voltage threshold value intrinsic to the component, the shunt reference IC 1 becomes conductive and the voltage at the base terminal of the PNP transistor P 2 falls. As a result, the PNP transistor P2 becomes conductive and ignites the thyristor P1.In principle, the circuit part of the crownbar circuit 1 described up to this point forms a functionality of triggering a short circuit if the comparator voltage at the comparator input of the shunt reference exceeds the predefined reference voltage threshold value. This functionality can also be realized by a large number of other circuit configurations without deviating from the crownbar circuit in the sense of the invention.In order to realize a triggering functionality in the crowbar circuit 1 in the event of an overvoltage between the input terminals, a first Zener diode NZ 1 can be provided, which applies a voltage potential, which is reduced by the breakdown voltage of the first Zener diode NZ 1 starting from the potential at the first input terminal, to the comparator input of the shunt reference IC 1. For this purpose, the cathode terminal of the Zener diode NZ 1 can be connected to the potential of the first input terminal E 1 and to the anode terminal via a fifth resistor R 5, to the comparator input of the shunt reference IC 1 and via a sixth resistor R 6 to the second input terminal E 2. For this purpose, the anode terminal of the Zener diode NZ 1 can be connected directly or via a further component to a first terminal of the sixth resistor R 6, which in turn is connected to the second output terminal directly or via a further component. The second terminal of the sixth resistor R 6 may be connected to the second input terminal directly or via another device.By means of the fifth resistor R 5, the comparator input of the shunt reference is protected from the second output terminal A 2.For limiting the load current, a resistor arrangement may be provided having a seventh resistor R 7, an eighth resistor R 8, the fifth resistor R 5 and the sixth resistor R 6. These can be connected in series as resistor dividers. The seventh resistor R 7 and the sixth resistor R 6 are in this case situated in the main current path of the load between the output terminals A 1, A 2, with the result that these cause a voltage drop in the voltage divider which acts on the comparator voltage. The first output terminal A 1 may further be connected to the comparator input of the shunt reference via the eighth resistor R 8.The resistance value of the seventh resistor R 7 may also be distributed among a plurality of resistors to realize the resistance value. This has the advantage of achieving a better distribution of the heat on the printed circuit board. The division into a plurality of resistors can of course also be carried out for the fifth, sixth and eighth resistors R5, R6, R8. When using multiple series resistors to realize the resistance of the seventh resistor R 7, the tap for the eighth resistor R 8 may be made after "each" of the resistors, but at least one resistor needs to be located between the tap and the first output terminal A 1.If the load current in the crowbar circuit 1 exceeds a threshold value, the output potential at the second output terminal A 2 which rises as a result across the sixth resistor R 6 pulls up the control voltage, provided the resistance value of the sixth resistor R 6 is less than the resistance value of the seventh resistor R 7. These are dimensioned such that when a predetermined triggering current is reached, the comparator voltage reaches or exceeds the reference voltage threshold value of the shunt reference. This leads to triggering of the thyristor by the functionality of triggering a short circuit.The interconnection described above makes it possible to set different threshold values for the triggering current and the maximum zener diode current without inserting an additional impedance in series with the zener diode NZ 1.Thus, through the seventh resistor R 7 at the first output terminal A 1, the output potential depends on the load current corresponding to the voltage drop across the seventh resistor R 7. At low output currents, the output potential at the first output terminal A 1 is high and the comparator input of the shunt reference IC 1 is biased with an offset. As a result, a lower current through the zener diode NZ 1 is sufficient to trigger the crownbar circuit 1. This leads to a lower thermal load on the Zener diode NZ 1 in the event of a fault, so that a smaller design and a lower-power design can be provided.At higher load currents, the potential at the first output terminal A 1 is lower and the shunt reference IC 1 is biased correspondingly lower, i.e. the contribution to the comparator voltage is lower. A higher load current is thus necessary in order to trigger the crowbar circuit 1. In the transition region, the bias voltage at the comparator input of the shunt reference IC 1 continuously decreases, thereby influencing the output characteristic curve.In the series circuit of the seventh resistor R 7, the eighth resistor R 8, the fifth resistor R 5 and the sixth resistor R 6, a further zener diode NZ 2 can optionally be provided between the eighth resistor R 8 and the fifth resistor R 5, said further zener diode being connected to the comparator input of the shunt reference IC 1.The optional further zener diode NZ 2 in series with the eighth resistor R 8 completely switches off the bias voltage of the shunt reference IC 1 from a specific output current, and the triggering threshold of the voltage depending on the load current in the medium and high load range can be increased further. In addition, the further zener diode NZ 2 prevents a current flow through the eighth and the fifth resistor past the load present at the output connections.The embodiment of FIG. 2 differs from the embodiment of FIG. 1 in that the shunt reference IC 1 is replaced by an NPN bipolar transistor P 3 or an N-channel MOSFET. This results in a base-emitter voltage of approximately 0.7 V for the reference voltage threshold value.FIG. 3 shows a further embodiment of the crowbar circuit 1, in which current limiting is realized within the current-carrying line. This current limiting can be designed, for example, by an NPN bipolar transistor P4 or as a self-blocking N-channel MOSFET. This can be controlled as a function of the cathode voltage at the cathode terminal of the shunt reference IC 1. In particular, the base terminal of the NPN bipolar transistor P 4 is connected via a ninth resistor to the voltage potential of the first input terminal and via a tenth resistor to the cathode terminal of the shunt reference IC 1.As long as the load current is less than the set triggering current of the crowbar circuit 1, no appreciable current flows through the shunt reference IC 1 and through the third and fourth resistors R 3, R 4. As soon as the set limit value of the load current through the first zener diode NZ 1 or the eighth resistor R 8 is exceeded, the shunt reference IC 1 becomes low-ohmic and a current flows through the tenth resistor R 10, which reduces the voltage at the base terminal of the NPN transistor P 4 and the voltage at the emitter of the NPN transistor P 4 likewise reduces. The current through the first zener diode NZ 1 and the seventh resistor R 9 decreases in sequence until the load current reaches the value of the trigger current again. This makes it possible to implement current limiting which initially prevents immediate triggering of the crowbar circuit 1.In order for the first transistor P1 to become conductive, a base-emitter voltage Ube<-0.6 V must be present at the transistor P3. The base-emitter voltage of the transistor P 3 corresponds to the voltage drop across the fourth resistor R 4: UBE(P 3)=U(R 4). The resistance of the third resistor R3 will typically be greater than the resistance of the fourth resistor R4, thus following U(R3+R4)=ca -2,... -5V as the triggering condition. For transistor P4 to conduct, its base-emitter voltage must be Ube(P4)>0.6 V, i.e., U(R10)>0.6 V+U(R3+R4). Thus, the ninth and tenth resistors R9 and R10 may be dimensioned such that the transistor P4 turns off before the transistor P3 turns on.In order to meet the requirements of explosion protection, it may be necessary in the above embodiments to design individual components of the crownbar circuit 1 or entire circuit blocks in a multiply redundant manner. In order to realize inertia during the triggering of the crowbar circuit 1, the resistor R 4 can be replaced by a low-pass filter, so that a time delay results during the triggering of the thyristor P 1.In principle, the above-mentioned method can be used. The circuit can be constructed in two embodiments that are complementary to one another, so that the short circuit is triggered when it is determined that the reference voltage threshold value is exceeded or that the reference voltage threshold value is not reached.Typical exemplary values for characteristic variables of the relevant components are: breakdown voltage of the first Zener diode NZ 1: 8-24 V (can also be depicted by 2-3 Zener diodes in series, e.g. 3 x 8 V for 24 V). Breakdown voltage of the second zener diode NZ2: Lower value than for NZ1, 4-12V.R5: 10-20 kiloohmsR6: 41-100 ohmsR7: typically greater than R6, 70-360 ohmsR8: typically greater than R5, 20-200 kiloohms

Claims

A crowbar circuit (1) for use in a voltage converter, comprising: - a trip device (P1) configured to trip or cause a short circuit across input terminals (E1, E2) depending on a control voltage; a comparator (IC1) which is designed to actuate the triggering device (P1) for triggering the short circuit as a function of a comparator voltage at a comparator input of the comparator (IC1) and as a function of a predefined reference voltage threshold value, a zener diode (NZ1) having a predefined breakdown voltage in series with a fifth resistor (R5) which are connected between one of the input terminals and the comparator input, so that, when the breakdown voltage is exceeded by the applied input voltage, the reference voltage threshold value is exceeded or undershot by the comparator voltage and the short circuit is thereby triggered or brought about; a series circuit of a seventh resistor (R7), an eighth resistor (R8), the fifth resistor (R5) and a sixth resistor (R6), wherein a load current at an output of the crowbar circuit (1) flows through the seventh and sixth resistors (R7, R6) and the comparator input is electrically connected to a node between the fifth and eighth resistors (R5, R8) and the series circuit is connected between the input terminals, so that when a predetermined threshold current is exceeded, the reference voltage threshold value is exceeded or undershot by the comparator voltage and the short circuit is thereby triggered.The crowbar circuit (1) according to claim 1, wherein the comparator (IC1) comprises a shunt reference (IC1) with a predetermined reference voltage threshold or a bipolar transistor, wherein the reference voltage threshold is determined by its base-emitter voltage.The crowbar circuit (1) according to claim 1 or 2, wherein a further zener diode (NZ2) is connected in reverse direction in series with the eighth resistor (R8).The crowbar circuit (1) according to any one of claims 1 to 3, wherein a low pass filter is provided between the trip device (P1) and the comparator (IC1), such that a time delay is provided when the short circuit is trip if it is determined that the comparator voltage exceeds or falls below the reference voltage threshold.The crowbar circuit (1) according to any one of claims 1 to 4, wherein the comparator input is connected via the fifth resistor (R5) to one of two output terminals at which a high and a low output potential is provided.The crowbar circuit (1) according to claim 5, wherein the fifth resistor (R5) is connected to the output terminal for the low output potential, wherein the resistance value of the seventh resistor (R7) is greater than the resistance value of the sixth resistor (R6).The crowbar circuit (1) according to claim 5, wherein the fifth resistor (R5) is connected to the output terminal for the high output potential, wherein the resistance value of the seventh resistor (R7) is lower than the resistance value of the sixth resistor (R6).The crowbar circuit (1) according to any one of claims 1 to 7, wherein the zener diode (NZ1) and the sixth resistor (R6) are connected in series between the input terminals (E1, E2).Crowbar circuit (1) according to one of Claims 1 to 5, wherein current limiting is realized within a line carrying load current which limits the flowing load current when the comparator voltage determines whether the reference voltage threshold value is exceeded or undershot.The crowbar circuit (1) according to any one of claims 1 to 9, wherein one of the input terminals is provided with a fuse (F1) that interrupts the current flow when the short circuit is triggered.

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