DE19816942A1 - Circuit for monitoring an electric circuit for conductor breaks particularly for input and output circuits of sensors - Google Patents

Abstract

The circuit monitors the voltage drop across a resistor in the sensor supply or the sensor output lead. To monitor the input circuit, a voltage divider (20) with two resistors (22,24) is located in the supply lead (32) to the sensor. The center point (42) between the resistors is connected through a filter (16) to two threshold detectors. One detector (44) is set to operate below a threshold that represents a specific minimum current indicating a conductor break. A second detector (52) monitors the logic state of the input signal to the detectors indicated by LEDs (48,56). To monitor the output circuit, a single detector monitors the voltage across a diode connected between the output lead and ground.

Description

The invention relates to a circuit arrangement for monitoring a current circuit on line break, the circuit at least one current and / or voltage source and at least one active or in the circuit via supply lines passive electrical unit such as B. has a signal generator or a load / actuator.

Input / output devices each have connections for connecting signal transmitters or actuators speaking input or output circuits. The inputs are ver different standards such as B. IEC 1131 type 1 and type 2 standardized, with inputs according to IEC 1131 type 1 z. B. for sensor signals from mechanical contacts such as relays, Push buttons as well as 3-wire proximity switches and inputs according to IEC 1131 type 2 for sensor signals from "solid state devices", from mechanical contacts and buttons as well as all 3- / 2- Wire proximity switches according to the IEC 947-5-2 standard are suitable.

The inputs differ essentially in their input voltage and Current ranges through which various signal states such as "on signal", "transition" and  "Zero signal" are set.

For the use of active 2-wire sensors that supply one to their internal logic Supply current, inputs of type IEC 1131 type 2 were previously used, which for the detection of an "on signal" a comparatively high input current of need at least 6 mA, which causes losses and a complex cable transfer watch requires.

For monitoring the supply lines for line breaks of passive analog Senso Ren like resistance thermometers, is a circuit arrangement from DE 43 22 490 A1 known with a constant current generator, the supply lines to the sensor connected is. This is used to monitor the supply lines for line breaks Potential in the output of the current generator with a predetermined value by means of a Threshold switch compared. However, such a circuit arrangement is not for Monitoring of supply lines suitable for digital signal transmitters, their input signals are monitored for logical states.

The connection of an input / output device is usually carried out by an outgoing conductor from the feeding input / output device to the actuator or the load and a return conductor is routed to a common busbar.

According to the prior art, wire break monitoring in the outgoing conductor is thereby realizes that a resistor is arranged in the course of the conductor, and one of one Voltage generated in the current conductor flowing voltage is monitored at the resistor. Here, the problem arises that when no current flows in the down conductor, that is, H. in off Condition, monitoring of the outgoing line is excluded. Furthermore, the voltage dropping across the measuring resistor fluctuates so that with respect to the Dimensioning of the resistance and high demands regarding the evaluation circuit are to be fulfilled.

Another option for monitoring a circuit in the off state is to  to briefly apply pulses to the circuit, at the same time taking a measurement gear is started. Here the problem arises that the impulses are so brief need, so that connected consumers such. B. Relay not during the pulse duration can attract. An evaluation of these short-term impulses is also high Associated effort.

The present invention addresses the problem of an input or output to further develop the circuit of the aforementioned type such that it is as low as possible Power loss a simple and safe monitoring of the supply lines Wire break is possible.

The problem is solved according to the invention in that a minimum current in the circuit It can be specified which one on at least one component located in the circuit Voltage drop caused by a threshold value associated with the component switch is monitored and compared with a predeterminable value. The described The basic principle can apply to both an input circuit and an output current circle can be used. The minimum current can be specified such that a the lowest possible power loss is generated in the circuit arrangement.

In a particularly preferred embodiment it is provided that the circuit is designed as an input circuit with a voltage divider, the voltage divider being connected to the first threshold switch for monitoring the input circuit for line break and to a second threshold switch for monitoring a logic input signal of the electrical unit such as signal transmitters is. A means tap of the voltage divider is connected on the one hand to a first preferably negative input of the first threshold switch and on the other hand to a first preferably positive input of the second threshold switch, with a second input of the threshold _switches each having a first or second threshold value such as reference voltage (U _ref1 , U _ref2 ) is connected.

The input circuit described above is particularly suitable for active 2-wire proximity switches that have a voltage drop of ≦ 8 V, a minimum operating current of ≦ 2 mA and a maximum current of ≦ 0.8 mA when switched off. It is necessary to specify a minimum input current in the input circuit, which is, for example, the minimum operating current of the connected sensor element. Via the first threshold switch, which can be designed as a comparator, the voltage U _{t applied} to the center _{tap is} compared with the first reference voltage U _ref1 and it is determined whether the input current is greater or less than the required minimum current. If the input current falls below the required minimum current, which, for. B. indicates a line break, the first threshold switch switches and indicates a line break.

For the optical display of the initial state of the first threshold switch is a Output of the comparator via a series connection of diode and resistor with ground potential connected. A corresponding output circuit also has the output of the second threshold switch. The circuit arrangement is particularly notable characterized in that the minimum current can be chosen so low that it is lower than a 0.3 mA of the majority of all 2-wire proximity switches. On in this way the proximity switches can be used directly. Be simple mechanical contacts of relays or pushbuttons used as signal generators are provided, that a resistor is arranged in parallel with the switching contact in order to achieve the minimum current in the Maintain input circuit.

Another special feature of the input circuit monitoring is that the Monitoring can be released or switched off. It is intended that the first Threshold switch has an open collector output and a resistor to Enable monitoring with positive potential and to block monitoring with Ground potential is connected. Furthermore, the output is with a driver module connected, at the output of which the logic signals of line monitoring when enabled are tapped. For the optical display of the output signal, the output of the driver Building block connected via a light emitting diode and a resistor with ground potential.

If the first threshold switch with an open collector output detects a signal that Line break corresponds, while the output of the threshold switch via the opp was at ground potential, the level at the output of the driver block is "Logical 0"  tapped. This functionality is useful if an input is unused or if is not connected.

Furthermore, between the first inputs of the first and second comparators and the Center tap of the voltage divider arranged a filter element. The filter element is as R-C member formed, with a resistance between the center tap and the on gears is arranged and from the inputs a capacitor against ground potential is switched.

In a further preferred circuit arrangement, it is provided that the circuit is designed as an output circuit with a switching element such as a transistor switch and a resistor connected in parallel therewith, the load and / or the actuator being connected in series with the switching element and the load and / or the actuator is connected via a diode, in particular a Schottky diode with reference potential, which is connected to a first input of the threshold switch, the second input of which is connected to a third reference _voltage (U _ref2 ). A special feature of this circuit arrangement is that the monitoring takes place in the return path and in relation to ground potential (0 V reference). As already stated above, with conventional wiring in control cabinets, only the outgoing conductor is led from the supplying module to the load, with the return conductor being guided on a common busbar. With such a circuit arrangement, only the much more difficult line monitoring in the outgoing conductor is possible.

A minimum is also established via the resistor arranged in parallel with the switching element current impressed so that even when the switching element or output driver is switched off Output circuit can be monitored. For so-called "current source" outputs is defines a maximum leakage current (off-state leakage current). That the imprinted minimum current does not exceed the maximum leakage current is another essential property the circuit arrangement. For an output with a nominal current of 0.5 A, for example maximum leakage current when switched off approx. 1 mA.

The diode via which the return conductor with the common ground potential is preferred is connected, designed as a Schottky diode. The Schottky diode causes the same thing Current is less dissipated than a normal diode. For filtering the on the bulkhead ky diode applied measurement signal is between the input of the comparator and the  An R-C element is provided on the anode connection of the Schottky diode.

As a protective circuit against undervoltage when switching inductive loads another diode connected in antiparallel to the Schottky diode.

Furthermore, it is provided that the positive input of the comparator is connected to ground potential (0 V reference) via a first resistor (R _n ), thereby preventing the comparator from "floating" in the event of a line break or unused output. The resistance (R _n ) at the positive input of the comparator should have a certain ratio to the resistance R _i lying parallel to the switching element, which is in the range of 0.1 × R _p <R _n <0.2 × R _p . It is also provided that the monitoring of the output circuit can be switched off. The positive input of the comparator is connected to a further resistor R _en , to which a voltage can be impressed in order to suppress the monitoring circuit. If there is a potential at the resistor R _en which is greater than the reference voltage, a "Logical 1" signal is present at the output of the comparator regardless of the voltage drop across the Schottky diode, which is associated with the information "no line break".

Further details, advantages and features of the invention result not only from the Claims, the features to be extracted from these - individually and / or in combination -, but also from the following description of one of the drawings Embodiment.

Show it:

Fig. 1 shows a circuit arrangement of an input monitoring circuit with open-circuit,

Fig. 2 shows a circuit arrangement according to Fig. 1 with disconnectable line break monitoring and

Fig. 3 shows a circuit arrangement of an output circuit with line break monitoring.

In Fig. 1, a circuit arrangement 10 is shown, having an input circuit 12 and an evaluation unit 14 which is coupled with an input filter 16 to the input circuit 12.

The input circuit 12 has an internal current branch 18 , which in the exemplary embodiment shown here is designed as a voltage divider 20 with the resistors 22 and 24 , and an external current branch 26 , which consists of a series connection of voltage source 28 and signal generator 30 . The signal generator 30 is connected via a first supply line 32 to a first input terminal Ix and via a second connecting line 34 to a first pole of the voltage source 28 . A second pole of the voltage source 28 is connected via a further supply line 36 to an input terminal Mges, which is internally connected to the ground potential.

Passive signal transmitters such as mechanical contacts of relays or push buttons or active signal transmitters such as 2-wire proximity switches can be used as signal transmitters 30 . The 2-wire proximity switches preferably have a voltage drop of ≦ 8 V, a minimum operating current of ≦ 2 mA and a maximum current of ≦ 0.8 mA when switched off.

The input filter 16 is designed as an RC element with a resistor 38 and a capacitor 40 , the first connection of the resistor 38 being connected to a center tap 42 of the voltage divider 20 and the second connection providing an output 42 at which one Measuring voltage U _{t can be} tapped. The output 42 is connected to ground potential via the capacitor 40 .

The evaluation unit 14 consists of a first threshold switch 44 , the negative input of which is connected to the output 42 of the input _filter 16 and the first input voltage U _ref1 is present at the positive input _thereof . At an output 46 of the threshold switch 44 , an output signal can be tapped, which provides information about the line status. The output 46 is connected to ground potential via a light emitting diode 48 and a resistor 50 .

Furthermore, a second threshold switch 52 is provided for monitoring the input signal of the signal generator 30 , which is connected with its positive input to the output 42 of the input _filter 16 and at whose negative input a second reference voltage U _ref2 is present. At an output 54 of the threshold switch 52 , an output signal can be tapped, which indicates the logical state of the input signal or the signal generator 30 . For optical display of the output signal, the output 54 is connected via a light-emitting diode 56 and a resistor 58 to ground potential.

In the exemplary embodiment shown here, a "Logical 1" signal can be tapped at the output 54 if the input voltage is in the range from +11 V to +30 V and a resulting input current I _{E is} in the range from 2.5 mA to 10 mA. A signal "Logical 0" can be tapped at the output 54 when the input voltage U _e in the range from -3 V to +5 V and a resulting input current I _E in the range ND. . . 10 mA.

Via the input voltage divider 20 and the input filter 16 , the input signal U _E , I _{E is} mapped into the measurement voltage U _t , which has a voltage range that is adapted to the inputs of the internal logic of the evaluation unit 14 . To determine the logic state of the input signal, the second comparator 52 compares the measurement voltage U _t with the second reference voltage U _ref2 . If the _measurement voltage U _{t is} above the second reference voltage U _ref2 , there is a "logic 1" signal at the output 54 , which is also indicated by the light-emitting diode 56 lighting up. If the _measurement voltage U _{t is} below the second reference voltage U _ref2 , the signal at output 54 is "logic 0".

The first comparator 44 is used for wire break or wire break monitoring in an input circuit 12 . For this it is necessary that the input current I _{E is provided} by a minimum input current I _Emin during operation. The first comparator 44 _{compares the measurement} voltage U _t with the first reference voltage U _ref1 , which is _lower than the second reference _voltage U _ref2 . In particular, it is determined whether the input current I _{E is} greater or less than the required minimum input current I _Emin . If the measuring voltage U _{t is} less than the reference voltage U _ref1 , ie the input current I _E is less than the minimum input current I _emin , a signal "Logical 1" is present at the output 46 of the first comparator 44 , as a result of which a wire break or line break is indicated. However, if the input current _exceeds the minimum current I _Emin , the measurement voltage U _{t is} greater than the first reference voltage U _ref1 , so that a "Logical 0" signal is present at the output 46 , as a result of which a perfect operating state, ie no wire break, is indicated.

In particular, the minimum _current I _emin can be chosen to be so low that it is lower than the minimum supply _current of approx. 0.3 mA of the majority of all 2-wire proximity switches. In this way, the proximity switches can be used immediately.

FIG. 2 shows a circuit arrangement 60 which essentially has the same structure as the circuit arrangement 10 according to FIG. 1. In the exemplary embodiment shown, a mechanical switch contact 62 of a relay or a push button is provided as the signal transmitter 30 . In order to maintain a minimum input current I _E during operation for wire monitoring, it is provided that a resistor 64 is connected in parallel with the switching contact 62 .

In order to enable or disable the output of the comparator 44 , it is provided that the comparator 44 in the circuit arrangement according to FIG. 2 has an "open collector" output 66 . A logic 68 or 5 V signal can be applied to the output via a resistor 68 to enable wire break monitoring and a logic 0 or 0 V signal to suppress wire break monitoring. The output 66 is followed by a driver module 70 , the output 71 of which is connected to ground potential via a diode 73 and a resistor 75 . The "Logical 0" signal is present at output 71 if no wire break is detected when monitoring is enabled, ie if the measurement voltage U _{t is} greater than the first reference _voltage U _ref1 or if the monitoring is not enabled.

Suppression of wire break monitoring is particularly advantageous if a Entrance is unused.

In Fig. 3 is a circuit arrangement 72 of an output circuit 74th The output circuit 74 of the circuit 74 consists essentially of a series circuit comprising a voltage source (not shown), a switching element 76 , a load 78 and a diode 80 which conducts in the operating current direction and is connected to ground potential on the cathode side. The load 78 is connected via a forward line 82 to a terminal OH and via a return line 84 to a terminal OR of an input / output device (not shown) in which the components 76 , 80 are arranged.

In parallel with the switching element 76 , which is preferably designed as a transistor with a control input 86 , there is a resistor 88 which , even when the switching element 76 is switched off, enables a current to flow in the output circuit 74 . The voltage source, not shown, can be designed as an external voltage source and connected to input terminals US and M. The input terminal M is internally connected to ground potential. The input terminal OR, which is connected on the output side to the return conductor 84 , is connected to ground potential via the diode 80 . On the anode side, the diode 80 is connected to an input 90 of an RC element 92 , which is connected on the output side to a positive input of a _{threshold switch} 94 , the negative input of which is connected to a third reference _voltage U _ref3 . By the threshold switch 94, the drop across the diode 80 or the voltage clamping is applied to the input OR potential with the reference voltage U _ref3 ver equalized. Due to the resistor 88 arranged in parallel with the switching element 76 , a minimum _current I _{Lmin flows} in the output _circuit 74 even when switched off. A maximum "off-state leakage" current is defined for what are known as "current source" outputs. In the embodiment shown here it is provided that the impressed minimum current does not exceed the maximum "off-state leakage" current. For an output with a nominal current of 0.5 A, for example, the maximum "off-state leakage" current is 1 mA.

As soon as the voltage falling across the Schottky diode 80 or the potential at the input OR _falls below the reference _voltage U _ref3 , the signal "logic 0" is present at the output 96 of the threshold switch 94 designed as a comparator, as a result of which a line break is signaled; because if the line breaks, the current I _L through the input circuit is 0 A.

The diode 80 is preferably designed as a Schottky diode, which generates a lower power loss at the same current compared to a normal diode. In order to ensure protection against undervoltage, which can occur when switching inductive loads, a further protective diode 98 is arranged antiparallel to the diode 80 .

Furthermore, it is provided that the positive input of the threshold switch 94 is connected to ground potential via a resistor 100 with the value R _n . This prevents the threshold switch from "floating" in the event of a line break or when the OH, OR output is not used. The resistor 100 with the value R _n should form a ratio in the range 01 × R _p ≦ R ≦ 0.2 × R _p to the resistor 88 with the value R _p .

Furthermore, it is provided that the output 96 of the threshold switch 94 can also be switched off in this circuit arrangement. In order to achieve this, the positive input of the threshold switch 94 is connected via a resistor 102 either to a signal "Logical 1", which is greater than the reference _voltage U _ref3 , in order to suppress the wire break monitoring and / or to a signal "Logical 0" connectable to enable monitoring.

Claims (19)

1. Circuit arrangement ( 10 , 72 ) for monitoring a circuit ( 12 , 74 ) for line break, in particular an input or output circuit comprising a current and / or voltage source ( 28 ) and at least one via supply lines ( 32 , 34 , 82 , 84 ) in the circuit ( 12 , 74 ) active or passive electrical unit ( 30 , 78 ) such as signal transmitter or a load / actuator, characterized in that in the circuit ( 12 , 74 ) a minimum current (I _Emin , I _Lmin ) It can be specified which causes a voltage drop on at least one component ( 22 , 24 , 80 ) in the circuit which is compared by at least one (first) threshold _switch ( 44 , 94 ) with a predefinable threshold value (U _ref1 , U _ref3 ). 2. Circuit arrangement according to claim 1, characterized in that the circuit ( 12 ) is designed as an input circuit with a voltage divider ( 20 ), the voltage divider ( 20 ) with the first threshold switch ( 44 ) for monitoring the input circuit for line break and with a second Threshold switch ( 52 ) for monitoring a logic input signal of the electrical unit ( 30 ) is connected as a signal generator. 3. Circuit arrangement according to claim 1 or 2, characterized in that a center tap ( 42 ) of the voltage divider ( 20 ) on the one hand with a first preferably negative input of the first threshold switch ( 44 ) and on the other hand with a first, preferably positive input of the second threshold switch ( 52 ) is connected, a second input of the respective threshold switch ( 44 , 52 ) being connected to a first or second threshold value such as reference _voltage (U _ref1 , U _ref2 ). 4. Circuit arrangement according to one or more of the preceding claims, characterized in that the minimum current I _{Emin is} less than or equal to the supply current of a signal generator ( 30 ) lying in an input circuit ( 12 ). 5. Circuit arrangement according to one or more of the preceding claims, characterized in that the value of the first reference _voltage U _{ref1 is} less than the value of the second reference _voltage U _ref2 . 6. Circuit arrangement according to one or more of the preceding claims, characterized in that the signal transmitter ( 30 ) is designed as an electrical switching contact ( 62 ) of a relay or pushbutton, a resistor ( 64 ) being switched in parallel with the switching contact ( 62 ). 7. Circuit arrangement according to one or more of the preceding claims, characterized in that the first threshold switch ( 44 ) is designed as a comparator with a "push-pull" output ( 46 ), which has a series connection of light-emitting diode ( 48 ) and resistor ( 50 ) is connected to ground potential. 8. Circuit arrangement according to one or more of the preceding claims, characterized in that the first threshold switch ( 44 ) has an "open collector" output ( 66 ) which, via a resistor ( 68 ), optionally with a signal "Logical 1" or a signal "Logical 0" can be connected to enable or suppress the output ( 66 ). 9. Circuit arrangement according to one or more of the preceding claims, characterized in that a filter element ( 16 ), preferably an RC element, is arranged between the center tap ( 42 ) of the voltage divider ( 20 ) and the first inputs of the threshold switches ( 44 , 52 ) . 10. The circuit arrangement according to claim 1, characterized in that the circuit is designed as an output circuit ( 74 ) with a switching element ( 76 ) and a resistor ( 88 ) connected in parallel therewith, that the peripheral as a load and / or as an actuator ( 78 ) Component in series with the switching element ( 76 ) and that the load and / or the actuator ( 78 ) is connected via a diode ( 80 ) with reference potential, the diode ( 80 ) having a first input of the threshold switch ( 94 ) is connected, the second input of which is connected to a third reference _voltage (U _ref2 ). 11. Circuit arrangement according to one or more of the preceding claims, characterized in that the diode ( 80 ) is designed as a Schottky diode. 12. Circuit arrangement according to one or more of the preceding claims, characterized in that the minimum _current I _Lmin flowing in the output _{circuit can be set} via the _resistor ( 88 ) and does not exceed a maximum predetermined leakage current. 13. Circuit arrangement according to one or more of the preceding claims, characterized in that the diode ( 80 ) on the anode side on the one hand with an input terminal OR for a return conductor ( 84 ) of the load and / or the actuator ( 78 ) and on the other hand with a first input of the threshold switch ( 94 ) is connected. 14. Circuit arrangement according to one or more of the preceding claims, characterized in that a voltage dropping at the diode ( 80 ) or a potential at the input OR is compared via the threshold switch ( 94 ) with the reference _voltage U _ref3 . 15. Circuit arrangement according to one or more of the preceding claims, characterized in that the preferably positive first input of the threshold switch ( 94 ) is preceded by an R-C filter element ( 92 ). 16. Circuit arrangement according to one or more of the preceding claims, characterized in that a protective diode ( 98 ) is connected antiparallel to the diode ( 80 ). 17. Circuit arrangement according to one or more of the preceding claims, characterized in that the preferably positive input of the threshold switch ( 94 ) is connected to ground potential via a resistor ( 100 ). 18. Circuit arrangement according to one or more of the preceding claims, characterized in that the preferably positive input of the threshold switch ( 94 ) via a resistor ( 100 ) is optionally connectable with a signal "Logical 1" or "Logical 0", with the Signal "Logical 1" the voltage monitoring is suppressed by the threshold comparator ( 94 ). 19. Circuit arrangement according to one or more of the preceding claims, characterized in that the resistor R _p ( 88 ) and the resistor R _n ( 100 ) are in a certain relationship to one another, preferably 0.1 × R _p ≦ R _n ≦ 0, 2 × R _t .