Connection circuit of field device and field device

Abstract

The invention relates to a connection circuit (1) for a field device (100). Comprising two connectors (2) forming a two-wire interface, a microcontroller (3) for operating a field device, a voltage converter (4) connected upstream of the microcontroller (3) and configured to operate the microcontroller (3) with an operating voltage, a power supply capacitor (5) connected upstream of the voltage converter (4) and configured to absorb electrical energy when the connection circuit (1) is activated and to use it to supply power to the voltage converter (4); a first current limiting element (6) connected upstream of the power supply capacitor (5) and designed to limit the input current below a permissible limiting current when the connection circuit (1) is activated, a first bridging element (8) connected in parallel with the first current limiting element (6) for bridging the first current limiting element (6) when a first criterion is met, and a test element (10) configured to check whether the first criterion is satisfied. The invention also relates to a field device.

Description

Technical Field

[0001] The invention relates to a connecting circuit for a field device and to a field device having the connecting circuit according to the invention. Background Art

[0002] In automation, in particular process automation, field devices for capturing and / or modifying process variables are often used. In order to detect process variables, sensors integrated into, for example, filling level measuring devices, flow meters, pressure and temperature measuring devices, pH-redox potentiometers, conductivity meters, etc. are used to detect corresponding process variables, such as filling level, flow, pressure, temperature, pH level or conductivity. Actuators such as, for example, valves or pumps are used to influence process variables. Therefore, the flow rate of a fluid in a pipeline section or the filling level in a container can be changed by means of an actuator. In principle, all devices that are process-oriented and provide or process process-related information are referred to as field devices. In connection with the present invention, "field devices" therefore also refer to remote I / O, radio adapters, or generally electronic measuring components that are arranged at the field level.

[0003] The field device is in particular selected from the group consisting of a flow meter, a fill level measuring device, a pressure measuring device, a temperature measuring device, a limit level measuring device and / or an analytical measuring device.

[0004] The flow meter is in particular a Coriolis flow meter, an ultrasonic flow meter, a vortex flow meter, a thermal and / or magnetic inductive flow meter.

[0005] The fill-level measuring device is in particular a microwave fill-level measuring device, an ultrasonic fill-level measuring device, a time domain reflectometry device, a radiometric fill-level measuring device, a capacitive fill-level measuring device, an inductive fill-level measuring device and / or a temperature-sensitive fill-level measuring device.

[0006] The pressure measuring device is in particular an absolute pressure device, a relative pressure device or a differential pressure device.

[0007] The temperature measuring device is in particular a measuring device having a thermocouple and / or a temperature-dependent resistor.

[0008] The limit level measuring device is in particular an electronic vibration limit level measuring device, an ultrasonic limit level measuring device and / or a capacitive limit level measuring device.

[0009] Analytical measuring devices are in particular pH sensors, conductivity sensors, oxygen and reactive oxygen species sensors, (spectro)photometric sensors and / or ion-selective electrodes.

[0010] In order to accommodate the connecting circuit, the field device of the described type also includes an electronics housing, which, as proposed, for example, in US 63 97 683 A and WO 00 / 36379 A1, can be arranged remote from the sensing element and connected to the sensing element only via a flexible cable, or, as shown, for example, in EP 903 651 A1 or EP 1 008 836 A1, is arranged directly on the sensing element or in a sensing element housing in which the sensing element is separately arranged. Usually, the electronics housing is also used to accommodate some mechanical parts of the sensing element, such as a membrane-like, rod-like, sleeve-like or tubular deformation body or a vibrating body that deforms under mechanical influence during operation, as shown, for example, in EP 984 248 A1, US 45 94 584 A1, US 47 16 770 A1 or US 63 52 000 B1; see also US 63 52 000 B1 mentioned at the beginning. Field devices of the type described are usually also connected to each other and / or to corresponding process control computers via a data transmission system connected to a connecting circuit, to which they send measured value signals, for example via a (4 mA to 20 mA) current loop and / or via a digital data bus, and / or receive operating data and / or control commands from these computers in a corresponding manner. The data transmission systems used here are in particular serial fieldbus systems, such as PROFIBUS-PA, FOUNDATION Fieldbus and corresponding transmission protocols. With the aid of the process control computer, the transmitted measured value signals can be further processed and visualized as corresponding measurement results on a monitor, for example, and / or converted into control signals for other field devices formed as actuators, such as solenoid valves, electric motors, etc.

[0011] Modern field devices are usually so-called two-wire field devices, i.e. field devices in which the connection circuit is electrically connected to an external power source via only one pair of cables (two-wire conductors) and in which the connection circuit also transmits current measurements via the single pair of cables to an evaluation unit provided in the external power source and / or electrically coupled to the external power source. The connection circuit comprises a current controller, via which the supply current is used to set and / or modulate (in particular time) the supply current, an internal operating and evaluation circuit for controlling the field device, and an internal power supply circuit, to which an internal input voltage of the field device electronics divided from the supply voltage is applied and which supplies the internal operating and evaluation circuit, the internal operating and evaluation circuit having at least one voltage regulator, through which a variable partial current of the supply current flows, and which provides an internal useful voltage in the field device electronics which is substantially constantly regulated at a predefinable voltage level. 65 74 515 B1, US 65 35 161 B1, US 65 12 358 B1, US 64 80 131 B1, US 63 11 136 B1, US 62 85 094 B1, US 62 69 701 B1, US 6140 940 A1, US 67 99 476 B1, US 6577 989 B2, US 66 62 120 B1, US 65 74 515 B1, US 65 35 161 B1, US 65 12 358 B1, US 64 80 131 B1, US 63 11 136 B1, US 62 85 094 B1, US 62 69 701 B1, US 6140 940 A1, US 60 14 100 A1, US 59 59 372 A1, US 57 42 225 A1, US 56 72 975A1, US 55 35 243 A1, US 54 16 723 A1, US 52 07 101 A1, US 50 68 592 A1, US 5065 152 A1, US 49 26 340 A1, US 46 56 353 A1, US 43 17 116 A1, EP 1 147 841A1, EP 1 058 093 A1, EP 591 926 A1, EP 525 920 A1, EP 415 655 A1, DE 44 12388 A1 or DE 39 34 007 A1.

[0012] Historically, such two-wire field devices have been primarily designed so that the instantaneous current intensity of the supply current currently flowing in the single pair of wires designed as a current loop is set to a value between 4 mA and 20 mA, while representing the measurement value currently generated by the field device or the setting value currently transmitted to the field device. As a result, a particular problem with such two-wire field devices is that the electrical power that can or will be converted by the connecting circuit at least nominally (hereinafter referred to as "available power") can fluctuate over a wide range in an almost unpredictable manner during operation. With this in mind, modern two-wire field devices, in particular modern two-wire measuring devices with a (4 mA to 20 mA) current loop, are usually designed so that their device functionality, which is implemented by means of a microcontroller provided in the evaluation and operating circuit, can be changed, and in this way, the operating and evaluation circuit, which usually implements little power, can be adapted to the currently available power.

[0013] Advanced Physical Layer (APL: ADVANCED PHYSICAL LAYER) is a new communication standard for field devices. It is based on single pair Ethernet (SPE: SINGLE PAIR ETHERNET) and should also allow intrinsically safe power supply (EX). Typically, multiple field devices are connected to a field switch, and in order not to exceed the specified available power when starting these field devices at the same time, and not to interrupt communication and / or operation when starting a single field device, the APL standard specifies rules for startup behavior. These rules include rules on inrush current and current peaks, as well as maximum current changes and maximum operating currents. These values ​​are limited, especially when starting field devices.

[0014] In order to ensure a stable internal voltage and power supply, especially in the two-wire version of APL field devices (i.e. powered via the APL cable), relatively high input capacitance is required as a so-called buffer. However, without countermeasures, this leads to very high inrush currents. If the input capacitance is too small to avoid this effect, high current peaks will occur on the supply cable when starting up various circuit components (e.g. buck-boost converter, MCU, etc.). However, rapid current changes ( ) and excessive current peaks (>55mA).

[0015] The obvious solution is therefore to limit the input current until the field device is powered on and ready for operation. For this purpose, so-called current limiters are used. A common, well-known circuit uses a transistor, which limits the current on the cable when a comparison value is exceeded. However, this has some disadvantages. For example, there is a power loss due to the need for a shunt resistor to measure the current. At startup (i.e. until the minimum voltage is applied to the circuit components), the comparison circuit is not yet fully operational and the permitted inrush current may be exceeded. In addition, due to the inertia of the circuit, rapid changes in the current may not be fully regulated. Summary of the invention

[0016] The object of the present invention is to provide an alternative solution.

[0017] This object is achieved by a connecting circuit for a field device according to claim 1 and by a field device according to claim 15 .

[0018] The connection circuit for a field device according to the present invention comprises:

[0019] - two connectors, which form a two-wire interface for connecting a two-wire cable, in particular an interface in accordance with Ethernet-APL (IEEE Std 802.3cg-2019), via which the field device can be supplied with electrical energy and measurement signals can be transmitted from the field device;

[0020] - microcontrollers for operating field devices;

[0021] - a voltage converter, which is connected upstream of the microcontroller,

[0022] wherein the voltage converter is configured to operate the microcontroller using the operating voltage;

[0023] - a supply capacitor, which is connected upstream of the voltage converter,

[0024] wherein the supply capacitor is configured to absorb electrical energy when the connection circuit is activated and use it to supply the voltage converter;

[0025] - a first current limiting element connected upstream of the supply capacitor,

[0026] wherein the first current limiting element is designed to limit the input current to below the permitted limiting current when starting the connection circuit;

[0027] a first bridging element connected in parallel with the first current limiting element and arranged to bridge the first current limiting element if a first criterion is met; and

[0028] - a test element configured to check whether a first criterion is fulfilled.

[0029] The supply capacitor allows stable internal voltages and power supplies with the aid of sufficiently large buffer capacitance. In addition, the charging current of the input capacitance of the field device is limited without delay at startup. The capacitance of the supply capacitor is typically greater than 50 µF.

[0030] The first current limiting element is configured to limit the charging current of the supply capacitor so that, for example, a maximum inrush current specified by a standard is not exceeded. The test element is configured to ensure that the charging current flows via the first current limiting element if the voltage supply is insufficient (i.e., when the supply capacitor is being charged). In addition, the test element is configured to bridge the first current limiting element via the first bridging element when the supply capacitor has reached a predetermined charging state.

[0031] Advantageous embodiments of the invention are subject matter of the dependent claims.

[0032] One embodiment provides that the test element is configured to compare a first voltage applied between the connection and the first current limiting element with a second voltage applied between the first current limiting element and the supply capacitor,

[0033] Therein, the first criterion is met when the supply capacitor reaches a predetermined charging state and / or the second voltage (particularly the sum of the second voltage and a preset voltage offset) is greater than the first voltage.

[0034] The voltage offset is selected such that this voltage (for bridging) is reliably reached while taking all tolerances into account and the difference between the bridged voltage and the available charging voltage is as small as possible so that the compensation current does not generate impermissible current peaks when bridging the current limiting element.

[0035] One embodiment provides that the connecting circuit comprises:

[0036] - a second current limiting element connected upstream of the supply capacitor,

[0037] Therein, the second current limiting element is designed to limit the temporal current change caused by the recharging of the capacitor to below the current change limit, in particular not later than 1000 milliseconds after the connection circuit has been activated.

[0038] The advantage of this embodiment is that it limits high current changes when all subsequent circuit components are enabled. This involves, for example, voltage converters, microcontrollers, memories and integrated circuits for enabling communication (APL-Phy). Therefore, the second current limiting element is a dI / dt limiter.

[0039] One embodiment provides that: at a permitted maximum operating voltage of 15V, the current change limit is less than 10mA / ms,

[0040] Among them, at the permitted maximum operating voltage of 50V, the current change limit is less than 100mA / ms.

[0041] One embodiment provides that the second current limiting element is designed to allow less than 7 current peak events within 1000 ms after the connection circuit is activated at the permitted maximum operating voltage of 15 V, in which the time current change is greater than or equal to 10 mA / ms,

[0042] The second current limiting element is designed to allow less than 7 current peak events within a sliding time interval of 1000ms after the connection circuit is started at a maximum permitted operating voltage of 50V, in which the time current change is greater than or equal to 100mA / ms.

[0043] One embodiment provides that the second current limiting element is designed such that at a permitted maximum operating voltage of 15 V, the maximum current jump is less than or equal to 50 mA,

[0044] The second current limiting element is designed such that, at a permitted maximum operating voltage of 50V, a maximum current peak current during a current peak event is less than or equal to 50mA.

[0045] One embodiment provides that the connecting circuit comprises:

[0046] - a second bridging element connected in parallel with the second current limiting element and intended for bridging the second current limiting element when a second criterion is met.

[0047] One embodiment provides that the microcontroller communicates with the second bridge element and is configured to transmit a signal to the second bridge element if the second criterion is met.

[0048] One embodiment provides that the second criterion is met when the microcontroller has reached an operating state ready for communication.

[0049] One embodiment provides that the microcontroller is configured to transmit a signal with a delay such that a current peak event generated during bridging by means of the first bridging element (8) does not coincide in time with a current peak event generated by starting the voltage converter (4).

[0050] One embodiment provides that at the permitted maximum operating voltage of 15 V, the current limit corresponds to 95 mA, in particular 55.56 mA,

[0051] Therein, at the permitted maximum operating voltage of 50 V, the current limit corresponds to 1250 mA.

[0052] One embodiment provides that the first current limiting element has at least one current limiting resistor , for which the following apply: .

[0053] Current limiting resistor Ensures the necessary charging current limit. Since the current limiting resistor Always present in the charging path, there is no reaction delay and the current is limited from start-up.

[0054] One embodiment provides that the second current limiting element has a current limiting resistor , for which the following apply: .

[0055] One embodiment provides that the first bridge element is designed such that the first current limiting element is inactive after activation, in particular when a predetermined voltage across the first current limiting element is exceeded.

[0056] As soon as the first current limiting element has been bridged, the test element automatically receives a value which ensures that the bridge always remains open - regardless of the subsequent charging state of the input capacitor. This means that the current limitation is only active when the field device is switched on and the voltage of the input capacitor is defined to be lower than the external voltage. If the field device is switched off only briefly and the capacitors in the connecting circuit are still charging, the startup process is correspondingly short.

[0057] The field device according to the present invention comprises:

[0058] - sensing element,

[0059] wherein the sensing element has a sensor for determining a process variable,

[0060] - Electronic housing,

[0061] Therein, the connecting circuit according to the invention is arranged in an electronics housing. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The present invention is explained in more detail with reference to the following drawings, in which:

[0063] Figure 1 : shows a simplified representation of a first embodiment of a connection circuit;

[0064] Figure 2 : shows part of a circuit diagram of a first embodiment of a connection circuit;

[0065] Figure 3 : shows a simplified representation of a second embodiment of the connection circuit;

[0066] Figure 4: shows part of a circuit diagram of a second embodiment of a connection circuit;

[0067] Figure 5 : shows the time course of the current after startup and the change of the current over time; and

[0068] Figure 6 : shows a field device according to the present invention. DETAILED DESCRIPTION

[0069] Figure 1 FIG. 1 shows a simplified representation of a first embodiment of a connection circuit 1 for a field device. This is only the part focusing on the first current limiting element 6, so Figure 1 The basic components are not shown, but Figure 3 The connection circuit 1 according to the present invention comprises two connections (such as Figure 6 ), which form a two-wire interface, in particular an interface compliant with Ethernet-APL (IEEE Standard 802.3cg-2019) for connecting a two-wire cable (such as Figure 6 As shown in FIG. 1 ), via the two-wire cable, the field device can be supplied with electrical energy from the voltage source 12 and the measurement signal can be transmitted from the field device to the outside, for example to a control system.

[0070] Furthermore, the connecting circuit 1 according to the invention comprises a microcontroller (such as Figure 3 A microcontroller within the meaning of this application is a single-chip computer system or semiconductor chip, which includes a processor and optionally necessary RAM.

[0071] According to the invention, the connection circuit has a voltage converter (such as Figure 3 As shown in FIG. 1 ), the voltage converter is connected upstream of the microcontroller and is configured to convert the input voltage into an operating voltage with which the microcontroller can operate. A commercially available voltage converter can be used as the voltage converter.

[0072] Figure 1 A supply capacitor 5 is shown connected upstream of the voltage converter. It is configured to absorb electrical energy when the connection circuit 1 is started and use it to supply the voltage converter. During start-up, the supply capacitor 5 is charged via the voltage source 12. The internal supply of the connection circuit 1 requires a sufficiently high supply capacitance (e.g. 220 µF).

[0073] The first current limiting element 6 connected upstream of the power supply capacitor 5 is designed to limit the input current below the allowed limit current when the connection circuit 1 is started. The first current limiting element 6 may include a field effect transistor, a relay, a bipolar transistor, a current limiting resistor or other electronic components that can be freely selected from the prior art and achieve the same function.

[0074] Since it is not desired that the first current limiting element 6 continue to limit the current during normal operation, a first bridging element 8 is provided in parallel with the first current limiting element 6. This serves to bridge the first current limiting element 6 or is configured to bridge the first current limiting element 6 if a first criterion is met. The first criterion may be a specification of the voltage present on the electronic component or the charge state of a supply capacitor.

[0075] Furthermore, a test element 10 is provided, which is configured to check whether the first criterion is met. The test element 10 may include a comparator or another suitable analog or digital circuit for comparing two voltages. The test element 10 is configured to compare a first voltage applied between the connection and the first current limiting element 6. and a second voltage applied between the first current limiting element 6 and the supply capacitor 5 For example, when the power supply capacitor 5 reaches a predetermined charging state and / or when the second voltage ——Especially the second voltage and the sum of the preset voltage offset OS - greater than the first voltage When , the first criterion is met. If the first criterion is met, the first bridge element 8 is activated and the first current limiting element 6 is deactivated. After the first criterion is met, the current flows via the first bridge element 8.

[0076] The first bridge element 8 is designed such that the first current limiting element 6 is inactive after startup, in particular when a predetermined voltage across the first current limiting element 6 is exceeded. Thus, the first bridge element 8 can be designed as a switch, or comprise at least one switching component, which bridges (or short-circuits) the first current limiting element 6 and thus becomes active only when a predetermined voltage has been established at the first current limiting element 6. Conventional transistors, field effect transistors (in particular metal oxide semiconductor field effect transistors), relays or analog switches are suitable for use as the first bridge element 8. It is also advantageous for a plurality of said switch components to work together.

[0077] The connection circuit 1 may have additional capacitors 13 (not to be confused with the supply capacitors 5) having a capacitance of less than 200 µJ or 500 µJ. These capacitors are not used to supply the microcontroller, but cause inrush current events.

[0078] Figure 2A part of a circuit diagram of a first embodiment of a connection circuit 1 is shown. The first current limiting element consists of two current limiting resistors 20, which have the references R234 and R235. Alternatively, the first current limiting element may consist of only one current limiting resistor or of more than two current limiting resistors. If the first current limiting element is a current limiting resistor, the following applies to the resistors: The required resistance can be distributed across multiple individual current limiting resistors as shown.

[0079] The test element comprises a comparator 21 which is configured to compare a voltage applied to a measuring resistor 22 arranged between the input and the first current limiting element with a voltage applied to a measuring resistor 23 arranged between the first current limiting element and the voltage converter. The comparator 21 is designed to transmit a voltage signal to the voltage converter via a signal conductor 26 and if a first criterion is met, i.e. the inequality , then two switches 24 (V208 and V209) are started.

[0080] Two damping components 25 are provided to minimize the current change that may occur when switching the two switches 24 (V208 and V209). The damping component 25 includes a resistor R242 (arranged between the two switches 24) and a capacitor C212 (connected in parallel with at least one switch 24). The resistance of R242 and the capacitance of C212 are selected so that the current change that occurs when switching the two switches 24 does not exceed a specified tolerance value. The resistance of R242 is preferably between 100 ohms and 1 megohm. The capacitance of capacitor C212 is preferably between 10nF and 100µF. Resistor R241 is arranged in parallel with capacitor C212 and is configured to safely discharge capacitor C219 when the power supply is turned off, thereby ensuring damping or delay at the next startup.

[0081] In the embodiment shown, the measuring resistor 22 comprises two individual resistors R233 and R228 connected in series, while the measuring resistor 23 comprises three resistors R238, R239 and R237 connected in series, wherein there is a node between the two resistors R239 and R237, which is connected to the first input of the comparator 21 via an electrical conductor. There is also a node between the resistors R233 and R228, which is connected to the second input of the comparator 21 via an electrical conductor. A resistor R236 is arranged between the emitter of the transistor V209 and the ground potential, which serves to reduce the discharge current in the capacitor C212 and is designed to make the switching of the transistor V209 occur more slowly. The resistor R236 also ensures that the base current is limited by the transistor V209.

[0082] The two switches 24 ( V208 and V209 ) comprise a (npn) bipolar transistor ( V209 ) whose base is connected to the output of the comparator 21 and the emitter is connected to ground potential and a p-MOSFET ( V208 ) whose gate is connected to the collector of the (npn) bipolar transistor via an electrical conductor.

[0083] Figure 3 A simplified representation of a second embodiment of a connection circuit 1 is shown. The depicted representation comprises Figure 1 (see the dotted border area). In addition, if the first criterion is met, the test element 10 is designed to transmit a signal (in particular a voltage signal) to the voltage converter. This prevents the current peak event that occurs when the voltage converter 4 is activated from coinciding with the current peak event when bridging by means of the first bridge element.

[0084] Apart from Figure 1 In addition to the components already shown in FIG. 1 , the connecting circuit also has a second current limiting element 7, which is connected upstream of the supply capacitor 5. This element is designed so that, in particular, the instantaneous change of the current caused by the recharging of the capacitor 5 no later than 1000 milliseconds after the connection circuit 1 is started, it remains below the current change limit. For a permitted maximum operating voltage of 15 V, the current change limit is less than 10 mA / ms, and for a permitted maximum operating voltage of 50 V, the current change limit is less than 100 mA / ms. In addition, the second current limiting element 7 is designed to allow less than 7 current peak events within 1000 milliseconds after the connection circuit 1 is started at a permitted maximum operating voltage of 15 V, in which the time current change is greater than or equal to 10 mA / ms. In the case of a permitted maximum operating voltage of 50 V of the connecting circuit 1, the second current limiting element 7 is designed so that within a sliding time interval of 1000 ms after the connection circuit is started, less than 7 current peak events are allowed, in which the time current change is greater than or equal to 100 mA / ms. It is also advantageous if the second current limiting element 7 is designed such that the maximum current jump is less than or equal to 50 mA at a permitted maximum operating voltage of 15 V, or alternatively, the second current limiting element 7 - for a permitted maximum operating voltage of 50 V - is designed such that when a current peak event occurs, the maximum value of the current peak is less than or equal to 50 mA. The second current limiting element 7 may include a field effect transistor, a relay, a bipolar transistor, a current limiting resistor or other electronic components that can be freely selected from the prior art and achieve the same function.

[0085] Furthermore, a second bridging element 9 is provided, which is connected in parallel with the second current limiting element 7 and is configured to bridge the second current limiting element 7 if a second criterion is met. The second criterion may be a specification of a voltage present on the electronic component or a charge state of a supply capacitor or an operating state of the microcontroller / a state in the program (software) of the microcontroller. The second criterion may be met if the microcontroller 3 has reached an operating state ready for communication. Thus, the second bridging element 9 may be designed as a switch or comprise at least one switching element, which bridges (or short-circuits) the second current limiting element 7 and thus becomes active only when the second criterion is met.

[0086] The microcontroller 3 communicates with the second bridge element 9 and is configured to transmit a signal, in particular a voltage signal, to the second bridge element 9 if the second criterion is met. It is advantageous if the current peak events are appropriately coordinated so that they do not coincide in time. For this purpose, the microcontroller 3 is configured to transmit a signal, in particular a voltage signal, with such a delay that a current peak event generated during bridging by means of the second bridge element 8 does not coincide in time with a current peak event generated by the starting voltage converter 4.

[0087] The Ethernet-APL (IEEE Standard 802.3cg-2019) standard specifies current limits. For example, at a permitted maximum operating voltage of 15V, the current limit is 95mA, specifically 55.56mA, and at a permitted maximum operating voltage of 50V, the current limit is 1250mA.

[0088] Figure 4 FIG. 1 shows a portion of a circuit diagram of a second embodiment of the connection circuit 1. This portion shows only the portion of the connection circuit 1 that is relevant to the description of the second current limiting element and the second bridge element. For the second current limiting element, its current limiting resistor In the range of 3 to 500 ohms. In the embodiment shown, the second current limiting element comprises a current limiting resistor 30. The second bridge element comprises two switches 33 of the same type (V211 and V212). Each of the switches V211 and V212 shown is a MOSFET. However, other switch components can also be used. The gate of the switch V212 is connected to the microcontroller via a signal conductor 31. The source is electrically connected to the ground potential and the drain is electrically connected to the gate of the other switch V211 by means of a conductor. A damping element 34, in this case a resistor R247 (100 ohms to 1 megohm), is connected in series. The second switch 33 is configured so that when the switch is activated, the second current limiting element is bridged (or short-circuited). A capacitor C219 is connected in parallel with the switch V211 and also has the function of the damping component 34. The capacitance of the capacitor C219 is between 10nF and 100µF, including the limit values.

[0089] The resistor R248 is arranged in parallel with the capacitor C219 and is configured to safely discharge the capacitor C219 when the power is turned off to ensure damping or delay at the next startup.

[0090] Figure 5 The time course of the current after startup and the time current change are shown. The current is measured at the input end of the connection circuit, and the current change is derived from the time derivative of the current. When turned on at 0 milliseconds, the current jumps to about 10mA in a step-like manner. The surge current surge causes the current change peak event I in the time current change. The maximum value of this event is about 5.46A / s, and is therefore lower than the specified 10A / s. After the first surge current surge, the current drops slightly. The first current surge comes from the charging current of the infinite capacitance before the current limit. As the voltage difference between the input and the large charging capacitor becomes balanced, the charging current decreases. At 600 milliseconds, a current peak event is visible, which has a maximum current of about 5mA. This is due to the bridging of the load resistor II. During this period, the current change is only minimal. The next larger current peak and current change are measured when starting auxiliary III. After about 900 milliseconds, another step-like current jump occurs. This is due to the bridging of the second current limiting element IV. The maximum deflection of the current change in this period is 3.46A / s. Therefore, the maximum value of all occurring coil current change peaks is below the limit V of 10 A / s.

[0091] Figure 6A field device 100 according to the invention is shown, which has a sensing element 101 with a sensor 102 for determining a process variable and an electronics housing 103. A connecting circuit 1 according to the invention is arranged in the electronics housing 103. The connecting circuit 1 has two connections 2, forming a two-wire interface, in particular an interface in accordance with Ethernet-APL (IEEE Standard 802.3cg-2019), for connecting a two-wire cable 105. The two-wire cable 105 is designed to supply electrical energy to the field device 100 and to transmit measurement signals from the field device 100 to a monitoring unit. The field device 100 shown is a vortex flowmeter. The sensing element 101 of the vortex flowmeter generally comprises a measuring tube 104, which has two connecting devices (e.g. flanges) arranged at the front end and in which a bluff body is arranged for forming a Karman vortex street (shielded by the measuring tube 104). The sensor 102 comprises a sensor blade (also shielded by the measuring tube) which projects into the Karman vortex street and a deformable body to which the sensor blade is attached and to which the movement of the sensor blade is transmitted. Examples of vortex flow meters are known in particular from US 2006 / 0230841, US 2008 / 0072686, US 2011 / 0154913, US 2011 / 0247430, US 2016 / 0123783, US 2017 / 0284841, US 2019 / 0094054, US 60 03 384, US 61 01 885, US 63 52 000, US 69 1 0387 or US 69 38496 and in particular by the applicant under the trade names “PROWIRLD 200”, “PROWIRLF 200”, “PROWIRLO 200”, “PROWIRLR 200”. 200" (https: / / www.de.endress.com / de / messgeraete-fuer-die-prozesstechnik / durchflussmessung-produktuebersicht / vortex-wirbelz%C3%A4hler-durchflussmessung).

[0092] Reference Symbols List

[0093] Connection circuit 1

[0094] Connector 2

[0095] Microcontroller 3

[0096] Voltage Converter 4

[0097] Power supply capacitor 5

[0098] The first current limiting element 6

[0099] Second current limiting element 7

[0100] First bridging element 8

[0101] Second bridging element 9

[0102] Test element 10

[0103] Capacitor 11

[0104] Voltage source 12

[0105] Additional capacitor 13

[0106] Current limiting resistor 20

[0107] Comparator 21

[0108] Measuring resistor 22

[0109] Measuring resistor 23

[0110] Switch 24

[0111] Damping component 25

[0112] Signal conductor 26 to voltage converter

[0113] Current limiting resistor 30

[0114] Signal conductor 31 from microcontroller

[0115] Switch 33

[0116] Damping component 34

[0117] Field Device 100

[0118] Sensing element 101

[0119] Sensor 102

[0120] Electronic housing 103

[0121] Surge current I

[0122] Bridge Load Resistor II

[0123] Startup Assistant III

[0124] Bridge the first / second current element IV

[0125] Limit V

Claims

1. A connection circuit (1) for a field device (100), comprising: - two connectors (2), the two connectors forming a two-wire interface for connecting a two-wire cable, in particular an interface conforming to Ethernet-APL (IEEE Std 802.3cg-2019), via which the field device (100) can be supplied with electrical energy and measurement signals can be transmitted from the field device (100); - a microcontroller (3) for operating the field device; - a voltage converter (4) connected upstream of the microcontroller (3), Wherein, the voltage converter (4) is configured to operate the microcontroller (3) using an operating voltage; a supply capacitor (5) connected upstream of the voltage converter (4), wherein the power supply capacitor (5) is configured to absorb electrical energy when the connection circuit (1) is activated, and use the electrical energy to supply power to the voltage converter (4); - a first current limiting element (6) connected upstream of the supply capacitor (5), The first current limiting element (6) is designed to limit the input current to below an allowed limiting current when starting the connection circuit (1); - a first bridging element (8) connected in parallel with the first current limiting element (6) and intended for bridging the first current limiting element (6) if a first criterion is met; and - a test element (10) configured to check whether the first criterion is fulfilled.

2. The connecting circuit (1) according to claim 1, in, The test element (10) is configured to compare a first voltage applied between the connection piece (2) and the first current limiting element (6) with a second voltage applied between the first current limiting element (6) and the supply capacitor (5), The first criterion is met when the supply capacitor (5) reaches a predetermined charging state and / or the second voltage, in particular the sum of the second voltage and a preset voltage offset, is greater than the first voltage.

3. The connection circuit (1) according to claim 1 or claim 2, comprising: - a second current limiting element (7) connected upstream of the supply capacitor (5), The second current limiting element (7) is designed to limit the temporal current change caused by the recharging of the capacitor (5) to below a current change limit, in particular not later than 1000 milliseconds after the connecting circuit (1) is activated.

4. The connecting circuit (1) according to claim 3, in, At the permitted maximum operating voltage of 15V, the current change limit is less than 10mA / ms, Wherein, at the permitted maximum operating voltage of 50V, the current change limit is less than 100mA / ms.

5. The connecting circuit (1) according to claim 3 or claim 4, in, The second current limiting element (7) is designed to allow less than 7 current peak events within 1000ms after the connection circuit is activated at a permitted maximum operating voltage of 15V, in which the time current change is greater than or equal to 10mA / ms. The second current limiting element (7) is designed to allow less than 7 current peak events within a sliding time interval of 1000ms after the connection circuit is started at a maximum permitted operating voltage of 50V, in which the time current change is greater than or equal to 100mA / ms.

6. The connecting circuit (1) according to at least one of claims 3 to 5, in, The second current limiting element (7) is designed so that at the permitted maximum operating voltage of 15V, the maximum current jump is less than or equal to 50mA, The second current limiting element (7) is designed such that, at a permitted maximum operating voltage of 50V, the maximum current peak current during a current peak event is less than or equal to 50mA.

7. The connection circuit (1) according to at least one of claims 3 to 6, comprising: - a second bridging element (9) connected in parallel with the second current limiting element (7) and intended for bridging the second current limiting element (7) if a second criterion is met.

8. The connecting circuit (1) according to claim 7, in, The microcontroller (3) communicates with the second bridge element (9) and is configured to transmit a signal to the second bridge element (9) if the second criterion is met.

9. The connecting circuit (1) according to claim 7 or claim 8, in, The second criterion is met when the microcontroller (3) has reached an operating state ready for communication.

10. The connecting circuit (1) according to at least one of the preceding claims, in, The test element (10) is configured to transmit a signal to the voltage converter (4) with a delay such that a current peak event generated during bridging by means of the first bridge element (8) does not coincide in time with a current peak event generated by starting the voltage converter (4).

11. The connecting circuit (1) according to at least one of the preceding claims, in, At the permitted maximum operating voltage of 15 V, the current limit corresponds to 95 mA, in particular 55.56 mA, Therein, at the permitted maximum operating voltage of 50 V, the current limit corresponds to 1250 mA.

12. The connecting circuit (1) according to at least one of the preceding claims, in, The first current limiting element (6) has the following applicable current limiting resistor : .

13. The connecting circuit (1) according to at least one of claims 3 to 12, in, The second current limiting element (7) has the following applicable current limiting resistor : .

14. The connecting circuit (1) according to at least one of the preceding claims, in, The first bridging element (8) is designed such that the first current limiting element (6) is inactive after activation, in particular when a predetermined voltage across the first current limiting element (6) is exceeded.

15. A field device (100), comprising: - sensing element (101), The sensing element (101) has a sensor (102) for determining a process variable. - Electronic housing (103), Therein, the connecting circuit (1) according to at least one of claims 1 to 14 is arranged in the electronics housing (103).

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