Intrinsically safe automation field device

Abstract

An intrinsically safe field device of automation technology, comprising: - connection terminals (30a, 30b) via which a current (Is) can be supplied; - a sensor element and / or actuator element (16); - a field device electronics (31, 32, 33, 34, 36) having a current path (50) between the connection terminals (30a, 30b) and a voltage regulator (36) incorporated into the current path (50), the voltage regulator being designed to provide a power supply (16) on the basis of the current (Is); - an explosion protection unit (35, 38) comprising at least two actively controllable switching elements (38a, 38b) and two threshold circuits (35a, 35b) incorporated in series into the current path (50), the threshold circuits (35a, 35b) being designed such that a first threshold circuit (35a) controls a first switching element (38a) according to a first threshold value and a second threshold circuit (35b) controls a second switching element (38b) according to a second threshold value, such that when the first threshold value and / or the second threshold value is reached, the current (Is) is limited to the first threshold value and / or the second threshold value, and the threshold circuits (35a, 35b) are connected in parallel to the voltage regulator (36).

Description

TECHNICAL FIELD

[0001] The present invention relates to an intrinsically safe field device for automation technology in explosion hazard areas. BACKGROUND

[0002] In automation technology, field devices for recording and / or modifying process variables are frequently used, in particular in process automation technology. Sensors, such as level measuring devices, flow meters, pressure and temperature measuring devices, pH redox potential meters, conductivity meters, etc., are used to record corresponding process variables, such as level, flow, pressure, temperature, pH level and conductivity. Actuators, such as, for example, valves or pumps, are used to influence process variables. Thus, the flow rate in a pipe section or the level in a container can be changed by means of actuators. In principle, all devices used in a process and supplying or processing process-related information are referred to as field devices. In the context of the present invention, field devices also include remote I / O, radio adapters and / or devices that are generally arranged at the field level.

[0003] A wide variety of such field devices are manufactured and sold by the Endress+Hauser company.

[0004] Many field devices can be used in a so-called 2-wire version. In this case, power is supplied to the field device by means of the same two wires that are used for communication.

[0005] In particular in the process industry, but also in automation technology, physical or technical variables often have to be measured or determined by field devices in areas in which there is a potential explosion risk, i.e. so-called explosion hazard areas. By means of appropriate measures in the field device and the evaluation system, for example voltage and current limitation, the electrical energy present in the signals to be transmitted can be limited so that an explosion cannot be triggered by the signals in any case (short circuit, interruption, thermal effects, etc.). For this purpose, corresponding protection principles have been defined in IEC EN DIN 60079-ff.

[0006] According to this standard, the design and circuitry measures of field devices used in explosion hazard areas are defined on the basis of the type of ignition protection to be applied. One of these types of ignition protection represents the type of ignition protection "intrinsically safe" (code Ex-i, IEC EN DIN 60079-11, published in June 2012).

[0007] The type of ignition protection "intrinsically safe" is based on the principle of current and voltage limitation in the circuit. The energy that can possibly ignite the explosive environment in the circuit is limited here so that the ignition of the surrounding explosive environment cannot occur either by means of a spark or by means of an inadmissible heating of the electrical components.

[0008] The ignition protection type "intrinsic safety" defines three protection classes: Ex-ia, Ex-ib and Ex-ic. In this case, the highest class is defined by class a, at which two countable error combinations do not lead to a failure, thus to an ignition (2-error safety). Class b defines one countable error which does not lead to a failure, thus to an ignition (1-error safety). In the case of class c, therefore, no error safety is defined, so that in the case of one failure, an ignition can already be triggered (0-error safety).

[0009] The critical area in which a possible ignition of the surrounding explosive environment can be triggered is the connection terminal of the field device to which the two-wire line is connected.

[0010] The short-circuit current, which can act on the connection terminal, is therefore usually limited by means of an explosion protection unit. The explosion protection units known from the prior art usually comprise one or more Zener diodes for power limitation for voltage limitation and one or more explosion protection resistors in series therewith for combination with a fuse for maximum current limitation, wherein, in addition, an additional diode is utilized to prevent a power supply-side backflow of energy from the inductor L and / or the capacitor C. The explosion protection unit is usually designed such that the one or more explosion protection resistors have a total resistance of approximately 330 ohms.

[0011] A disadvantage of these explosion protection units is that, during normal operation, for example, with a terminal voltage Uk = 24 V and an error current of 22 mA, a voltage drop of 7.26 V (with a 330 ohm resistance) will occur. This corresponds to a power loss at the explosion protection resistor of 1 / 3 of the available total power.

[0012] Due to this performance loss, only energy-saving components can be used in the device circuit. The development of the field device electronics also becomes more complex and thus more expensive. In addition, functional disadvantages, such as, for example, a loss in measurement performance and in the visual or digital representation of the measured values (for example, by means of a display or via Bluetooth, etc.), also occur.

[0013] It is therefore an object of the present invention to overcome the above-mentioned disadvantages. SUMMARY

[0014] According to the invention, this object is achieved by an intrinsically safe automation field device.

[0015] An intrinsically safe automation field device for an explosion hazard area comprises:

[0016] - a first connection terminal and a second connection terminal for connecting a two-wire line via which a current can be supplied;

[0017] - a sensor element and / or an actuator element for capturing and / or setting a process variable;

[0018] - a field device electronics connected to the first connection terminal and to the second connection terminal, the field device electronics conducting an electrical current Is via an electrical current path from the first connection terminal to the second connection terminal, the electrical current Is being supplyable via the two-wire line, wherein the field device electronics has a voltage regulator incorporated into the electrical current path, the voltage regulator being designed to provide at least a sensor element and / or an actuator element with an electrical power supply on the basis of the supplied electrical current Is, wherein the field device electronics is further designed to transmit a process variable captured via the sensor element by setting the electrical current Is to a corresponding value and / or to receive a process variable to be set by the actuator element by reading the electrical current Is and to adjust the actuator element accordingly;

[0019] - an explosion protection unit comprising at least a first and a second actively controllable switching element incorporated in series into the electrical current path, and at least a first and a second threshold circuit, the first and the second threshold circuit being designed such that the first threshold circuit controls the first controllable switching element according to a first threshold value of the electrical current Is and the second threshold circuit controls the second controllable switching element according to a second threshold value of the electrical current Is, such that the electrical current Is is limited to the first threshold value and / or to the second threshold value when the first threshold value and / or the second threshold value is reached, and wherein the threshold circuits are connected in parallel to the voltage regulator incorporated into the electrical current path.

[0020] According to the application, instead of one or more ohmic explosion protection resistors in the explosion protection unit, actively controllable switching elements within the explosion protection unit are incorporated as current limitation into the electrical current path, which are controlled in each case via a threshold circuit.

[0021] An advantageous embodiment of the field device according to the application provides that the explosion protection unit further comprises a third actively controllable switching element incorporated in series into the electrical current path with the first and the second switching element, and a third threshold circuit, the third threshold circuit being designed such that the third threshold circuit controls the third controllable switching element according to a third threshold value of the electrical current Is, wherein the threshold circuits are designed such that the electrical current Is is limited to the first threshold value, to the second threshold value and / or to the third threshold value when the first threshold value, the second threshold value and / or the third threshold value is reached, and wherein the third threshold circuit is also connected in parallel to the voltage regulator incorporated into the electrical current path.

[0022] A further advantageous embodiment of the field device according to the application provides that the first threshold value, the second threshold value and / or the third threshold value of the electrical current Is is greater than 22 mA, in particular in the range of 32 to 40 mA.

[0023] A further advantageous embodiment of the field device according to the application provides that the first threshold circuit, the second threshold circuit and / or the third threshold circuit are designed such that at least two of the three threshold values, preferably all threshold values, are substantially identical.

[0024] An alternative embodiment of the field device according to the application provides that the first threshold circuit, the second threshold circuit and / or the third threshold circuit are designed such that at least two of the three threshold values of the current Is differ from one another.

[0025] A further advantageous embodiment of the field device according to the application provides that the actively controllable switching element comprises a semiconductor switching element or a transistor. The advantage of using semiconductor switching elements, for example in the form of field-effect transistors (MOSFETs), is that in normal operation they generate a very small voltage drop of < 1 V. The power drop is also very small (< 5%).

[0026] A further advantageous embodiment of the field device according to the application provides that the threshold circuit comprises at least two transistors in each case.

[0027] A further advantageous embodiment of the field device according to the application provides that the field device electronics also has a shunt resistor which is incorporated into the current path in series with the voltage regulator, and the field device electronics is also configured to read back the current Is via the shunt resistor, and wherein the first threshold value, the second threshold value and / or the third threshold value of the current Is achieved by the first threshold circuit, the second threshold circuit and / or the third threshold circuit is achieved by at least one voltage tap across the shunt resistor. In particular, this embodiment can provide that the at least one threshold circuit, preferably all threshold circuits, comprise at least two transistors in each case, and the at least two transistors are connected to one another in each case and to the respective actively controllable switching element and to the shunt resistor, such that the respective switching threshold value is substantially determined by the shunt resistor.

[0028] A further advantageous embodiment of the field device according to the application provides that the at least one threshold circuit, preferably all threshold circuits, comprise at least one comparator, a transistor and a voltage divider, which are connected to one another and to the respective actively controllable switching element in each case, such that the respective switching threshold value is substantially determined by the voltage divider. BRIEF DESCRIPTION OF DRAWINGS

[0029] The application is explained in more detail on the basis of the following drawings. The following is shown:

[0030] Figure 1 : Schematic representation of a field device which is connected to a higher-level unit via a two-wire line for signal and power transmission,

[0031] Figure 2 : a first variant of an explosion-proof unit designed according to the application is shown in detail,

[0032] Figure 3 : a second variant of an explosion-proof unit designed according to the application is shown in detail, and

[0033] Figures 4a to 4c : a circuit simulation of an explosion-proof unit. DETAILED DESCRIPTION

[0034] Figure 1 A schematic representation of a field device 10 is shown, which is connected via a first connection terminal 30a and a second connection terminal 30b to a two-wire line 14 for signal and energy transmission. The two-wire line 14 is connected in turn at the other end to a higher-level unit 12. In the example shown, the field device 10 is a measurement point at which a measurement value or process variable (for example temperature, pressure, humidity, level, flow) is captured by means of a sensor 16. However, the field device can also be an actuator point, in which a process variable is set by means of an actuator.

[0035] The field device 10 does not contain its own energy source, but obtains the supply current required for its operation via the two-wire line 14. This can be provided, for example, by means of a voltage source 18 contained in the higher-level unit 12. A measurement value signal representing the measurement value just measured is transmitted from the field device 10 to the higher-level unit 12 via the same two-wire line 14. According to conventional technology, the measurement value signal is a signal current Is flowing via the two-wire line 14, which can vary between two pre-specified values (typically current values of 4 mA and 20 mA). The voltage source 18 supplies a DC voltage Uv, and the measurement current Is is a direct current.

[0036] In order to acquire the measurement value, the field device 10 contains the already mentioned sensor 16 and connected thereto a measurement transducer circuit 20, which controls a controllable current source via a control line 22, so that the measurement current Is is set to a value representing the captured measurement value (signal current).

[0037] The higher-level unit 12 contains an evaluation circuit 26, which obtains the measurement value information from the signal current Is transmitted via the two-wire line 14. To this end, a measurement resistor 28 is inserted into the two-wire line, at which a voltage UM is generated, which is proportional to the signal current Is transmitted via the two-wire line, and is fed to the evaluation circuit 26.

[0038] By means of the internal field device electronics, a signal current Is is guided in the field device 10 from the first connection terminal 30a to the second connection terminal 30b by means of a current path 50. The current Is can be set via a controllable current regulator incorporated into the current path 50 or the current sink 32. The current regulator is correspondingly controlled by the signal output at the output by the measurement transducer circuit 20, which is supplied as a control signal to the current regulator 32 via the control line 22. Depending on the measurement values captured in each case, the signal current Is flowing in the two-wire line is thus set by corresponding control of the current regulator or the current sink 32. The current regulator or the current sink can comprise, for example, a transistor, which is regulated by the control signal from the measurement transducer circuit 20. In the case of a field device designed as an actuator, i.e. with an actuator element instead of a sensor element, the current regulator is omitted.

[0039] The field device further comprises a low-impedance shunt resistor 33 via which the signal current Is is read back by the measurement transducer circuit 20 by means of the read-back line 23. According to Ohm's law, a voltage U_shunt = R_shuntIs drops at the shunt resistor 33. The voltage U_shunt is thus proportional to the current Is flowing through the field device. In order to regulate the signal current Is to be set, the voltage drop across the shunt resistor 33 is supplied to the measurement transducer circuit. A shunt resistor 33 of this type is indispensable for regulating the current signal, corresponds to the measurement values determined by the sensor of the field device, and usually has a resistance value in the range of 5 to 40 ohms, preferably 7 to 30 ohms, particularly preferably in the range of 10 to 25 ohms.

[0040] As can be seen from Figure 1 The field device 10 also contains a voltage regulator 36, for example in the form of a switching regulator or a linear regulator, the task of which is to generate an operating voltage for the measurement transducer circuit 20 and the sensor 16 that is as constant as possible. The input voltage of the voltage regulator 36 can be provided, for example, by a voltage source 34, in particular in the form of a capacitor. The voltage source 34 supports an input voltage or terminal voltage Uk, which is provided by a voltage source 18 contained in the higher-level unit 12. The voltage source 34 thus serves as a "source" for the circuit part connected thereto, in particular for the voltage regulator 36.

[0041] The combined use of the voltage regulator 36 with the voltage source 34 makes it possible to always provide the highest possible power for the measurement transducer circuit 20 and the sensor 16. Although its input voltage Ue increases, the voltage regulator 36 ensures that the operating voltage of the measurement transducer circuit 20 and the sensor 16 remains at a constant value, so that a higher output power is available by increasing the input voltage Ue at the voltage regulator 36, thus also achieving a higher output power.

[0042] If the measured value captured by the sensor 16 is at the lower end of the measured value range, the signal current Is is also assumed to be the lower limit value of the signal current range. Thus, in the technology of typically 4 to 20 mA, the value is 4 mA. If the measured value captured by the sensor 16 is at the upper end of the measured value range, the signal current Is is assumed to be the upper limit value of the signal current range. Thus, in the technology of typically 4 to 20 mA, the value is 20 mA.

[0043] For voltage limitation, the voltage limiting circuit 31 is able to be connected in parallel between the first connection terminal and the second connection terminal with an external voltage source. The voltage limiting circuit 31 can be formed, for example, by a Zener diode.

[0044] Furthermore, the field device comprises an explosion protection unit 35, 38 for current limitation designed according to the application. In Figure 1 , the explosion protection unit is indicated by way of example only. Figure 2 and Figure 3 A first variant and a second variant of the explosion protection unit are shown in detail.

[0045] The explosion protection unit 35, 38 comprises at least a first actively controlled or controllable switching element 38a and a second actively controlled or controllable switching element 38b and a first threshold circuit 35a and a second threshold circuit 35b, each threshold circuit controlling a switching element. The explosion protection unit is used for current limitation.

[0046] Depending on the desired protection class and in accordance with the requirements of the standard IEC EN DIN 60079-11 published in June 2012, the explosion protection unit can also have a third switching element 38c and a third threshold circuit 35c, as Figure 2 and 3 are shown by way of example.

[0047] The actively controlled or controllable switching elements 38a, 38b and 38c can be implemented, for example, as semiconductor switching elements, such as field effect transistors, in particular MOSFETs, or as transistors, in particular bipolar transistors.

[0048] The circuit elements 38a, 38b, 38c, which are in series with one another and in series with the voltage regulator 36, are incorporated into the current path 50. The threshold circuits 35a, 35b, 35c, on the other hand, are incorporated in parallel with the voltage regulator 36, which can be said to act as a load.

[0049] In the case of the switching elements 38a, 38b, 38c being designed as field effect transistors, this offers the advantage compared to conventional current limiters that the desired gate-source voltage can be dispensed with. Here, the stroke is implemented via the voltage regulator 36 as a load.

[0050] The threshold circuits are basically constructed in such a way that they actuate the corresponding switching elements according to a threshold value of the current Is flowing in the current path 50, respectively. The actuation occurs in such a way that, upon reaching the respective threshold value, the current Is flowing in the current path 50 will be limited to the respective threshold value. The threshold value of the current Is is greater than 22 mA (Is > 22 mA), in particular lies in the range of 32 to 40 mA. The threshold circuits 35a, 35b, 35c can now be designed in such a way that they all have the same threshold value, or alternatively, the threshold values differ at least in some cases.

[0051] In order not to have to generate an additional voltage drop and thus to produce a power loss, the voltage tap 21 required for determining the current value can be implemented across the shunt resistor 33.

[0052] In detail, the threshold circuits can be designed differently. Thus, as Figure 2 is shown, the first threshold circuit 35a, the second threshold circuit 35b and the third threshold circuit 35c can in each case consist of a transistor circuit having at least two transistors, in particular bipolar transistors. The transistor circuits 35a, 35b, 35c are designed in such a way that the respective switching element 38a, 38b, 38c is controlled via a collector-emitter path of a first transistor 41a, 41b, 41c, wherein a control signal is supplied to the base of the first transistor 41b, 41b, 41c via a collector-emitter path of a second transistor 42a, 42b, 42c. In this case, the voltage is tapped at the shunt resistor 33 via the emitter of the second transistor. If necessary, as Figure 2 is shown, a further resistor can be connected between the tapping point and the emitter of the second transistor. According to this variant, the respective switching threshold is basically determined by the shunt resistor (33) or the resistance value of the shunt resistor.

[0053] Figure 3 A further variant of the explosion-proof unit 35, 38 designed according to the application is shown. The first threshold circuit 35a, the second threshold circuit 35b and the third threshold circuit 35c have in each case a comparator 39a, 39b, 39c and a transistor 41a, 41b, 41c. In addition, the explosion-proof unit has a voltage divider 40. The voltage tap across the shunt resistor 33 is here implemented by the voltage divider. The center tap of the voltage divider 40 is connected to the inverting input of the comparator, so that the respective switching threshold is basically determined by the voltage divider 40, i.e. by the resistance ratio of the two resistors of the voltage divider.

[0054] Figures 4a to 4cThe circuit simulation of the explosion-proof unit 35, 38 designed according to the application is shown, which has an active control semiconductor switching element in comparison to the explosion-proof units known from the prior art, which consist of passive components such as resistors, for example.

[0055] It is clear from Figure 4a and 4b that the explosion-proof unit 35, 38 with a semiconductor switching element for current limitation generates a significantly lower voltage drop, and thus a significantly lower power loss, in comparison to the explosion-proof units known from the prior art with resistors or passive components (indicated by dashed ellipses in Figure 4a and 4b . This means that significantly more energy can be provided accordingly within the field device.

[0056] A further advantage of the explosion-proof unit 35, 38 designed according to the application is shown in Figure 4c , in which the circuit is limited to a defined current, as already mentioned, which can be seen in the following figures. In the case of ohmic limitation, the current rises to > 50 mA, and at the semiconductor limitation, to ≤ 40 mA.

[0057] List of reference signs

[0058] 10 field device

[0059] 12 higher-level unit, for example programmable logic controller (PLC)

[0060] 14 two-wire line

[0061] 16 sensor

[0062] 20 transducer circuit

[0063] 21 voltage tap

[0064] 22 control line

[0065] 23 read-back line

[0066] 24 output of the measuring transducer circuit

[0067] 30a, 30b connection terminal

[0068] 31 voltage-limiting circuit ((multiple) Zener diode)

[0069] 32 controllable power supply

[0070] 33 shunt resistor

[0071] 34 voltage source, for example capacitor

[0072] 35 threshold circuit

[0073] 35a first threshold circuit

[0074] 35b second threshold circuit

[0075] 35c third threshold circuit

[0076] 36 voltage regulator, e.g. switching regulator or linear regulator 37

[0078] 38 active switching element, in particular semiconductor switching element or transistor

[0079] 38a first active switching element, in particular semiconductor switching element or transistor

[0080] 38b second active switching element, in particular semiconductor switching element or transistor

[0081] 38c third active switching element, in particular semiconductor switching element or transistor

[0082] 39a, 39b, 39c comparator

[0083] 40 voltage divider

[0084] 41a, 41b, 41c transistor

[0085] 50 current path

[0086] Is measured current

[0087] Uk terminal voltage

[0088] U_shunt voltage across shunt resistor

Claims

1. Intrinsically safe field device for automation technology in explosion-hazardous areas, comprising: - a first connection terminal and a second connection terminal (30a, 30b) for connecting a two-wire line (14) via which a current can be supplied; - a sensor element and / or an actuator element for capturing and / or setting a process variable; - a field device electronics connected to the first and second connection terminals (30a, 30b), which conducts a current (Is) via a current path (50) from the first to the second connection terminal (30a, 30b), which can be supplied via the two-wire line, wherein the field device electronics has a voltage regulator (36) incorporated into the current path (50), which is designed to provide at least the sensor element and / or the actuator element with a power supply based on the supplied current (Is), wherein the field device electronics is further designed to transmit the process variable captured via the sensor element by setting the current (Is) to a corresponding value, and / or to receive a process variable to be set by the actuator element by reading the current (Is) and to adjust the actuator element accordingly; - an explosion protection unit (35, 38) comprising at least a first and a second actively controllable switching element (38a, 38b) incorporated in series into the current path (50), and at least a first and a second threshold circuit (35a, 35b), which are designed such that the first threshold circuit (35a) controls the first actively controllable switching element (38a) according to a first threshold value of the current (Is) and the second threshold circuit (35b) controls the second actively controllable switching element (38b) according to a second threshold value of the current (Is) such that the current (Is) is limited to the first and / or the second threshold value when the first and / or the second threshold value is reached, and wherein the first and second threshold circuits (35a, 35b) are connected in parallel to the voltage regulator (36) incorporated into the current path (50). ​ ​ ​ ​ 2. The intrinsically safe field device of claim 1, wherein, The explosion protection unit (35, 38) further comprises a third actively controllable switching element (38c) which is connected in series with the first actively controllable switching element (38a) and the second actively controllable switching element (38b) into the current path (50), and a third threshold circuit (35c) which is designed such that it controls the third actively controllable switching element (38c) in dependence on a third threshold value of the current (Is), wherein the first threshold circuit (35a), the second threshold circuit (35b) and the third threshold circuit (35c) are designed such that, upon reaching the first threshold value, the second threshold value and / or the third threshold value, the current (Is) is limited to the first threshold value, the second threshold value and / or the third threshold value, and wherein the third threshold circuit (35c) is also connected in parallel to the voltage regulator (36) which is incorporated into the current path (50).

3. The intrinsically safe field device of claim 2, wherein, The first threshold value, the second threshold value and / or the third threshold value of the current (Is) is greater than 22 mA.

4. The intrinsically safe field device of claim 3, wherein, The first threshold value, the second threshold value and / or the third threshold value of the current (Is) is in the range from 32 to 40 mA.

5. An intrinsically safe field device of automation technology according to any one of claims 2 to 4, wherein, The first threshold circuit, the second threshold circuit and / or the third threshold circuit are designed such that at least two of the three threshold values are substantially identical.

6. The intrinsically safe field device of claim 5, wherein, The first threshold circuit, the second threshold circuit and / or the third threshold circuit are designed such that all of the three threshold values are substantially identical.

7. An intrinsically safe field device of automation technology according to any one of claims 2 to 4, wherein, The first threshold circuit, the second threshold circuit and / or the third threshold circuit are designed such that at least two of the three threshold values of the current (Is) differ from one another.

8. An intrinsically safe field device of automation technology according to any one of claims 2 to 4, wherein, The first actively controllable switching element (38a), the second actively controllable switching element (38b) and the third actively controllable switching element (38c) have semiconductor switching elements or transistors.

9. An intrinsically safe field device of automation technology according to any one of claims 2 to 4, wherein, The first threshold circuit (35a), the second threshold circuit (35b) and the third threshold circuit (35c) each comprise at least two transistors.

10. An intrinsically safe field device of automation technology according to any one of claims 2 to 4, wherein, The field device electronics further comprise a shunt resistor (33) which is incorporated into the current path (50) in series with the voltage regulator (36), and the field device electronics are further configured to read back the current (Is) via the shunt resistor (33), and wherein the first threshold value, the second threshold value and / or the third threshold value of the current (Is) which is reached is achieved by at least one voltage tap across the shunt resistor (33) by the first threshold circuit, the second threshold circuit and / or the third threshold circuit.

11. The intrinsically safe field device of claim 10, wherein, At least one threshold circuit of the threshold circuits comprises at least two transistors each, and the at least two transistors are connected to each other and to the respective actively controllable switching element and to the shunt resistor (33) in each case, such that the respective switching threshold is essentially determined by the shunt resistor (33).

12. The intrinsically safe field device of claim 11, wherein, All threshold circuits of the threshold circuits comprise at least two transistors each.

13. An intrinsically safe field device of automation technology according to any one of claims 2 to 4, wherein, At least one threshold circuit of the first threshold circuit (35a), the second threshold circuit (35b) and the third threshold circuit (35c) comprises at least one comparator (39a, 39b, 39c), a transistor and a voltage divider (40) in each case, which are connected to each other and to the respective actively controllable switching element, such that the respective switching threshold is essentially determined by the voltage divider (40).

14. The intrinsically safe field device of claim 13, wherein, All threshold circuits of the first threshold circuit (35a), the second threshold circuit (35b) and the third threshold circuit (35c) comprise at least one comparator (39a, 39b, 39c), a transistor and a voltage divider (40) in each case.

Images