WO2023010342A1

WO2023010342A1 - Intrinsically safe circuit for load

Info

Publication number: WO2023010342A1
Application number: PCT/CN2021/110601
Authority: WO
Inventors: Wilfred Fernando BALUJA; Chang Liu; Yasong YU
Original assignee: Abb Schweiz Ag
Priority date: 2021-08-04
Filing date: 2021-08-04
Publication date: 2023-02-09

Abstract

Embodiments of present disclosure relate to an intrinsically safe circuit for a load. The intrinsically safe circuit comprises an input adapted to be connected to a power supply; an output adapted to be connected to the load; a switching unit connected between the input and the output and configured to deliver an energy from the input to the output; a voltage clamping unit connected to the input and the switching unit, and configured to turn off the switching unit when an input voltage at the input exceeds a threshold voltage and clamp the input voltage to the threshold voltage, and configured to limit a leakage current through the voltage clamping unit below a predetermined leakage current level when the input voltage is in the vicinity of the threshold voltage; and a current limiting unit connected between the switching unit and the output and configured to limit a current to the output so as to limit the energy delivered to the output.

Description

INTRINSICALLY SAFE CIRCUIT FOR LOAD

FIELD

Embodiments of the present disclosure generally relate to the field of intrinsically safe circuits, and more particularly, to an intrinsically safe circuit for a load.

BACKGROUND

Intrinsically safe circuits are commonly used together with sensors operating in explosive atmospheres to prevent explosion by limiting the energy in the sensor to a level below which can cause ignition by either sparking or heating effects. However, there is an excessive leakage current flowing through a parallel path formed by clamping devices or protection circuits in conventional intrinsically safe circuits, resulting in an inaccurate measurement of some sensors or a damage to other devices in the intrinsically safe circuits. Thus, there is a need for an intrinsically safe circuit with a low leakage current so as to guarantee the measurement accuracy of the sensor and achieve a higher energy limitation.

SUMMARY

In view of the foregoing problems, various example embodiments of the present disclosure provide an intrinsically safe circuit for a load and a sensor circuit comprising the same to better limit the energy and perform a measurement in a manner of high accuracy and high safety.

In a first aspect of the present disclosure, example embodiments of the present disclosure provide an intrinsically safe circuit for a load. The intrinsically safe circuit comprises an input adapted to be connected to a power supply; an output adapted to be connected to the load; a switching unit connected between the input and the output and configured to deliver an energy from the input to the output; a voltage clamping unit connected to the input and the switching unit, and configured to turn off the switching unit when an input voltage at the input exceeds a threshold voltage and clamp the input voltage to the threshold voltage, and configured to limit a leakage current through the voltage clamping unit below a predetermined leakage current level when the input voltage is in the vicinity of the threshold voltage; and a current limiting unit connected between the switching unit and the output and configured to limit a current to the output so as to limit the energy delivered to the output. With these embodiments, the voltage and the current of the intrinsically safe circuit are limited, such that the energy is limited to a predetermined energy level. Moreover, the leakage current of the intrinsically safe circuit is limited to a predetermined leakage level.

In some embodiments, the voltage clamping unit comprises a first resistor, a second resistor, a PNP transistor, a diode and a Zener diode; a first terminal of the first resistor is connected to the input; an emitter of the PNP transistor is connected to the input, a collector of the PNP transistor is connected to a control terminal of the switching unit; a first terminal of the second resistor is connected to a second terminal of the first resistor, and a second terminal of the second resistor is connected to a base of the PNP transistor; an anode of the first diode is connected to the first terminal of the second resistor; and a cathode of the Zener diode is connected to a cathode of the first diode, and an anode of the Zener diode is connected to the ground. With these embodiments, the voltage is clamped by the sum of the voltages across the Zener diode, the diode D1, the resistor R2 and the base-emitter junction of transistor Q1, and the leakage current is limited by the first resistor and the diode.

In some embodiments, the switching unit comprises a PMOS transistor and a third resistor, a source of the PMOS transistor is connected to the input, a drain of the PMOS transistor is connected to the output, a gate of the PMOS transistor is connected to the third resistor, and the third resistor is connected to the ground. With these embodiments, the excessive energy can be prevented from being delivered to the output.

In some embodiments, the current limiting unit comprises a fuse. With these embodiments, the current can be limited reliably and at a low cost.

In some embodiments, the intrinsically safe circuit further comprises: a soft starting unit connected between the voltage clamping unit and the switching unit and configured to soft start the intrinsically safe circuit. With these embodiments, the inrush current can be prevented reliably.

In some embodiments, the soft starting unit comprises a capacitor, wherein the capacitor is connected between the base of the PNP transistor and the control terminal of the switching unit. With these embodiments, the soft start can be achieved at a low cost.

In some embodiments, the intrinsically safe circuit further comprises: a switching hysteresis unit connected between the voltage clamping unit and an output terminal of the switching unit and configured to generate a switching hysteresis signal that prevents repetitive turn on and turn off the switching unit. With these embodiments, the repetitive turn on and turn off the switching unit can be prevented, and the service life of the circuit can be prolonged.

In some embodiments, the switching hysteresis unit comprises a fourth resistor, wherein the fourth resistor is connected between the base of the PNP transistor and the output terminal of the switching unit. With these embodiments, the switching hysteresis can be achieved at a low cost.

In some embodiments, the intrinsically safe circuit further comprises a reverse polarity protection unit connected between the switching unit and the current limiting unit and configured to prevent a reverse voltage from being applied to the load. With these embodiments, the load can be protected in the case that the reverse voltage is applied to the intrinsically safe circuit.

In some embodiments, the reverse polarity protection unit comprises at least one diode, wherein an anode of the at least one diode is connected to an output terminal of the switching unit, and a cathode of the at least one diode is connected to the current limiting unit. With these embodiments, the reverse polarity protection can be achieved at a low cost.

In a second aspect of the present disclosure, example embodiments of the present disclosure provide a sensor circuit. Wherein the sensor circuit comprises: a flexible probe configured to measure a height of a liquid level; and an intrinsically safe circuit according to the first aspect of the present disclosure, the intrinsically safe circuit being electrically connected to the flexible probe and configured to supply energy to the flexible probe. The sensor circuit comprises the intrinsically safe circuit according to the first aspect of the present disclosure, thus may provide the same advantages.

In some embodiments, the flexible probe comprises a housing made of thermoplastic construction or flexible membranes. With these embodiments, the cost of material and installation of the sensor circuit can be reduced.

It is to be understood that the Summary section is not intended to identify key or essential features of embodiments of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description.

DESCRIPTION OF DRAWINGS

Through the following detailed descriptions with reference to the accompanying drawings, the above and other objectives, features and advantages of the example embodiments disclosed herein will become more comprehensible. In the drawings, several example embodiments disclosed herein will be illustrated in examples and in a non-limiting manner, wherein:

FIG. 1 is a schematic view illustrating a working scenario of a conventional intrinsically safe circuit;

FIG. 2 is a schematic block diagram of an intrinsically safe circuit in accordance with an embodiment of the present disclosure;

FIG. 3 is a schematic circuit diagram of an intrinsically safe circuit in accordance with an embodiment of the present disclosure; and

FIG. 4 is a schematic block diagram of a sensor circuit in accordance with an embodiment of the present disclosure.

Throughout the drawings, the same or similar reference symbols are used to indicate the same or similar elements.

DETAILED DESCRIPTION OF EMBODIEMTNS

Principles of the present disclosure will now be described with reference to several example embodiments shown in the drawings. Though example embodiments of the present disclosure are illustrated in the drawings, it is to be understood that the embodiments are described only to facilitate those skilled in the art to better understand and thereby implement the present disclosure, rather than to limit the scope of the disclosure in any manner.

The term “comprises” or “includes” and its variants are to be read as open terms that mean “includes, but is not limited to. ” The term “or” is to be read as “and/or” unless the context clearly indicates otherwise. The term “based on” is to be read as “based at least in part on. ” The term “being operable to” is to mean a function, an action, a motion or a state that can be achieved by an operation induced by a user or an external mechanism. The term “one embodiment” and “an embodiment” are to be read as “at least one embodiment. ” The term “another embodiment” is to be read as “at least one other embodiment. ” The terms “first, ” “second, ” and the like may refer to different or same objects. Other definitions, explicit and implicit, may be included below. A definition of a term is consistent throughout the description unless the context clearly indicates otherwise.

According to embodiments of the present disclosure, a voltage clamping unit and a current limiting unit are provided in the intrinsically safe circuit so as to limit the energy output to the load, and the leakage current of the intrinsically safe circuit is limited to a predetermined leakage level by the voltage clamping unit. The above idea may be implemented in various manners, as will be described in detail in the following paragraphs.

First, operational principles and problems of a conventional intrinsically safe circuit will be described in detail with reference to FIG. 1. FIG. 1 is a schematic view illustrating a working scenario of the conventional intrinsically safe circuit. As shown in FIG. 1, the intrinsically safe circuit is formed by an intrinsically safe barrier 101. The intrinsically safe barrier 101 is connected between a power supply 102 and a load 103 (such as, a sensor) to limit an energy delivered from the power supply 102 to the load 103.

As shown in FIG. 1, the intrinsically safe barrier 101 comprises a fuse F0, three Zener diodes Z0, and a resistor R0. The fuse F0 is used to prevent an overcurrent from flowing into the load 103 and the Zener diodes Z0 are used to clamp a voltage applied to the load 103. When the current exceeds a predetermined current level, the fuse F0 may be disconnected, and the current provided to the load 103 will become 0. When the voltage of the power supply 102 exceeds a predetermined voltage level, the Zener diodes Z0 are conducted, and the voltage applied to the load 103 may be clamped to a breakdown voltage of the Zener diodes Z0. By clamping the voltage and limiting the current, the energy delivered to the load 103 may be limited.

When the load 103 is a sensor, it will use the current flowing therethrough as a sensor signal. The value of the current represents the measuring result. For example, for a sensor used to measure a height of a liquid level, the current value, such as in a range of 4-20 mA, of the sensor represents the height of the liquid level. In this situation, the current value should be accurate enough to guarantee the measurement accuracy of the sensor.

However, the intrinsically safe barrier 101 results in a significant leakage current of 10-20 μA per Zener diode in a reverse direction when the voltage of the power supply 102 is in the vicinity of the breakdown voltage of the Zener diode. As a result, the leakage current will flow back to the power supply 102 and influence the accuracy of the sensor. This situation becomes more critical in the case of explosive atmospheres where a triple redundancy of the Zener circuit is required to meet the ‘ia’ protection type. Additionally, the excessive leakage current will permanently damage the fuse F0.

Although a well-regulated voltage source can be used as the power supply 102 to output a stable voltage below the breakdown voltage of the Zener diode, the cost of such a power supply 102 will be relatively high, and voltage spikes sometimes are inevitable.

Thus, there is a need for an intrinsically safe circuit with a low leakage current so as to guarantee the measurement accuracy and achieve a higher energy limitation.

Hereinafter, the principles of the intrinsically safe circuit in accordance with embodiments of the present disclosure will be described in detail with reference to FIGs. 2-3. FIG. 2 is a schematic block diagram of an intrinsically safe circuit in accordance with an embodiment of the present disclosure, and FIG. 3 is a schematic circuit diagram of an intrinsically safe circuit in accordance with an embodiment of the present disclosure. As shown in FIG. 2, the intrinsically safe circuit 200 generally includes a switching unit 201, a voltage clamping unit 202 and a current limiting unit 203.

As shown in FIG. 2, the switching unit 201 is connected between an input and an output of the intrinsically safe circuit 200. The input of the intrinsically safe circuit 200 is adapted to be connected to a power supply, and the output of the intrinsically safe circuit 200 is adapted to be connected to a load, for example, a sensor. The switching unit 201 is used to deliver energy from the input to the output in a normal situation, and cut off the energy delivering in an over-voltage situation.

As shown in FIG. 3, in an embodiment, the switching unit 201 comprises a PMOS transistor Q2 and a resistor R3. A source of the PMOS transistor Q2 is connected to the input, a drain of the PMOS transistor Q2 is connected to the output, a gate of the PMOS transistor Q2 is connected to the resistor R3, and the resistor R3 is connected to the ground. In other embodiments, the switching unit 201 can include other components. The scope of the present disclosure is not intended to be limited in this respect.

Returning to FIG. 2, the voltage clamping unit 202 is connected to the input and the switching unit 201. The voltage clamping unit 202 is used to turn off the switching unit 201 when an input voltage at the input of the intrinsically safe circuit 200 exceeds a threshold voltage and clamp the input voltage to the threshold voltage. By doing this, the energy delivering can be cut off when an over-voltage occurs, such that the voltage of the sensor is clamped. Furthermore, the voltage clamping unit 202 is used to limit a leakage current through the voltage clamping unit 202 below a predetermined leakage current level when the input voltage is in the vicinity of the threshold voltage. By doing this, the accuracy of a current type sensor can be guaranteed.

As shown in FIG. 3, in an embodiment, the voltage clamping unit 202 comprises a resistor R1, a resistor R2, a PNP transistor Q1, a diode D1 and a Zener diode Z1. The resistor R1 is connected between the input and an anode of the diode D1. A cathode of the diode D1 is connected to a cathode of the Zener diode Z1, and an anode of the Zener diode Z1 is connected to the ground. An emitter of the PNP transistor Q1 is connected to the input, a collector of the PNP transistor Q1 is connected to the gate of the PMOS transistor Q2, and a base of the PNP transistor Q1 is connected to the anode of the diode D1 via the resistor R2. In other embodiments, the voltage clamping unit 202 can include other components. The scope of the present disclosure is not intended to be limited in this respect.

In some embodiments, the breakdown voltage of the Zener diode Z1 is 30 V. In other embodiments, the breakdown voltage of the Zener diode Z1 can be other values, for example, 28V, 32V. The scope of the present disclosure is not intended to be limited in this respect.

In some embodiments, the leakage current of the Zener diode Z1 is limited to 5 nA. In other embodiments, the leakage current of the Zener diode Z1 can be limited to other values, for example, 10 nA, 20 nA. The scope of the present disclosure is not intended to be limited in this respect.

Returning to FIG. 2, the current limiting unit 203 is connected between the switching unit 201 and the output. The current limiting unit 203 is used to limit the current flowing into the load. When the current flowing into the load exceeds a threshold value, the current limiting unit 203 cuts off the current. As a result, the value of current in the load is limited below the threshold value.

As shown in FIG. 3, in an embodiment, the current limiting unit 203 comprises a fuse F1. When the current exceeds the threshold value, the fuse will be disconnected. In other embodiments, the current limiting unit 203 can include other components. The scope of the present disclosure is not intended to be limited in this respect.

In some embodiments, the fusing current of the fuse F1 is 28mA. In other embodiments, the fusing current of the fuse F1 can be other values, for example, 26 mA, 30mA. The scope of the present disclosure is not intended to be limited in this respect.

Returning to FIG. 2, in some embodiments, the intrinsically safe circuit 200 further comprises a soft starting unit 204. The soft starting unit 204 is connected between the voltage clamping unit 202 and the switching unit 201. The soft starting unit 204 is used to soft start the intrinsically safe circuit 200 so as to prevent an inrush current during the power on of the intrinsically safe circuit 200.

As shown in FIG. 3, in an embodiment, the soft starting unit 204 comprises a capacitor C1. The capacitor C1 is connected between the base of the PNP transistor Q1 and the gate of the PMOS transistor Q2. In other embodiments, the soft starting unit 204 can include other components. The scope of the present disclosure is not intended to be limited in this respect.

Returning to FIG. 2, in some embodiments, the intrinsically safe circuit 200 further comprises a switching hysteresis unit 205. The switching hysteresis unit 205 is connected between the voltage clamping unit 202 and an output terminal of the switching unit 201. The switching hysteresis unit 205 is used to generate a switching hysteresis signal that prevents repetitive turning on and turning off the switching unit 201 due to the noise in the intrinsically safe circuit or the resistance in the power supply.

As shown in FIG. 3, in an embodiment, the switching hysteresis unit 205 comprises a resistor R4. The resistor R4 is connected between the base of the PNP transistor Q1 and the drain of the PMOS transistor Q2. In other embodiments, the switching hysteresis unit 205 can include other components. The scope of the present disclosure is not intended to be limited in this respect.

In some embodiments, the resistance value of the resistor R4 is 120 times the resistance of the resistor R2. In other embodiments, the ratio of the resistor R4 and the resistor R2 can be other values, for example, 100, 150, 200. The scope of the present disclosure is not intended to be limited in this respect.

Returning to FIG. 2, in some embodiments, the intrinsically safe circuit 200 further comprises a reverse polarity protection unit 206. The reverse polarity protection unit 206 is connected between the switching unit 201 and the current limiting unit 203. The reverse polarity protection unit 206 is used to prevent a reverse voltage from being applied to the load, such as the sensor.

As shown in FIG. 3, in an embodiment, the reverse polarity protection unit 206 comprises two diodes D2 and D3. The diodes D2 and D3 are connected in series between the drain of the PMOS transistor Q2 and the fuse F1. When the input voltage is reversed, the input voltage will not be applied to the sensor due to the diodes D2 and D3. In other embodiments, the reverse polarity protection unit 206 can include other components. The scope of the present disclosure is not intended to be limited in this respect.

In some embodiments, the reverse polarity protection unit 206 comprises more than two diodes. In other embodiments, the reverse polarity protection unit 206 comprises only one diode. The scope of the present disclosure is not intended to be limited in this respect.

Hereinafter, the principles of the intrinsically safe circuit 200 will be described in detail with reference to FIG. 3.

At the beginning, when the input is connected to a power supply, the voltage on the base of the PNP transistor Q1 is 0V, such that the PNP transistor Q1 is turned on and the gate of the PMOS transistor Q2 is connected to the input via the PNP transistor Q1. As a result, the PMOS transistor Q2 is turned off at this point.

Then, the capacitor C1 is charged through the resistors R1, R2, R3, and the voltage on the base of the PNP transistor Q1 is increased. When the base-emitter voltage of the PNP transistor Q1 is below the turn on voltage, the PNP transistor Q1 is turned off, and the gate of the PMOS transistor Q2 is connected to the ground via the resistor R3. The PMOS transistor Q2 is turned on. Thus, the soft starting of the intrinsically safe circuit 200 is achieved.

In a normal situation, the energy is delivered from the input to the output via the PMOS transistor Q2. When the input voltage exceeds the sum of the breakdown voltage of the Zener diode Z1 (Vz1, for example, 30V) , the forward voltage of the diode D1 (VfD1, for example, 0.7V) and the base-emitter saturation voltage of the PNP transistor Q1, an over-voltage occurs. At this time, the base-emitter voltage of the PNP transistor Q1 exceeds the turn on voltage, such that the PNP transistor Q1 is turned on. As a result, the PMOS transistor Q2 is turned off, such that the energy delivering is cut off.

At the same time, the Zener diode Z1 is reverse conducted. The voltage V1 at a node N1 is clamped. Meanwhile, the leakage current of the Zener diode Z1 is limited by the resistor R1 and the diode D1. Compared with the leakage current in the conventional intrinsically safe circuit, the leakage current in intrinsically safe circuit 200 can be reduced to a thousandth of the leakage current in the conventional intrinsically safe circuit.

Still at this time, since the PMOS transistor Q2 is turned off, the voltage V2 at a node N2 generally is equal to the sum of the forward voltage of the diode D2 (VfD2, for example, 0.7V) and the forward voltage of the diode D3 (VfD3, for example, 0.7V) , because the node N2 is pulled to ground via the diodes D2, D3 and the load. The voltage Vb on the base of the PNP transistor Q1 is calculated as:

Vb = (V2 –V1) *R2 / (R2 + R4) = (VfD2+VfD3-Vz1-VfD1) *R2 / (R2 + R4)

Because V2 is smaller than V1, this creates a negative offset. As a result, the PNP transistor Q1 is turned on reliably, and thus the PMOS transistor Q2 is turned off reliably. Accordingly, a switching hysteresis is achieved.

With the intrinsically safe circuit as shown in FIG. 3, the current of the sensor is limited, the voltage applied to the sensor is clamped, and the leakage current of the Zener diode is limited. As a result, the energy is well limited, and the measurement accuracy of the sensor can be guaranteed.

Hereinafter, the principles of a sensor circuit will be described in detail with reference to FIG. 4. FIG. 4 is a schematic block diagram of a sensor circuit in accordance with an embodiment of the present disclosure. As shown in FIG. 4, the sensor circuit 400 generally includes a flexible probe 410 and an intrinsically safe circuit 200 according to embodiments of the present disclosure.

As shown in FIG. 4, the intrinsically safe circuit 200 is electrically connected to the flexible probe 410 and used to supply energy to the flexible probe 410. The flexible probe 410 is used to measure a height of a liquid level, for example, gas. Different current value output by the flexible probe 410 indicates different height of the liquid level. In some embodiments, the flexible probe 410 comprises a housing made of thermoplastic construction or flexible membranes. In other embodiments, the flexible probe comprises other kinds of housings. The scope of the present disclosure is not intended to be limited in this respect.

With the sensor circuit as shown in FIG. 4, as the energy of the flexible probe 410 is well limited by the intrinsically safe circuit 200, the flexible probe 410 can be used in explosive atmospheres. Compared with the sensors that carry Explosion Proof (XP) or Flameproof (Exd) protection types, the sensor circuit 400 can reduce material and installation costs while meeting the requirement of Intrinsically Safe.

While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

Claims

An intrinsically safe circuit (200) for a load, comprising:

an input adapted to be connected to a power supply;

an output adapted to be connected to the load;

a switching unit (201) connected between the input and the output and configured to deliver an energy from the input to the output;

a voltage clamping unit (202) connected to the input and the switching unit (201) , and configured to turn off the switching unit (201) when an input voltage at the input exceeds a threshold voltage and clamp the input voltage to the threshold voltage, and configured to limit a leakage current through the voltage clamping unit (202) below a predetermined leakage current level when the input voltage is in the vicinity of the threshold voltage; and

a current limiting unit (203) connected between the switching unit (201) and the output and configured to limit a current to the output so as to limit the energy delivered to the output.
The intrinsically safe circuit (200) according to claim 1, wherein the voltage clamping unit (202) comprises a first resistor (R1) , a second resistor (R2) , a PNP transistor (Q1) , a diode (D1) and a Zener diode (Z1) ;

a first terminal of the first resistor (R1) is connected to the input;

an emitter of the PNP transistor (Q1) is connected to the input, a collector of the PNP transistor (Q1) is connected to a control terminal of the switching unit (201) ;

a first terminal of the second resistor (R2) is connected to a second terminal of the first resistor (R1) , and a second terminal of the second resistor (R2) is connected to a base of the PNP transistor (Q1) ;

an anode of the first diode (D1) is connected to the first terminal of the second resistor (R2) ; and

a cathode of the Zener diode (Z1) is connected to a cathode of the first diode (D1) , and an anode of the Zener diode (Z1) is connected to the ground.
The intrinsically safe circuit (200) according to claim 1, wherein the switching unit (201) comprises a PMOS transistor (Q2) and a third resistor (R3) , a source of the PMOS transistor (Q2) is connected to the input, a drain of the PMOS transistor (Q2) is connected to the output, a gate of the PMOS transistor (Q2) is connected to the third resistor (R3) , and the third resistor (R3) is connected to the ground.
The intrinsically safe circuit (200) according to claim 1, wherein the current limiting unit (203) comprises a fuse (F1) .
The intrinsically safe circuit (200) according to claim 2, further comprising:

a soft starting unit (204) connected between the voltage clamping unit (202) and the switching unit (201) and configured to soft start the intrinsically safe circuit (200) .
The intrinsically safe circuit (200) according to claim 5, wherein the soft starting unit (204) comprises a capacitor (C1) , wherein the capacitor (C1) is connected between the base of the PNP transistor (Q1) and the control terminal of the switching unit (201) .
The intrinsically safe circuit (200) according to claim 2, further comprising:

a switching hysteresis unit (205) connected between the voltage clamping unit (202) and an output terminal of the switching unit (201) and configured to generate a switching hysteresis signal that prevents repetitive turn on and turn off the switching unit (201) .
The intrinsically safe circuit (200) according to claim 7, wherein the switching hysteresis unit (205) comprises a fourth resistor (R4) , wherein the fourth resistor (R4) is connected between the base of the PNP transistor (Q1) and the output terminal of the switching unit (201) .
The intrinsically safe circuit (200) according to claim 1, further comprising a reverse polarity protection unit (206) connected between the switching unit (201) and the current limiting unit (203) and configured to prevent a reverse voltage from being applied to the load.
The intrinsically safe circuit (200) according to claim 9, wherein the reverse polarity protection unit (206) comprises at least one diode, wherein an anode of the at least one diode is connected to an output terminal of the switching unit (201) , and a cathode of the at least one diode is connected to the current limiting unit (203) .
A sensor circuit (400) , comprising:

a flexible probe (410) configured to measure a height of a liquid level; and

an intrinsically safe circuit (200) according to any of claims 1-10, the intrinsically safe circuit (200) being electrically connected to the flexible probe (410) and configured to supply energy to the flexible probe (410) .
The sensor circuit according to claim 11, wherein the flexible probe (410) comprises a housing made of thermoplastic construction or flexible membranes.