A SIL2 safety level 4-20mA input detection circuit and method

Abstract

The present invention discloses a 4-20mA input detection circuit and method of a SIL2 safety level, which belongs to the field of input detection technology, including a main control module, a constant current generating module, a 4-20mA detection module, and a 4-20mA input module; the constant current generating module, the 4-20mA detection module, and the 4-20mA input module are all communicated with the main control module, and the constant current generating module and the 4-20mA input module are all communicated with the 4-20mA detection module, and the constant current generating module and the 4-20mA input module are controlled by the main control module, and the current of 4-20mA is detected. The present invention adds a verification circuit to the 4-20mA input module on the basis of the detection circuit of the 4-20mA input module, and uses an arbitration-controlled detection method to verify whether the 4-20mA input module is normal on the basis of real-time detection of the 4-20mA input, ensuring that the detection circuit of the 4-20mA input module meets the SIL2 safety level.

Description

Technical Field

[0001] The present invention relates to the field of input detection technology, and in particular to a 4-20mA input detection circuit and method with a SIL2 safety level. Background Art

[0002] The signal current of general instruments is 4-20mA, with a minimum current of 4mA and a maximum current of 20mA. When transmitting signals, it is important to consider the resistance of the wire. If voltage is used for transmission, a certain voltage drop will occur in the wire, which will cause a certain error in the signal at the receiving end. Therefore, current signals are used as the standard transmission signal for transmitters.

[0003] In order to make the 4-20mA input module meet the SIL2 safety level, it is necessary to be able to verify whether the module itself is normal. Currently, there are many detection circuits for 4-20mA input modules, but no suitable design method has been found based on the SIL2 safety level design. The above problem needs to be solved urgently. Therefore, a 4-20mA input detection circuit and method with SIL2 safety level are proposed. Summary of the Invention

[0004] The technical problem to be solved by the present invention is how to realize real-time detection of a 4-20mA input module and module function self-test, and provides a 4-20mA input detection circuit with a SIL2 safety level.

[0005] The present invention solves the above technical problems through the following technical solutions. The present invention includes a main control module, a constant current generating module, a 4-20mA detection module, and a 4-20mA input module; the constant current generating module, the 4-20mA detection module, and the 4-20mA input module are all communicatively connected to the main control module, and the constant current generating module and the 4-20mA input module are all communicatively connected to the 4-20mA detection module. The main control module controls the constant current generating module and the 4-20mA input module, and detects the 4-20mA current.

[0006] Furthermore, the main control module is a microcontroller U1, which realizes the output enable control and output current switching of the constant current generating module through two GPIO pins, and controls the input enable of 4-20mA in the 4-20mA input module through another GPIO pin.

[0007] Furthermore, the constant current generating module includes a current selection circuit, a primary constant current circuit, and a secondary constant current circuit; three different current output selections are realized through the current selection circuit, and the three different current outputs are zero current, a first self-test current (I1), and a second self-test current (I2). The constant current output is realized through the primary constant current circuit, and a drive is provided for the secondary constant current circuit, and the final current output is realized through the secondary constant current circuit.

[0008] Furthermore, the current selection circuit includes resistors R1 to R5, transistor Q1, and transistor Q2. The microcontroller U1 is connected to the base of the transistor Q1 through the first GPIO pin, the resistor R1 is arranged between the first GPIO pin and the base of the transistor Q1, the emitter of the transistor Q1 is grounded, the collector of the transistor Q1 is connected to the primary constant current circuit, the collector of the transistor Q1 is connected to the power supply end through the resistor R3, and a first test point TP1 is provided at the collector of the transistor Q1. The microcontroller U1 is connected to the base of the transistor Q2 through the second GPIO pin, the resistor R2 is arranged between the second GPIO pin and the base of the transistor Q2, the emitter of the transistor Q2 is grounded, the collector of the transistor Q2 is connected to the collector of the transistor Q1 through the resistor R4, and the collector of the transistor Q2 is grounded through the resistor R5.

[0009] Furthermore, the primary constant current circuit includes an error amplifier chip U2, a capacitor C1, a MOS tube Q3, resistors R6 and R7. The non-inverting input terminal of the error amplifier chip U2 is connected to the collector of the transistor Q1, the inverting input terminal is connected in parallel with the drain of the MOS tube Q3 and then grounded through the resistor R6, the output terminal is connected to the gate of the MOS tube Q3, the capacitor C1 is connected between the inverting input terminal and the output terminal of the error amplifier chip U2, the source of the MOS tube Q3 is connected to the power supply terminal through the resistor R7, and is connected to the secondary constant current circuit.

[0010] Furthermore, the secondary constant current circuit includes an error amplifier chip U3, a capacitor C2, a MOS tube Q4, a resistor R8, and a diode D1. The non-inverting input terminal of the error amplifier chip U3 is connected to the source of the MOS tube Q3, the inverting input terminal is connected in parallel with the source of the MOS tube Q4 and then connected to the power supply terminal through the resistor R8, the output terminal is connected to the gate of the MOS tube Q4, the capacitor C2 is connected between the inverting input terminal and the output terminal of the error amplifier chip U3, and the drain of the MOS tube Q4 is connected to the 4-20mA detection module through the diode D1.

[0011] Furthermore, the 4-20mA detection module includes an ADC chip U4, an input filter circuit, an impedance matching circuit, and a sampling resistor R9; the input filter circuit includes a resistor R10 and a capacitor C3, and the impedance matching circuit includes an operational amplifier U5. One end of the ADC chip U4 is connected to the microcontroller U1, and the other end is connected to one end of the resistor R10. The other end of the resistor R10 is connected to the output end of the operational amplifier U5. One end of the capacitor C3 is connected between the resistor R10 and the ADC chip U4, and the other end is grounded. The inverting input end of the operational amplifier U5 is connected to the output end to form a follower, and the non-inverting input end is connected in parallel with the diode D1 and grounded through the resistor R9.

[0012] Furthermore, the 4-20mA input module includes a resistor R13, a transistor Q6, a signal relay, a freewheeling diode D4, a TVS diode D3, and an external 4-20mA input interface. The microcontroller U1 is connected to the base of the transistor Q6 through the third GPIO pin. The resistor R13 is arranged between the third GPIO pin and the base of the transistor Q6. The emitter of the transistor Q6 is grounded, and the collector is connected to the negative end of the signal relay coil. The positive end of the signal relay coil is connected to the power supply end. One end of the freewheeling diode D4 is connected to the power supply end, and the other end is connected to the collector of the transistor Q6. The external 4-20mA input interface is connected to the inverting input end of the operational amplifier U5 through the first contact (contact 1) of the signal relay. One end of the TVS diode D3 is grounded, and the other end is connected to the second contact (contact 2) of the signal relay. The second contact of the signal relay is connected to the non-inverting input end of the operational amplifier U5.

[0013] The present invention also provides a 4-20mA input detection method with a SIL2 safety level, which uses the above circuit to implement the 4-20mA input detection work with a SIL2 safety level, including the following steps:

[0014] S1: Set the first GPIO pin to a high level, the second GPIO pin to a low level, and the third GPIO pin to a high level, turn off the constant current generating module, and close the two contacts of the signal relay. The external signal is connected to the 4-20mA detection module through the external 4-20mA input interface. At this time, the 4-20mA input detection circuit is in the external signal detection state;

[0015] S2: In order to periodically verify whether the function of the 4-20mA input detection circuit is normal, after the 4-20mA input detection circuit runs for a specified time, the first GPIO pin is set to a low level, the second GPIO pin is set to a high level, and the third GPIO pin is set to a low level, the constant current generating module is turned on, the first self-test current is output, and the two contacts of the signal relay are disconnected, and the external 4-20mA input signal is turned off. At this time, the 4-20mA input detection circuit device is in the first self-test state; the microcontroller U1 verifies whether the difference between the read input current and the first self-test current is within the set threshold range, and then determines whether the 4-20mA input detection circuit is normal. If it is normal, it enters step S3, otherwise it determines that the 4-20mA input detection circuit is abnormal and reports an error;

[0016] S3: Set the first GPIO pin to a low level, the second GPIO pin to a low level, and the third GPIO pin to a low level, turn on the constant current generating module, output the second self-test current, and disconnect the two contacts of the signal relay, turn off the external 4-20mA input signal, and the 4-20mA input detection circuit is in the second self-test state; the microcontroller U1 verifies whether the difference between the read input current and the second self-test current is within the set threshold range, and determines whether the 4-20mA detection circuit is normal. If normal, enter step S1, otherwise it is determined that the 4-20mA input detection circuit is abnormal and an error is reported.

[0017] Compared with the prior art, the present invention has the following advantages: the 4-20mA input detection circuit with a SIL2 safety level adds a verification circuit for the 4-20mA input module on the basis of the detection circuit of the 4-20mA input module, and uses an arbitration-controlled detection method to verify whether the 4-20mA input module is normal on the basis of real-time detection of the 4-20mA input, thereby ensuring that the detection circuit of the 4-20mA input module complies with the SIL2 safety level and is worthy of being promoted and used. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a block diagram of a 4-20mA input detection circuit with a SIL2 safety level according to an embodiment of the present invention;

[0019] Figure 2 1 is a circuit diagram of a constant current generating module according to an embodiment of the present invention;

[0020] Figure 3 This is a circuit diagram of a 4-20mA detection module in an embodiment of the present invention;

[0021] Figure 4 4-20mA input module circuit diagram of the embodiment of the present invention;

[0022] Figure 5This is a schematic diagram of a 4-20mA input detection circuit with a SIL2 safety level according to an embodiment of the present invention;

[0023] Figure 6 This is a flowchart of arbitration control in an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process. However, the protection scope of the present invention is not limited to the following embodiment.

[0025] This embodiment provides a technical solution: a 4-20mA input detection circuit with a SIL2 safety level, comprising a main control module 1, a constant current generating module 2, a 4-20mA detection module 3, and a 4-20mA input module 4;

[0026] The main control module 1 is mainly a microcontroller U1, which can realize the control of the constant current generation module, the 4-20mA input module, the 4-20mA current detection, and the detection process control of the 4-20mA input detection circuit;

[0027] The constant current generation module is controlled mainly through the two GPIO pins (first GPIO pin and second GPIO pin) of the U1 chip to realize the output enable control of the constant current generation module and the output current switching of the constant current generation module;

[0028] The 4-20mA input module is controlled mainly through one GPIO pin (the third GPIO pin) of U1 to control the 4-20mA input enable, which is used to turn on or off the 4-20mA input;

[0029] 4-20mA current detection is the most basic current detection function, which converts the ADC sampling value of the U4 chip into a 4-20mA current value;

[0030] The detection process control of the 4-20mA input detection circuit realizes the cyclic switching of the constant current generation module and the 4-20mA input module through a detection process to determine whether the module itself is normal, thereby realizing the 4-20mA input detection of SIL2 safety level.

[0031] For detailed process, see Figure 6 As shown in the flowchart; the process is designed with two states: working state and self-test state;

[0032] S1: In the working state, first determine whether the waiting time has been reached and enter the self-test state; if the time has not been reached, set GPIO1 to high level, GPIO2 to low level, and GPIO3 to high level. At this time, the external input 4-20mA current is collected;

[0033] S2: In the working state, if it is determined that the waiting time has reached the time to enter the self-test state, the self-test state will be entered immediately;

[0034] S3: In the self-test state, first set GPIO1 to low level, GPIO2 to high level, and GPIO3 to low level. At this time, the self-test current I1 is collected. If the self-test current is judged to be wrong, it will exit directly and report an error;

[0035] S4: If the self-test current I1 is correct, GPIO1, GPIO2 and GPIO3 are set to low level. At this time, the self-test current I2 is collected. If the self-test current is wrong, the program exits directly and reports an error. Otherwise, the program enters the working state.

[0036] The constant current generating module includes a current selection circuit, a primary constant current circuit, and a secondary constant current circuit. The current selection circuit includes resistors R1 to R5 and transistors Q1 and Q2. The primary constant current circuit includes a chip U2, a capacitor C1, a MOS tube Q3, resistors R6 and R7. The secondary constant current circuit includes a chip U3, a capacitor C2, a MOS tube Q4, a resistor R8, and a diode D1.

[0037] The current selection circuit primarily implements three different current output options: zero current, I1 current, and I2 current. The zero current output control switches the output current on and off. The I1 and I2 currents can be configured to 25% and 75% of full scale (any value is acceptable without affecting functionality).

[0038] GPIO1 (first GPIO pin) controls the on / off of transistor Q1 to achieve TP1 (first test point) level U TP1 Whether to switch to 0 potential, thereby realizing whether to enable output; GPIO2 (second GPIO pin) controls transistor Q2 to realize TP1 level U TP1 The two potentials V1 and V2 are switched. When GPIO2 is high, Q2 is turned on. The voltage at TP1 is set to V1. The voltage value of V1 is When GPIO2 is low, Q2 is closed. If the voltage at TP1 is V2, then the voltage value of V2 is

[0039] The primary constant current circuit realizes the primary constant current output, mainly providing drive for the secondary constant current circuit.

[0040] U2 chip is the error amplifier module of the first-level constant current circuit. Using the virtual short principle of U2, the positive and negative input voltages of U2 are the same, then the voltage U at TP1 is TP1Equal to the voltage difference across resistor R6, the current flowing through resistor R6 is constant and always equal to U TP1 / R6; using the virtual disconnect principle of U2, the input current of the positive and negative input terminals of U2 is close to 0, then the current flowing through the MOS tube Q3 is equal to the current flowing through the resistor R6, thereby achieving the constant current function.

[0041] The output current value I of the primary constant current circuit level1 for:

[0042]

[0043] C1 is used to make the circuit more stable and prevent self-oscillation. There are many ways to prevent self-oscillation. The present invention only lists this method but is not limited to this method.

[0044] MOS tube Q3 is the driver stage of the primary constant current circuit. The U2 chip adjusts the change of Q3's VGS voltage by changing the output voltage to achieve the change of Q3's on-resistance. The change of Q3's on-resistance can control the current flowing through Q3 and adjust the voltage on resistor R6, thereby controlling the U2 chip to adjust the output voltage and realize closed-loop control of the entire constant current circuit.

[0045] The resistor R7 is used to provide a reference input for the secondary constant current circuit, because the current flowing through R7 is I level1 , then the voltage value on the resistor R7 is U R7 =R7*I level1 .

[0046] The secondary constant current circuit realizes the final current output. U3 chip is the error amplifier module of the secondary constant current circuit. Because the voltages of the positive input and negative input of U3 are equal (using the virtual short principle of U3 chip, the voltages of the positive input and negative input of U3 are equal), the current on resistor R7 is the constant current I output by the primary constant current circuit. level1 (equal to I1 or I2 or 0 current), then the voltage at the inverting input of U3 is R7*I level1 , the current flowing through the resistor R8 is equal to the current value I output by the secondary constant current circuit level2 :

[0047]

[0048] The function of C2 is the same as that of C1, which is to prevent the U3 chip from self-oscillating. The function of MOS tube Q4 is the same as that of Q3, which is to participate in the feedback control of the constant current circuit and realize the driving output of the secondary constant current circuit.

[0049] Diode D1 utilizes the unidirectional conduction characteristic of the diode. When the 4-20mA input is detected, the constant current generating module does not affect the 4-20mA input detection.

[0050] The 4-20mA detection module mainly includes U4 (ADC chip), ADC input filter circuit composed of RC, impedance matching circuit composed of operational amplifier U5, and sampling resistor R9.

[0051] The sampling resistor R9 realizes the conversion between current and voltage. Depending on the accuracy requirements, a precision resistor with a temperature coefficient of less than 25ppm can be selected to prevent inaccurate detection current caused by temperature changes and discrete resistor types. If the reference source of the ADC is Vref, the linear range of the ADC is increased. The selection of resistor R9 can be calculated according to the following formula:

[0052]

[0053] The 20mA in the above formula is the maximum current value of 4-20mA. The coefficient k is to prevent a certain error in the output current. The value range of the coefficient k is between 0.8 and 1.0.

[0054] U5 is an operational amplifier that forms a follower to convert the high impedance at R9 into a low impedance at the output of the operational amplifier, so that the ADC sampling module can obtain the voltage at R9 more accurately.

[0055] R10 and C3 form an ADC filter circuit to filter out current noise at the input end. The resistance value of R10 should not be too large and should be selected between 22R and 220R. The value of capacitor C3 is determined according to the response speed of the current and the noise frequency of the input signal. If the response time of the current is required to be above 0.1S, the value of C3 can be selected between 1nF and 100nF. If the response time of the current is required to be below 0.1S, the capacitance value of C3 is recommended to be in the range of 1000pF.

[0056] U4 is an ADC sampling chip that converts analog voltage values into digital quantities. After being acquired by the U1 chip, the actual input current value can be calculated. The sampling accuracy of the ADC is determined according to the accuracy requirements of the current sampling.

[0057] The 4-20mA input module inputs the 4-20mA current value from an external module. This module implements a switch function, which can be controlled by the U1 chip to switch between the self-test state and the measurement state.

[0058] The base current limiting resistor R13, transistor / N-channel MOS tube Q6, signal relay RLY1, and freewheeling diode D4 together form a current switching module, which controls the on and off of Q5 by controlling the on and off of transistor Q6, thereby achieving on-off control of the input current.

[0059] R13 is the base current limiting resistor of transistor Q6. It controls the switching state of RLY1 by controlling the on and off of Q6. Q6 can use a transistor or an N-channel MOS tube.

[0060] RLY1 is a signal relay. Any circuit that can realize the on-off circuit of controlling the input current can replace RLY1, so RLY1 can also use an analog switch.

[0061] D2 is a general-purpose diode that uses its unidirectional conduction function to isolate the constant current generating module and the 4-20mA input module to prevent mutual interference between the signals of the two modules;

[0062] D3 is a TVS diode, which prevents the voltage of the external input signal from exceeding the common-mode input voltage of U5 in the 4-20mA detection module and damaging the device;

[0063] D4 is the freewheeling diode of RLY1, which can ensure that the current in the relay coil can be continuously released to the power supply end after Q6 is turned off, preventing Q6 from being broken down.

[0064] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A 4-20mA input detection method with SIL2 safety level, characterized in that: A SIL2 safety-level 4-20mA input detection circuit is used to implement SIL2 safety-level 4-20mA input detection work. The SIL2 safety-level 4-20mA input detection circuit includes a main control module, a constant current generating module, a 4-20mA detection module, and a 4-20mA input module; the constant current generating module, the 4-20mA detection module, and the 4-20mA input module are all communicatively connected to the main control module, and the constant current generating module and the 4-20mA input module are all communicatively connected to the 4-20mA detection module. The main control module controls the constant current generating module and the 4-20mA input module, and detects the 4-20mA current. The SIL2 safety-level 4-20mA input detection method includes the following steps: S1: Set the first GPIO pin to a high level, the second GPIO pin to a low level, and the third GPIO pin to a high level, turn off the constant current generating module, and close the two contacts of the signal relay. The external signal is connected to the 4-20mA detection module through the external 4-20mA input interface. At this time, the 4-20mA input detection circuit is in the external signal detection state; S2: After the 4-20mA input detection circuit runs for a specified time, the first GPIO pin is set to a low level, the second GPIO pin is set to a high level, and the third GPIO pin is set to a low level, the constant current generating module is turned on, the first self-test current is output, and the two contacts of the signal relay are disconnected, and the external 4-20mA input signal is turned off. At this time, the 4-20mA input detection circuit is in the first self-test state; the microcontroller U1 verifies whether the difference between the read input current and the first self-test current is within the set threshold range, and then determines whether the 4-20mA input detection circuit is normal. If it is normal, it enters step S3; otherwise, it determines that the 4-20mA input detection circuit is abnormal and reports an error; S3: Set the first GPIO pin to a low level, the second GPIO pin to a low level, and the third GPIO pin to a low level, turn on the constant current generating module, output the second self-test current, and disconnect the two contacts of the signal relay, turn off the external 4-20mA input signal, and the 4-20mA input detection circuit is in the second self-test state; the microcontroller U1 verifies whether the difference between the read input current and the second self-test current is within the set threshold range, and determines whether the 4-20mA detection circuit is normal. If normal, enter step S1, otherwise it is determined that the 4-20mA input detection circuit is abnormal and an error is reported.

2. The SIL2 safety level 4-20mA input detection method according to claim 1, characterized in that: The main control module is a microcontroller U1, which realizes the output enable control and output current switching of the constant current generating module through two GPIO pins. The microcontroller U1 controls the input enable of 4-20mA in the 4-20mA input module through another GPIO pin.

3. The SIL2 safety level 4-20mA input detection method according to claim 2, characterized in that: The constant current generating module includes a current selection circuit, a primary constant current circuit, and a secondary constant current circuit; Three different current output selections are achieved through the current selection circuit, and the three different current outputs are zero current, the first self-test current, and the second self-test current. The constant current output is achieved through the first-level constant current circuit, which provides drive for the second-level constant current circuit, and the final current output is achieved through the second-level constant current circuit.

4. The SIL2 safety level 4-20mA input detection method according to claim 3, characterized in that: The current selection circuit includes resistors R1 to R5, a transistor Q1, and a transistor Q2. The microcontroller U1 is connected to the base of the transistor Q1 through a first GPIO pin. The resistor R1 is arranged between the first GPIO pin and the base of the transistor Q1. The emitter of the transistor Q1 is grounded. The collector of the transistor Q1 is connected to the primary constant current circuit. The collector of the transistor Q1 is connected to the power supply end through a resistor R3. A first test point TP1 is arranged at the collector of the transistor Q1. The microcontroller U1 is connected to the base of the transistor Q2 through a second GPIO pin. The resistor R2 is arranged between the second GPIO pin and the base of the transistor Q2. The emitter of the transistor Q2 is grounded. The collector of the transistor Q2 is connected to the collector of the transistor Q1 through a resistor R4. The collector of the transistor Q2 is grounded through a resistor R5.

5. The SIL2 safety level 4-20mA input detection method according to claim 4, characterized in that: The primary constant current circuit includes an error amplifier chip U2, a capacitor C1, a MOS transistor Q3, and resistors R6 and R7. The non-phase input terminal of the error amplifier chip U2 is connected to the collector of the transistor Q1, the inverting input terminal is connected in parallel with the drain of the MOS transistor Q3 and then grounded through the resistor R6, the output terminal is connected to the gate of the MOS transistor Q3, the capacitor C1 is connected between the inverting input terminal and the output terminal of the error amplifier chip U2, the source of the MOS transistor Q3 is connected to the power supply terminal through the resistor R7, and is connected to the secondary constant current circuit.

6. The SIL2 safety level 4-20mA input detection method according to claim 5, characterized in that: The secondary constant current circuit includes an error amplifier chip U3, a capacitor C2, a MOS transistor Q4, a resistor R8, and a diode D1. The non-phase input terminal of the error amplifier chip U3 is connected to the source of the MOS transistor Q3, the inverting input terminal is connected in parallel with the source of the MOS transistor Q4 and then connected to the power supply terminal through the resistor R8, the output terminal is connected to the gate of the MOS transistor Q4, the capacitor C2 is connected between the inverting input terminal and the output terminal of the error amplifier chip U3, and the drain of the MOS transistor Q4 is connected to the 4-20mA detection module through the diode D1.

7. The SIL2 safety level 4-20mA input detection method according to claim 6, characterized in that: The 4-20mA detection module includes an ADC chip U4, an input filter circuit, an impedance matching circuit, and a sampling resistor R9; the input filter circuit includes a resistor R10 and a capacitor C3, and the impedance matching circuit includes an operational amplifier U5. One end of the ADC chip U4 is connected to the microcontroller U1, and the other end is connected to one end of the resistor R10. The other end of the resistor R10 is connected to the output end of the operational amplifier U5. One end of the capacitor C3 is connected between the resistor R10 and the ADC chip U4, and the other end is grounded. The inverting input end of the operational amplifier U5 is connected to the output end to form a follower, and the non-inverting input end is connected in parallel with the diode D1 and then grounded through the resistor R9.

8. The SIL2 safety level 4-20mA input detection method according to claim 7, characterized in that: The 4-20mA input module includes a resistor R13, a transistor Q6, a signal relay, a freewheeling diode D4, a TVS diode D3, and an external 4-20mA input interface. The microcontroller U1 is connected to the base of the transistor Q6 through the third GPIO pin. The resistor R13 is arranged between the third GPIO pin and the base of the transistor Q6. The emitter of the transistor Q6 is grounded, and the collector is connected to the negative end of the signal relay coil. The positive end of the signal relay coil is connected to the power supply end. One end of the freewheeling diode D4 is connected to the power supply end, and the other end is connected to the collector of the transistor Q6. The external 4-20mA input interface is connected to the inverting input end of the operational amplifier U5 through the first contact of the signal relay. One end of the TVS diode D3 is grounded, and the other end is connected to the second contact of the signal relay. The second contact of the signal relay is connected to the non-inverting input end of the operational amplifier U5.

Images