Constant current device and fault detection method

Abstract

The system detects abnormalities where the actual current signal differs from the set current, thereby improving the reliability of the constant current device. [Solution] The error amplifier Q2 generates a current signal which is a constant current. The signal processing circuit 102 inputs an input voltage to the error amplifier Q2 to generate a current signal having a target current value, and detects a fault by comparing the voltage value of the voltage output from the error amplifier Q2 with a judgment voltage value.

Description

Technical Field

[0001] The present invention relates to a constant current device and a fault detection method.

Background Art

[0002] A two-wire transmitter is a device that is connected to an external circuit via two transmission lines, converts a physical quantity acquired from a sensor or the like into a current signal while using the external circuit as a power source, and outputs the signal to the external circuit. Since a two-wire transmitter does not require dedicated power wiring and can be installed at a low cost, it is widely used as a field device such as a differential pressure / pressure transmitter or a temperature transmitter in various plants using oil, petrochemicals, chemicals, gas, etc. A two-wire transmitter used as a field device converts a physical quantity into a direct current of 4 to 20 mA, which is a world standard as a signal of the field device, and transmits the current to an external circuit.

[0003] A two-wire transmitter is connected to an external circuit by two transmission lines and operates using the external circuit as a power source. Further, the two-wire transmitter includes an error amplifier such as an operational amplifier that operates to compare a voltage based on a physical quantity acquired from a sensor with a reference voltage and make them coincide, and a transistor whose current flowing is controlled by an output from the error amplifier. A constant current circuit is provided. Then, the two-wire transmitter outputs a predetermined current signal of 4 to 20 mA indicating a physical quantity acquired from a sensor or the like to an external circuit (for example, Patent Document 1). The external circuit supplies a power supply voltage to the two-wire transmission line and reads a voltage corresponding to a predetermined current signal sent from the two-wire transmission line to acquire the physical quantity measured by the two-wire transmitter.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in two-wire transmitters, circuit failures or insufficient power supply voltage can cause abnormalities where the transmitted current signal differs from the set current. For example, if a transistor that supplies the current to generate a predetermined current signal representing a physical quantity fails, a current different from the set current based on the operational amplifier's control will flow through the transistor, resulting in the current signal representing the physical quantity no longer showing an accurate value. In such cases, conventional two-wire transmitters have difficulty detecting the abnormality where the transmitted current signal differs from the set current, making it difficult to transmit the failure to external circuits. Therefore, conventional two-wire transmitters, including constant current circuits, are at risk of continuing to operate despite a failure, potentially leading to reduced reliability.

[0006] One aspect of the present invention is to improve the reliability of a constant current device by detecting an anomaly where the actual current signal flowing differs from the set current. [Means for solving the problem]

[0007] The constant current device relating to one side has the following parts: An error amplifier generates a current signal which is a constant current. A signal processing circuit inputs an input voltage to the error amplifier to generate the current signal having a target current value, and detects a fault by comparing the voltage value of the voltage output from the error amplifier with a determination voltage value. [Effects of the Invention]

[0008] According to the present invention, the reliability of a constant current device can be improved by detecting an abnormality in which the actual current signal flowing differs from the set current. [Brief explanation of the drawing]

[0009] [Figure 1] This is a circuit diagram of a two-wire transmitter according to the first embodiment. [Figure 2] This is a flowchart of the fault detection process using a two-wire transmitter according to the first embodiment. [Figure 3] This is a circuit diagram of a two-wire transmitter according to the second embodiment. [Figure 4] This is a circuit diagram of a two-wire transmitter according to the third embodiment. [Figure 5] This is a circuit diagram of a two-wire transmitter according to the fourth embodiment. [Figure 6] This is a circuit diagram of a two-wire transmitter according to the fifth embodiment. [Modes for carrying out the invention]

[0010] The embodiments of the constant current device and fault detection method will be described below with reference to the drawings. The same elements are denoted by the same reference numerals, and redundant explanations are omitted as appropriate. Furthermore, each embodiment can be combined as appropriate within the bounds of consistency.

[0011] (First Embodiment) (Overall structure) Figure 1 is a circuit diagram of a two-wire transmitter according to the first embodiment. As shown in Figure 1, the two-wire transmitter 100 is connected to the external circuit 10 by transmission lines L1 and L2. The two-wire transmitter 100 operates using the external circuit 10 as its power source.

[0012] The external circuit 10 has a power supply voltage Eb and a detection resistor R connected in series with the transmission lines L1 and L2. The external circuit 10 supplies power to the two-wire transmitter 100 using the power supply voltage Eb as its power source. The external circuit 10 also reads the voltage across the detection resistor R to obtain the physical quantity measured by the two-wire transmitter 100.

[0013] The two-wire transmitter 100 is a field device such as a differential pressure transmitter or a temperature transmitter, and outputs a predetermined current signal indicating a physical quantity to the external circuit 10. The two-wire transmitter 100 includes a sensor 101, a signal processing circuit 102, a reference voltage processing circuit 103, a comparator circuit 104, a shunt regulator circuit 105, a constant current circuit 106, a diode D1, a feedback resistor R1, and an AD (Analog Digital) conversion circuit 120.

[0014] (Composition of each part) Sensor 101 measures physical quantities such as pressure and temperature in a plant. Then, sensor 101 converts the measured physical quantity into an electrical signal and outputs it to signal processing circuit 102.

[0015] Signal processing circuit 102 receives an input of an electrical signal indicating a physical quantity from sensor 101. Then, signal processing circuit 102 performs predetermined processing such as linearity correction (strain correction) and noise removal on the acquired electrical signal. Next, signal processing circuit 102 converts the electrical signal subjected to the predetermined processing into a PWM (Pulse Width Modulation) signal for a current signal. Then, signal processing circuit 102 outputs the generated PWM signal for the current signal to switch SW1 as a switching control signal. Here, the current targeted for control based on the PWM signal for the current signal generated by signal processing circuit 102 is called "control current".

[0016] Signal processing circuit 102 holds an upper clamping determination value and a lower clamping determination value for the control current. The upper clamping determination value is a threshold for determining that the voltage is clamped to the circuit voltage which is the upper limit value. The upper determination threshold value is set to, for example, 2.95V when the circuit voltage is 3V. The lower clamping determination value is a threshold for determining that the voltage is clamped to the circuit common voltage which is the lower limit value. The lower determination threshold value is set to, for example, 0.05V when the circuit common voltage is 0V. Signal processing circuit 102 determines normal operation when the voltage is between the upper clamping determination value and the lower clamping determination value. This upper clamping determination value and lower clamping determination value are an example of "determination voltage values".

[0017] Signal processing circuit 102 receives an input of an abnormality notification signal obtained by converting the output voltage of error amplifier Q2 into a digital signal from AD conversion circuit 120. Then, signal processing circuit 102 compares the voltage value indicated by the abnormality notification signal with the upper clamping determination value and the lower clamping determination value.

[0018] When the voltage value indicated by the abnormal notification signal is greater than the upper clamping determination value, the signal processing circuit 102 determines that the voltage is clamped to the circuit voltage that is the upper limit value. Then, the signal processing circuit 102 changes the PWM for the current signal and controls the current signal so that the current value is a value on the smaller side outside the normal range. For example, when the normal range is 4 mA to 20 mA, the signal processing circuit 102 controls the current value of the current signal to 3 mA. Thereby, the signal processing circuit 102 notifies the external circuit 10 of an abnormality.

[0019] Also, when the voltage value indicated by the abnormal notification signal is less than the lower clamping determination value, the signal processing circuit 102 determines that the voltage is clamped to the circuit common voltage that is the lower limit value. Then, the signal processing circuit 102 changes the PWM for the current signal and controls the current signal so that the current value is a value on the larger side outside the normal range. For example, when the normal range is 4 mA to 20 mA, the signal processing circuit 102 controls the current value of the current signal to 23 mA. Thereby, the signal processing circuit 102 notifies the external circuit 10 of an abnormality.

[0020] However, the signal processing circuit 102 may not use anything other than the PWM signal for the current signal as the switching control signal. For example, when the signal processing circuit 102 can output an analog voltage, the output of the signal processing circuit 102 may be directly connected to the line L3.

[0021] In this way, the signal processing circuit 102 inputs an input voltage for generating a current signal having a target current value to the error amplifier Q2, and compares the voltage value of the voltage output from the error amplifier Q2 with the upper clamping determination value and the lower clamping determination value which are the determination voltage values to detect a failure. In particular, the signal processing circuit 102 according to the present embodiment detects the occurrence of a failure based on the digital signal input from the AD conversion circuit 120. Further, when the signal processing circuit 102 detects a failure, it notifies the occurrence of the failure by controlling the input voltage to the error amplifier Q2 so that the current value of the current signal is a value outside the predetermined normal range, that is, outside the range of 4 mA to 20 mA in this embodiment.

[0022] Furthermore, the signal processing circuit 102 is connected to a reference voltage output unit 110. The signal processing circuit 102 outputs a predetermined electrical signal to the reference voltage output unit 110 in accordance with the electrical signal from the sensor 101. For example, the signal processing circuit 102 simply amplifies the electrical signal and outputs the resulting signal as a predetermined electrical signal to the reference voltage output unit 110.

[0023] Furthermore, the signal processing circuit 102 is connected to the comparator circuit 104. When the signal processing circuit 102 receives notification of an abnormality from the comparator circuit 104, it performs processing such as saving the current value of the electrical signal input from the sensor 101.

[0024] Switch SW1 has one fixed contact connected to the positive electrode of a reference voltage source P1 having a first output voltage. Switch SW1 also has the other fixed contact connected to the positive electrode of a reference voltage source P2 having a second output voltage. Furthermore, one end of line L3 is connected to the movable contact of switch SW1. Here, the voltage level of the first output voltage is denoted as VR1(V), and the voltage level of the second output voltage is denoted as VR2(V).

[0025] The movable contact of switch SW1 switches between reference voltage source P1, whose voltage level is controlled by VR1, and reference voltage source P2, whose voltage level is controlled by VR2, in response to changes in the voltage level of the PWM signal for the current signal. When the movable contact of switch SW1 switches, an electrical signal flows through line L3 connected to the movable contact of switch SW1, with the voltage level changing between VR1 and VR2.

[0026] The other end of line L3 is connected to the constant current circuit 106. The constant current circuit 106 determines the value of the current signal output to the external circuit 10 according to the electrical signal flowing through line L3, i.e., the electrical signal output from sensor 101. The constant current circuit 106 includes an LPF (Low Pass Filter) 160, a buffer amplifier Q1, resistors R3 to R6, an error amplifier Q2, transistors Q3 and Q4, a resistor R7, and a diode D2. Diode D2 can also be replaced with a resistor. The operation of the constant current circuit 106 is described in detail below.

[0027] The LPF160, which includes resistor R2 and capacitor C2, receives the electrical signal input through line L3. The LPF160 then smooths the input electrical signal to convert it into an analog signal. After that, the LPF160 outputs the converted analog signal to buffer amplifier Q1.

[0028] The buffer amplifier Q1 has its output terminal connected to resistor R4, which is connected to the error amplifier Q2. The buffer amplifier Q1 receives the input electrical signal, which has been converted to an analog signal, from the LPF160. The buffer amplifier Q1 then buffers the input electrical signal and outputs it from its output terminal to the error amplifier Q2 via resistor R4.

[0029] Here, the buffer amplifier Q1, resistor R2, and capacitor C2 do not need to be mounted on the two-wire transmitter 100 if the signal processing circuit 102 is configured to output PWM, for example, if it is configured to output DC voltage using a DA converter.

[0030] The error amplifier Q2 is, for example, an operational amplifier. However, it is also possible to use a method other than an operational amplifier for the error amplifier Q2. The voltage difference between the output voltage from the buffer amplifier Q1 and the feedback voltage generated across the feedback resistor R1 is divided by resistors R3 and R4 and feedback resistor R1, and input to the non-inverting input terminal of the error amplifier Q2. These resistors R3 and R4 are examples of "first and second resistors". In addition, the voltage of VR1(V) of the reference voltage source P1 is divided by resistors R5 and R6, and input to the inverting input terminal of the error amplifier Q2.

[0031] Error amplifier Q2 has its output terminal connected to the base of transistor Q3. Error amplifier Q2 controls the collector current of transistor Q3 by inputting the output voltage to the base of transistor Q3. More specifically, error amplifier Q2 detects the voltage error in the inputs to the non-inverting and inverting input terminals, and adjusts the output voltage so that they match, thereby controlling the current flowing through the circuit together with transistors Q3 and Q4.

[0032] In other words, the error amplifier Q2 detects the error between the voltage it outputs, divided by the feedback resistors R1, R3, and R4, and the voltage obtained by dividing the output voltage of the reference voltage source P1, divided by the resistors R5 and R6. The error amplifier Q2 then controls the current flowing through transistors Q3 and Q4 so that the input voltages match. Specifically, the first and second resistors, R3 and R4, play a role in adjusting the input voltage so that the current value of the current signal approaches the target current value, based on the voltage derived from the potential difference across the feedback resistor R1.

[0033] Furthermore, if resistor R7 or transistor Q4 fails, a difference will occur between the current signal actually flowing through transmission line L1 and the control current set by signal processing circuit 102, resulting in a potential difference between the voltage at the negative terminal and the voltage at the positive terminal of error amplifier Q2. When a potential difference occurs, because the gain of error amplifier Q2 is large, the voltage value output by error amplifier Q2 will be fixed at or near the circuit voltage (upper limit) or the circuit common voltage (lower limit). More specifically, if the current value of the current signal actually flowing through transmission line L1 is greater than the control current value, the voltage output by error amplifier Q2 will be fixed at or near the circuit common voltage. Conversely, if the current value of the current signal actually flowing through transmission line L1 is smaller than the control current value, the voltage output by error amplifier Q2 will be fixed at or near the circuit voltage.

[0034] Transistor Q3 is an npn type transistor. The collector of transistor Q3 is connected to the base of transistor Q4. The emitter of transistor Q3 is connected to diode D2. The voltage input to the base of transistor Q3 is adjusted by the error amplifier Q2, which changes the current flowing through it and thereby controls the base current of transistor Q4. This transistor Q3 is an example of a "second transistor".

[0035] Transistor Q4 is a pnp type transistor. The emitter and collector of transistor Q4 are connected to the transmission line L1. The emitter of transistor Q4 is connected to the output terminal of diode D1, through which the current output from the external circuit 10 flows. A resistor R7 is also connected between the emitter and collector of transistor Q4. Resistor R7 is the starting resistor.

[0036] Transistor Q4 draws current from the external circuit 10 via transmission line L1 to its collector, in accordance with the control of the base current by the collector of transistor Q3. This current drawn by transistor Q4 from the external circuit 10 corresponds to the electrical signal output by sensor 101, i.e., a current signal of 4mA to 20mA. This current signal is output to the detection resistor R of the external circuit 10 via transmission line L2, allowing the external circuit 10 to obtain the measurement result of a physical quantity based on sensor 101. Transistor Q4 is an example of a "first transistor".

[0037] In this way, the operational amplifier error amplifier Q2 generates a current signal flowing on the transmission line L1 by controlling the current flowing through the second transistor, transistor Q4, based on the input voltage. More specifically, the second transistor, transistor Q3, adjusts the current value of the current flowing through the first transistor, transistor Q4, by the current it flows.

[0038] The reference voltage output unit 110 is connected to the signal processing circuit 102 and the reference voltage processing circuit 112. The reference voltage output unit 110 receives a predetermined electrical signal input from the signal processing circuit 102. Then, according to the predetermined electrical signal input, the reference voltage output unit 110 outputs a reference voltage signal to the reference voltage processing circuit 103 for generating the reference voltage of the shunt regulator circuit 105. The reference voltage is the voltage that serves as the reference for constant voltage control by the shunt regulator circuit 105. In this embodiment, the reference voltage output unit 110 outputs a reference voltage PWM signal with a varied duty cycle as the reference voltage signal.

[0039] The reference voltage processing circuit 103 performs predetermined processing on the PWM signal for the reference voltage between the reference voltage output unit 110 and the shunt regulator circuit 105. The reference voltage processing circuit 103 includes an LPF 130, an error amplifier Q5, and resistors R10 and R11.

[0040] The LPF130 has a resistor R8 and a capacitor C1. The LPF130 receives the PWM signal for the reference voltage as input from the reference voltage output unit 110. The LPF130 then smooths the PWM signal for the reference voltage using the resistor R8 and capacitor C1 and converts it into an analog signal.

[0041] Error amplifier Q5 receives the signal obtained by converting the PWM signal for the reference voltage into an analog signal from LPF130. Error amplifier Q5 then performs negative feedback amplification using resistors R9 and R10 to amplify the input signal and generate the reference voltage. Error amplifier Q5 outputs the reference voltage to the shunt regulator circuit 105. Here, the reference voltage is referred to as Vref.

[0042] Here, the reference voltage processing circuit 103 does not need to be mounted on the two-wire transmitter 100 if the signal processing circuit 102 is configured to output PWM, for example, if it is configured to output DC voltage using a DA converter.

[0043] The shunt regulator circuit 105 performs constant voltage control to stabilize the circuit operation. In particular, the two-wire transmitter 100 dynamically controls the circuit voltage according to the output current signal. The shunt regulator circuit 105 determines the circuit voltage of the two-wire transmitter 100 according to Vref, which is a reference voltage output from the error amplifier Q5. This ensures that sufficient dissipable power is available within the circuit even when the current supplied from the external circuit 10 (4mA to 20mA) is small. The shunt regulator circuit 105 includes an error amplifier Q6, a transistor Q7, and resistors R11 and R12.

[0044] Error amplifier Q6 receives the reference voltage Vref, input from the reference voltage processing circuit 103, as its non-inverting input terminal. Error amplifier Q6 also receives the voltage obtained by dividing the circuit voltage by resistors R11 and R12 as its inverting input terminal. Error amplifier Q6 detects the voltage error between the inputs at the non-inverting and inverting input terminals and, together with transistor Q7, controls the circuit voltage so that the errors match.

[0045] Transistor Q7 is a P-channel MOSFET. The reference voltage output unit 110 outputs a PWM signal for reference voltage with a higher duty cycle the smaller the electrical signal output from the signal processing circuit 102, i.e., the smaller the electrical signal output from the sensor 101. This means that the smaller the current supplied from the external circuit 10, i.e., the current signal flowing through the transmission line L1, the higher the reference voltage Vref of the error amplifier Q6 becomes, and the more the gate voltage of transistor Q7 shifts to the positive side.

[0046] In other words, the current flowing through transistor Q7 decreases proportionally as the current signal flowing through transmission line L1 decreases. When the current flowing through it decreases, the voltage drop in the circuit voltage due to transistor Q7 is suppressed. As a result, the circuit voltage increases as the current signal flowing through transmission line L1 decreases. The voltage input to the inverting input terminal of error amplifier Q6 also increases, and the circuit voltage stabilizes when it finally equals the reference voltage Vref input to the non-inverting input terminal.

[0047] The negative feedback operation by the shunt regulator circuit 105 described above can be expressed by the following equation (1).

[0048] Circuit voltage = (1 + (resistance of resistor R11 / resistance of resistor R12)) × Vref···(1)

[0049] The comparator circuit 104 detects abnormal conditions in the circuit voltage. The comparator circuit 104 has a comparator Q8 that detects a drop in circuit voltage as an abnormal condition.

[0050] Comparator Q8 receives a voltage corresponding to the PWM signal for the reference voltage at its inverting input terminal. Additionally, comparator Q8 receives a voltage obtained by dividing the circuit voltage by resistors R13 and R14 at its non-inverting input terminal. Comparator Q8 compares the voltage input to the inverting input terminal with the voltage input to the non-inverting input terminal. If the voltage at the non-inverting input terminal drops, it notifies the signal processing circuit 102 of the abnormality by inverting its output.

[0051] In this case, if detection of an abnormality in the circuit voltage is not required, the comparator circuit 104 does not need to be mounted on the two-wire transmitter 100.

[0052] The AD conversion circuit 120 is connected to the output terminal of the error amplifier Q2 via the diagnostic line L10. The AD conversion circuit 120 receives the voltage output from the error amplifier Q2 via the diagnostic line L10. The AD conversion circuit 120 then converts the analog signal of the input voltage into a digital signal. After that, the AD conversion circuit 120 outputs the voltage output from the error amplifier Q2, which has been converted into a digital signal, to the signal processing circuit 102. In this way, the AD conversion circuit 120 converts the voltage output from the error amplifier Q2, which is an operational amplifier, into a digital signal.

[0053] (Fault detection) Next, the types of faults detectable by the two-wire transmitter 100 according to this embodiment will be described. The two-wire transmitter 100 detects faults based on changes in the current signal flowing through the transmission line L1. Regarding the two-wire transmitter 100, the points that affect the current signal flowing through the transmission line L1 are the feedback resistor R1, resistor R7, transistor Q4, transistor Q3, and diode D2. Regarding the external circuit 10, the points that affect the current signal flowing through the transmission line L1 are the power supply voltage Eb and the detection resistor R. Therefore, the possibility of fault detection for each of these will be explained below.

[0054] Let's explain what happens when a fault occurs in resistor R7. If a fault occurs in resistor R7 that causes its resistance to decrease, more current will flow through the transmission line L1 than the control current. In that case, the voltage output from the error amplifier Q2 will stick to or be close to the lower limit of the circuit common voltage. Therefore, the signal processing circuit 102 can confirm that the voltage value of the current signal actually flowing through the transmission line L1 is smaller than the lower limit sticking judgment value of the control current, and can detect the occurrence of a fault and notify the external circuit 10 of the fault.

[0055] Furthermore, if a short circuit occurs in resistor R7, a current greater than the control current will flow through the transmission line L1. In this case, the voltage output from error amplifier Q2 will stick to or be close to the lower limit of the circuit common voltage. Therefore, the signal processing circuit 102 can detect the occurrence of a fault because it can confirm that the voltage value of the current signal actually flowing through the transmission line L1 is smaller than the lower limit sticking judgment value of the control current.

[0056] Furthermore, if a fault occurs in resistor R7 that increases its resistance or causes it to become open, the current flowing through resistor R7 decreases. In that case, the signal processing circuit 102 can increase the current flowing through transistor Q4 to match the current signal flowing through transmission line L1 to the control current. Therefore, the signal processing circuit 102 can notify the external circuit 10 of the correct physical quantity.

[0057] Next, we will explain what happens when a failure occurs in transistor Q4. If a failure occurs in transistor Q4 that causes more than the control current to flow through the transmission line L1, the current signal flowing through the transmission line L1 will be more than the control current. In that case, the voltage output from the error amplifier Q2 will stick to the lower limit, the circuit common voltage, or be close to the circuit common voltage. Therefore, the signal processing circuit 102 can confirm that the voltage value of the current signal actually flowing through the transmission line L1 is smaller than the lower limit sticking judgment value of the control current, and can detect the occurrence of a failure and notify the external circuit 10 of the failure.

[0058] Furthermore, if a fault occurs in transistor Q4 where the current flowing through transmission line L1 is less than the control current, the current signal flowing through transmission line L1 will be less than the control current. In that case, the voltage output from error amplifier Q2 will stick to or be close to the circuit voltage, which is the upper limit. Therefore, the signal processing circuit 102 can confirm that the voltage value of the current signal actually flowing through transmission line L1 is greater than the upper limit sticking judgment value of the control current, and can detect the occurrence of a fault and notify the external circuit 10 of the fault.

[0059] Next, we will explain what happens when transistor Q3 fails. In the case of transistor Q3, there are two possible failures: one in which the current flowing through the transmission line L1 exceeds the control current, and another in which the current flowing is less than the control current. In both cases, the signal processing circuit 102 operates in the same way as transistor Q4, and can detect the occurrence of the failure and notify the external circuit 10 of the failure.

[0060] Furthermore, if a fault occurs in which the feedback resistor R1 becomes open, the current signal flowing through the transmission line L1 will be less than the control current. In that case, the voltage output from the error amplifier Q2 will stick to or be close to the circuit voltage, which is the upper limit. Therefore, the signal processing circuit 102 can confirm that the voltage value of the current signal actually flowing through the transmission line L1 is greater than the upper limit sticking judgment value of the control current, and can detect the occurrence of a fault and notify the external circuit 10 of the fault.

[0061] Furthermore, if a short circuit occurs in the feedback resistor R1, a current greater than the control current will flow through the transmission line L1. In this case, the error amplifier Q2 attempts to increase the output current, so the output voltage will stick to or be close to the circuit voltage, which is its upper limit. Therefore, the signal processing circuit 102 can confirm that the voltage value of the current signal actually flowing through the transmission line L1 is greater than the upper limit sticking judgment value of the control current, and thus can detect the occurrence of a fault.

[0062] Next, we will explain what happens when a fault occurs in the power supply voltage Eb. When a fault occurs that increases the power supply voltage Eb, the current signal flowing through the transmission line L1, determined by the power supply voltage Eb, resistor R7, and feedback resistor R1, will be greater than the control current. In this case, the voltage output from the error amplifier Q2 will stick to or be close to the lower limit of the circuit common voltage. Therefore, the signal processing circuit 102 can confirm that the voltage value of the current signal actually flowing through the transmission line L1 is smaller than the lower limit sticking judgment value of the control current, and can detect the occurrence of a fault and notify the external circuit 10 of the fault.

[0063] Furthermore, if a fault occurs that causes the power supply voltage Eb to drop, the collector-emitter voltage of transistor Q4 decreases as the power supply voltage Eb drops, and the current signal flowing through transmission line L1 becomes smaller than the control current. In this case, the voltage output from error amplifier Q2 will stick to or be close to the circuit voltage, which is its upper limit. Therefore, the signal processing circuit 102 can confirm that the voltage value of the current signal actually flowing through transmission line L1 is greater than the lower limit sticking judgment value of the control current, and thus can detect the occurrence of a fault and notify the external circuit 10 of the fault.

[0064] Next, we will explain what happens when a fault occurs in the detection resistor R. In the case of the detection resistor R, as with the power supply voltage Eb, there are two possibilities: either the current flowing through the transmission line L1 is greater than or equal to the control current, or it is less than or equal to the control current. In both cases, the signal processing circuit 102 operates in the same way as in the case of the power supply voltage Eb, and can detect the occurrence of a fault and notify the external circuit 10 of the fault.

[0065] (Examples) Next, fault detection of the two-wire transmitter 100 according to this embodiment will be explained using specific numerical values. Here, the relevant parameters are set as follows: The gain of the error amplifier Q2 is 1000 times. The resistance value of the feedback resistor R1 is 100 Ω. The resistance values ​​of resistors R3 to R6 are all 100 kΩ. The voltage of the reference voltage source P1 is 0.2 V. The indicator voltage, which is the voltage output from the error amplifier Q2 according to the physical quantity obtained from the sensor 101, is 1.4 V.

[0066] As described above, the error amplifier Q2 detects the error between the voltage obtained by dividing its own output voltage by the feedback resistors R1, R3, and R4, and the voltage obtained by dividing the output voltage of the reference voltage source P1 by the resistors R5 and R6. Based on the detected error, the error amplifier Q2 controls the current flowing through transistors Q3 and Q4 so that the two input voltages match. Therefore, the voltage at the inverting input terminal and the voltage at the non-inverting input terminal of the error amplifier Q2 become the same.

[0067] Using the parameters described above, the current value of the current signal is calculated when no fault occurs. The voltage at the inverting input terminal of the error amplifier Q2 is obtained by the following equation (2).

[0068] The voltage at the inverting input terminal of error amplifier Q2 = Voltage of reference voltage source P1 × Resistance of resistor R6 / (Resistance of resistor R6 + Resistance of resistor R7) ... (2)

[0069] In other words, with the parameters described above, the voltage at the inverting input terminal of the error amplifier Q2 will be 0.1V.

[0070] Furthermore, the feedback voltage generated across the feedback resistor R1 can be calculated using the following formula (3).

[0071] The feedback voltage across the feedback resistor R1 = negative terminal voltage of error amplifier Q2 - ((indicating voltage - voltage at the inverting input terminal of error amplifier Q2) / resistance value of resistor R3) × resistance value of resistor R4 ... (3)

[0072] In other words, with the parameters described above, the feedback voltage across the feedback resistor R1 is -1.2V.

[0073] The current value of the current signal can then be determined by the following equation (4).

[0074] Current value of current signal = Feedback voltage across feedback resistor R1 / Resistance of resistor R3 ... (4)

[0075] In other words, with the parameters described above, the current value of the current signal will be 12mA.

[0076] Next, we will explain the case where a predetermined fault occurs and the current signal changes to 13mA. In this case, the feedback voltage across the feedback resistor R1 becomes -1.3V. The voltage at the non-inverting input terminal of the error amplifier Q2 can then be calculated using the following formula (5).

[0077] The voltage at the non-inverting input terminal of error amplifier Q2 = Feedback voltage across feedback resistor R1 + (Indicator voltage - Feedback voltage across feedback resistor R1) × Resistance of resistor R4 / (Resistance of resistor R3 / Resistance of resistor R4) ... (5)

[0078] In other words, with the parameters described above, the voltage at the non-inverting input terminal of error amplifier Q2 is 0.05V. Here, the output voltage of error amplifier Q2 is calculated as the voltage at the non-inverting input terminal of error amplifier Q2 minus the voltage at the inverting input terminal of error amplifier Q2 = -50V, but in reality it is the lower limit of the circuit common voltage.

[0079] Thus, when a current greater than the set current flows, the output voltage of the error amplifier Q2 sticks to the lower limit, the circuit common voltage, or becomes close to the circuit common voltage. Therefore, the signal processing circuit 102 can detect the abnormality of the increasing current by monitoring the output voltage of the error amplifier Q2. The signal processing circuit 102 can then notify the external circuit 10 of the abnormality by setting the current value of the current signal to a larger value outside the normal range. Here, we have explained the case where the current value of the current signal is higher than the control current, but even when the current value of the current signal is lower than the control current, the two-wire transmitter 100 performs the same operation, although the polarity is reversed.

[0080] (Fault detection process flow) Figure 2 is a flowchart of the fault detection process using the two-wire transmitter according to the first embodiment. Next, the flow of the fault detection process using the two-wire transmitter 100 according to this embodiment will be explained with reference to Figure 2.

[0081] A predetermined fault occurs in the two-wire transmitter 100, such as a decrease in the resistance value of resistor R7 or a failure in which more than the control current flows through transistor Q4 (step S1).

[0082] A difference occurs between the current signal flowing through transmission line L1 and the control current, resulting in a potential difference between the voltage at the inverting input terminal and the voltage at the non-inverting input terminal of error amplifier Q2 (step S2).

[0083] The voltage output from the error amplifier Q2 is fixed at or near the upper limit circuit voltage or the lower limit circuit common voltage (step S3).

[0084] The signal processing circuit 102 receives the input of the voltage output from the error amplifier Q2, which has been converted into a digital signal by the AD conversion circuit 120 (step S4).

[0085] Next, the signal processing circuit 102 determines whether the voltage value output from the error amplifier Q2 is greater than the upper limit sticking determination value (step S5). Specifically, if the voltage value output from the error amplifier Q2 is near the circuit voltage which is the upper limit, the signal processing circuit 102 determines that the voltage value output from the error amplifier Q2 is greater than the upper limit sticking determination value. In other words, the signal processing circuit 102 determines that the voltage value output from the error amplifier Q2 is stuck to the circuit voltage.

[0086] If the voltage value output from the error amplifier Q2 is greater than the upper limit setting judgment value (step S5: affirmative), the signal processing circuit 102 controls the current value of the current signal to a smaller value outside the normal range (step S6). As a result, the signal processing circuit 102 notifies the external circuit 10 of the fault.

[0087] If the voltage value output from the error amplifier Q2 is less than or equal to the upper limit sticking judgment value (step S5: negative), the signal processing circuit 102 determines whether the voltage value output from the error amplifier Q2 is less than the lower limit sticking judgment value (step S7). Specifically, if the voltage value output from the error amplifier Q2 is near the lower limit, which is the circuit common voltage, the signal processing circuit 102 determines that the voltage value output from the error amplifier Q2 is less than the lower limit sticking judgment value. In other words, the signal processing circuit 102 determines that the voltage value output from the error amplifier Q2 is stuck to the circuit common voltage.

[0088] If the voltage value output from the error amplifier Q2 is smaller than the lower limit setting judgment value (step S7: affirmative), the signal processing circuit 102 controls the current value of the current signal to a value on the larger side of the normal range (step S8). As a result, the signal processing circuit 102 notifies the external circuit 10 of the fault.

[0089] Here, the determination based on the upper or lower limit of the voltage output from the error amplifier Q2 is an example of "comparing the voltage value output from the error amplifier with the determination voltage value."

[0090] In contrast, if the voltage value output from the error amplifier Q2 is greater than or equal to the lower limit setting judgment value (step S7: negative), the signal processing circuit 102 controls the current signal so that it becomes the target current value (step S9). In this case, the operation of the signal processing circuit 102 is normal operation.

[0091] (effect) As described above, the two-wire transmitter 100 according to this embodiment monitors changes in the voltage output from the error amplifier Q2 and detects when a predetermined fault occurs and the voltage value output from the error amplifier Q2 becomes stuck at the upper or lower limit. As a result, the two-wire transmitter 100 according to this embodiment detects the fault and provides notification.

[0092] This allows the external circuit 10 to transmit an abnormality to the two-wire transmitter 100 if an internal fault in the circuit or insufficient power supply voltage causes the current signal to deviate from the set value. This improves the reliability of the two-wire transmitter 100 and contributes to the stable operation of the plant.

[0093] Here, a simple method for detecting an anomaly where the current signal differs from the set due to an internal circuit fault or insufficient power supply voltage is to directly measure the current flowing through the feedback resistor R1. However, the two-wire transmitter 100 according to this embodiment can be realized with a simpler circuit configuration than the method of directly measuring the current flowing through the feedback resistor R1. Therefore, the two-wire transmitter 100 according to this embodiment has advantages such as reduced component costs, low current consumption, and reduced difficulty in board wiring design. For example, since the current flowing into the two-wire transmitter 100 is small, low current consumption is required, and the two-wire transmitter 100 according to this embodiment can meet such requirements.

[0094] (modified version) Next, a modified example of the two-wire transmitter 100 according to the first embodiment will be described. The signal processing circuit 102 in this modified example has an AD conversion circuit. In this modified example, the AD conversion circuit 120 does not need to be mounted on the two-wire transmitter 100.

[0095] The signal processing circuit 102 is connected to the output terminal of the error amplifier Q2 via the diagnostic line L10. The signal processing circuit 102 receives the voltage output from the error amplifier Q2 as input. The signal processing circuit 102 then uses its own AD conversion circuit to convert the analog signal of the input voltage into a digital signal.

[0096] Subsequently, the signal processing circuit 102 determines whether the voltage value output from the error amplifier Q2 is greater than the upper limit threshold or less than the lower limit threshold, and controls the current value of the current signal to a value outside the normal range based on the determination result, thereby notifying the external circuit 10 of the fault. In this way, the signal processing circuit 102 according to this embodiment converts the voltage output from the error amplifier Q2, which is an operational amplifier, into a digital signal, and detects a fault based on the converted digital signal.

[0097] (effect) As explained above, it is also possible to use a signal processing circuit 102 that has an AD conversion circuit. In this case, the AD conversion circuit 120 can be omitted, thus reducing the component mounting area and component cost.

[0098] (Second Embodiment) Figure 3 is a circuit diagram of a two-wire transmitter according to the second embodiment. In this embodiment, the two-wire transmitter 100 uses comparators 121 and 122 instead of the AD conversion circuit 120 in the first embodiment to notify the signal processing circuit 102 of the voltage value information of the voltage output from the error amplifier Q2. In the following description, the operation of each part, which is the same as in the first embodiment, will be omitted.

[0099] Comparator 121 has its non-inverting input terminal connected to the connection path between error amplifier Q2 and transistor Q3 via diagnostic line L11. Furthermore, comparator 121 has its inverting input terminal connected to the connection point between resistor R21 and resistor R22, which are among the resistors R21-R23 connected in series between the input and output of reference voltage source P3. Finally, comparator 121's output terminal is connected to signal processing circuit 102.

[0100] Comparator 121 receives the voltage output by error amplifier Q2 as input. Comparator 121 then compares the input voltage value with the voltage value obtained by dividing the voltage of the reference voltage source P3 by resistors R21, R22, and R23. Comparator 121 outputs 1 if the input voltage value is greater than or equal to the divided voltage value of the reference voltage source P3. Comparator 121 outputs 0 if the input voltage value is less than the divided voltage value of the reference voltage source P3.

[0101] In other words, if the input voltage is greater than or equal to the divided voltage value of the reference voltage source P3, the comparator 121 determines that the voltage output by the error amplifier Q2 is stuck at the upper limit of the circuit voltage and outputs 1. Also, if the input voltage is less than the divided voltage value of the reference voltage source P3, the comparator 121 determines that the voltage output by the error amplifier Q2 is stuck at the normal range or the lower limit of the circuit common voltage and outputs 0. This comparator 121 is an example of a "first comparator". The divided voltage value of the reference voltage source P3 is an example of an "upper limit threshold". The comparator 121 determines whether the voltage output from the operational amplifier error amplifier Q2 is greater than or equal to the upper limit threshold.

[0102] Comparator 122 has an inverting input terminal connected between the error amplifier Q2 and transistor Q3 and the connection path via diagnostic line L11. Comparator 122 also has a non-inverting input terminal connected to the connection point between resistors R22 and R23, which are part of the resistors R21-R23 connected in series between the input and output of the reference voltage source P3. Furthermore, the output terminal of comparator 122 is connected to the signal processing circuit 102.

[0103] Comparator 122 receives the voltage output by error amplifier Q2 as input. Comparator 122 then compares the input voltage value with the voltage value obtained by dividing the voltage of the reference voltage source P3 by resistors R21 and R22 and resistor R23. Comparator 122 outputs 0 if the input voltage value is greater than or equal to the divided voltage value of the reference voltage source P3. Comparator 122 outputs 1 if the input voltage value is less than the divided voltage value of the reference voltage source P3.

[0104] In other words, if the input voltage is greater than or equal to the divided voltage value of the reference voltage source P3, the comparator 122 determines that the voltage output by the error amplifier Q2 is stuck at the normal range or the upper limit of the circuit voltage and outputs 0. If the input voltage is less than the divided voltage value of the reference voltage source P3, the comparator 122 determines that the voltage output by the error amplifier Q2 is stuck at the lower limit of the circuit common voltage and outputs 1. This comparator 122 is an example of a "second comparator". The divided voltage value of the reference voltage source P3 is an example of a "lower threshold". The comparator 122 determines whether the voltage output from the operational amplifier error amplifier Q2 is less than the lower threshold.

[0105] In this embodiment, both comparators 121 and 122 use logic that outputs 1 in the event of an abnormality. However, any logic other than the logic that outputs 1 in the event of an abnormality may be used, as long as the configuration can notify of an abnormality when a sticking to the circuit voltage or circuit common voltage is detected.

[0106] The signal processing circuit 102 receives the output signals from comparators 121 and 122, respectively. When the output signal from comparator 121 is 1, the signal processing circuit 102 controls the current value of the current signal to a smaller value outside the normal range. When the output signal from comparator 122 is 1, the signal processing circuit 102 controls the current value of the current signal to a larger value outside the normal range. As a result, the signal processing circuit 102 notifies the external circuit 10 of the fault.

[0107] (effect) As described above, the two-wire transmitter 100 according to this embodiment uses comparators 121 and 122 instead of the AD conversion circuit 120 to detect faults from the voltage information output by the error amplifier Q2. Based on the fault detection results from comparators 121 and 122, the two-wire transmitter 100 causes the signal processing circuit 102 to adjust the current value of the current signal to notify of the fault. This simplifies the complex processing of the AD conversion circuit 120, enables fault detection with a simpler configuration, and makes it possible to reduce component costs and the difficulty of board wiring design.

[0108] (Third embodiment) Figure 4 is a circuit diagram of a two-wire transmitter according to the third embodiment. The two-wire transmitter 100 according to this embodiment does not have an AD conversion circuit 120, nor does the signal processing circuit 102 have an AD conversion circuit. With this configuration, the two-wire transmitter 100 according to this embodiment detects when the voltage output from the error amplifier Q2 sticks to the circuit voltage, which is the upper limit of the voltage value. In the following description, the operation of each part, which is the same as in the first embodiment, will be omitted.

[0109] The path connecting the output terminal of the error amplifier Q2 and the base of the transistor Q3 is connected to the transmission line L2 via resistors R21 and R22, which are arranged in series.

[0110] The signal processing circuit 102 is connected via the diagnostic line L10 to the connection point between resistors R31 and R32. The signal processing circuit 102 receives the voltage value obtained by dividing the voltage output from the error amplifier Q2 by resistors R31 and R32 as an analog signal via the diagnostic line L10.

[0111] The signal processing circuit 102 has a voltage threshold set in advance for detecting when the circuit voltage is stuck to its upper limit. This voltage threshold may be the same as the upper limit sticking threshold in the first embodiment. The signal processing circuit 102 then uses the voltage input from the diagnostic line L10 as an analog signal to determine whether the voltage value exceeds the voltage threshold. If the voltage value of the voltage input from the diagnostic line L10 exceeds the voltage threshold, the signal processing circuit 102 determines that the voltage value of the voltage output from the error amplifier Q2 has stuck to the circuit voltage, which is its upper limit, and controls the current value of the current signal to a smaller value outside the normal range. As a result, the signal processing circuit 102 notifies the external circuit 10 of the fault.

[0112] As described above, the signal processing circuit 102 according to this embodiment receives an input of an analog signal, which is a voltage, output from the error amplifier Q2, which is an operational amplifier, and compares the voltage value of the acquired analog signal with a predetermined threshold voltage value. The signal processing circuit 102 then detects a fault if the voltage value of the voltage is equal to or greater than the threshold voltage value.

[0113] (effect) As described above, the two-wire transmitter 100 according to this embodiment can detect and notify of faults without using an AD conversion function. This reduces the component mounting area and component costs.

[0114] (Fourth Embodiment) Figure 5 is a circuit diagram of a two-wire transmitter according to the fourth embodiment. In this embodiment, the two-wire transmitter 100 notifies the external circuit 10 of a fault by shutting down the power supply to the two-wire transmitter 100 via the shutdown circuit 201 and stopping the operation of the two-wire transmitter 100. In the following description, the operation of each part, which is the same as in the second embodiment, will be omitted. The two-wire transmitter 100 according to this embodiment has a shutdown circuit 201 and a switch 202.

[0115] The signal processing circuit 102 compares the voltage value output from the error amplifier Q2 with the upper limit and lower limit determination values ​​to determine whether a fault has occurred. If it determines that a fault has occurred, the signal processing circuit 102 sends a notification of the fault to the shutdown circuit 201.

[0116] Switch 202 is a switch for interrupting the power supply to the signal processing circuit 102, the constant current circuit 106, etc. In this embodiment, switch 202 is placed on the transmission line L1 and interrupts the power supply to the signal processing circuit 102, the constant current circuit 106, etc. by disconnecting the transmission line L1.

[0117] The shutdown circuit 201 is connected to the signal processing circuit 102 via the communication line L20. The shutdown circuit 201 is also connected to the switch 202. Furthermore, the shutdown circuit 201 operates by receiving power from a separate power source from the signal processing circuit 102, the constant current circuit 106, etc.

[0118] The shutdown circuit 201 receives a notification of a fault from the signal processing circuit 102. Upon receiving the notification of the fault, the shutdown circuit 201 operates the switch 202 to disconnect the transmission line L1. This causes the two-wire transmitter 100 to stop transmitting signals indicating physical quantities. The shutdown circuit 201 has a latching function and maintains the switch 202 in the disconnected state even after the signal processing circuit 102 has stopped operating, thus continuing to cut off the power supply from the power source. When the two-wire transmitter 100 stops transmitting signals indicating physical quantities, the external circuit 10 determines that the signal current value has become 0 and detects a fault in the two-wire transmitter 100.

[0119] In this manner, the signal processing circuit 102 notifies the shutdown circuit 201 of the fault detection when it detects a fault. The shutdown circuit 201, upon receiving the fault detection notification from the signal processing circuit 102, notifies the signal processing circuit 102 of the fault by cutting off the power supply to the signal processing circuit 102 and stopping its operation.

[0120] (effect) As described above, the two-wire transmitter 100 according to this embodiment notifies of a fault by using the shutdown circuit 201 to stop the power supply to the signal processing circuit 102, constant current circuit 106, etc., thereby stopping the operation of transmitting a signal indicating a physical quantity. By stopping the operation of the two-wire transmitter 100 in the event of a fault, it is possible to reduce power consumption.

[0121] (Fifth embodiment) Figure 6 is a circuit diagram of a two-wire transmitter according to the fifth embodiment. In this embodiment, the two-wire transmitter 100 adds transistor Q9 in parallel with transistor Q3 and diode D2 to notify of a failure even when transistor Q3 or diode D2 fails. In the following description, the operation of each part, which is the same as in the second embodiment, will be omitted. The two-wire transmitter 100 according to this embodiment has transistor Q9 and resistor R41.

[0122] Transistor Q9 is connected to the collector of transistor Q3 via a path branched off from the path connecting the collector of transistor Q3 and the base of transistor Q4. Furthermore, the base of transistor Q9 is connected to the signal processing circuit 102 via the diagnostic line L30. The emitter of transistor Q9 is also connected to resistor R41.

[0123] This transistor Q9 is an example of a "third transistor." Transistor Q9 is arranged in parallel with the second transistor, transistor Q3, and adjusts the current value of the first transistor, transistor Q4, by the current it supplies.

[0124] One end of resistor R41 is connected to the emitter of transistor Q9. The other end of resistor R41 is connected to transmission line L2.

[0125] The signal processing circuit 102 performs the following operations. For example, if a failure occurs in which the control of transistor Q3 is not effective and the amount of current flowing through it decreases, the voltage output from error amplifier Q2 will stick to the circuit voltage, which is the upper limit, or be close to the circuit voltage. In this case, the signal processing circuit 102 determines that the voltage value output from error amplifier Q2 is greater than the upper limit sticking judgment value. In this case, the signal processing circuit 102 controls the current value of the current signal to a smaller value outside the normal range by reducing the current flowing through transistor Q9, and notifies the external circuit 10 of the failure.

[0126] Similarly, if diode D2 fails and current stops flowing, the signal processing circuit 102 can also notify the external circuit 10 of the failure.

[0127] Furthermore, if, for example, a failure occurs in which the control of transistor Q3 fails and the amount of current flowing through it increases, the voltage output from error amplifier Q2 will stick to the lower limit, the circuit common voltage, or be close to the circuit common voltage. In this case, the signal processing circuit 102 determines that the voltage value output from error amplifier Q2 is smaller than the lower limit sticking judgment value. In this case as well, the signal processing circuit 102 increases the current flowing through transistor Q9, thereby controlling the current value of the current signal to a value on the larger side outside the normal range, and notifies the external circuit 10 of the failure.

[0128] Thus, when the signal processing circuit 102 detects a fault in the second transistor, transistor Q3, based on the voltage value output from the operational amplifier, error amplifier Q2, it operates as follows: The signal processing circuit 102 controls the input voltage to the error amplifier Q2 by controlling the current flowing through the third transistor, transistor Q9, so that the current value of the current signal becomes a predetermined value, thereby notifying the circuit of the fault.

[0129] (effect) As described above, the two-wire transmitter 100 according to this embodiment can notify the external circuit 10 of a fault by placing the current signal outside the normal range even if transistor Q3 or diode D2 fails. This makes it possible to improve the coverage of fault detection.

[0130] In the above explanation, fault detection and notification were described using the two-wire transmitter 100 as an example. However, fault detection and notification can be performed in a similar configuration for other devices that use operational amplifiers to configure a constant current circuit, such as the constant current circuit 106.

[0131] A device that uses an operational amplifier to construct a constant current circuit preferably has, at a minimum, an error amplifier Q2, transistors Q4 and Q3, feedback resistors R1, R3 and R4. With a configuration including these components, it is possible to control the current. If the voltage information output from the error amplifier Q2 is input to the signal processing circuit 102, the signal processing circuit 102 can detect and notify of a fault in the device. For example, if the signal processing circuit 102 can output an analog voltage, it is also possible to operate by inputting the output of the signal processing circuit 102 to resistor R3.

[0132] Furthermore, the two-wire transmitter 100 described in each embodiment can be modified as follows. For example, resistor R7 can be replaced with an active circuit. Also, resistors R6 and R7 are used as bias for connecting to the transmission line L2, and can be connected directly to ground. In addition, the buffer amplifier Q1 and LPF160 convert the PWM to an analog voltage, and can be replaced with a DA converter.

[0133] Furthermore, the processing procedures, control procedures, specific names, and various data and parameters shown in the above documents and drawings may be changed at will unless otherwise specified.

[0134] Furthermore, the components of each illustrated device are functionally conceptual and do not necessarily need to be physically configured as shown. In other words, the specific forms of distribution and integration of each device are not limited to those shown. That is, all or part of them can be functionally or physically distributed and integrated in any unit according to various loads and usage conditions.

[0135] Some examples of the combinations of technical features that will be disclosed are listed below.

[0136] (1) An error amplifier that generates a current signal which is a constant current, A signal processing circuit inputs an input voltage to the error amplifier to generate the current signal having a target current value, and detects a fault by comparing the voltage value of the voltage output from the error amplifier with a determination voltage value. A constant current device characterized by being equipped with (2) The constant current device according to (1), characterized in that, when a fault is detected, the signal processing circuit controls the input voltage so that the current value of the current signal falls outside a predetermined normal range, thereby notifying the occurrence of a fault. (3) A first transistor arranged on the transmission line of the current signal, A second transistor adjusts the current value of the current flowing through the first transistor by the current it flows through, A feedback resistor placed on the transmission line, Based on the voltage derived from the potential difference across the feedback resistor, a first resistor and a second resistor adjust the input voltage so that the current value of the current signal approaches the target current value. Furthermore, The error amplifier generates the current signal flowing on the transmission line by controlling the current flowing through the second transistor based on the input voltage. A constant current device according to (1) or (2), characterized in that it is a constant current device according to (1) or (2). (4) The system further includes an AD conversion circuit that converts the voltage output from the error amplifier into a digital signal. The signal processing circuit detects the occurrence of a fault based on the digital signal input from the AD conversion circuit. A constant current device according to any one of (1) to (3), characterized by the above. (5) The constant current device according to any one of (1) to (3), characterized in that the signal processing circuit converts the voltage output from the error amplifier into a digital signal and detects a fault based on the converted digital signal. (6) A first comparator that determines whether the voltage output from the error amplifier is above an upper threshold, The system further comprises a second comparator that determines whether the voltage output from the error amplifier is less than a lower threshold, The signal processing circuit detects a fault based on the determination results of the first comparator and the second comparator. A constant current device according to any one of (1) to (3), characterized by the above. (7) The constant current device according to any one of (1) to (3), characterized in that the signal processing circuit receives an input of an analog signal voltage output from the error amplifier, compares the voltage value of the acquired analog signal voltage with a predetermined threshold voltage value, and detects a fault if the voltage value of the voltage is equal to or greater than the threshold voltage value. (8) The system further includes a shutdown circuit that, upon receiving notification of a fault detection from the signal processing circuit, cuts off the power supply to the signal processing circuit and stops its operation, thereby notifying of the fault. The signal processing circuit, upon detecting a fault, notifies the shutdown circuit of the fault detection. A constant current device according to any one of (1) to (7), characterized by the above. (9) The system further comprises a third transistor arranged in parallel with the second transistor, which adjusts the current value of the current flowing through the first transistor by the current it flows through itself. When the signal processing circuit detects a fault based on the voltage value output from the error amplifier due to a failure of the second transistor, it controls the input voltage by controlling the current flowing through the third transistor so that the current value of the current signal becomes a predetermined value, thereby notifying the circuit of the fault. The constant current device according to (3), characterized in that (10) A fault detection method for a constant current circuit equipped with an error amplifier that generates a current signal which is a constant current, The signal processing circuit, The process involves inputting an input voltage to the error amplifier to generate the current signal having a target current value, and then comparing the voltage value of the voltage output from the error amplifier with a determination voltage value to detect a fault. A fault detection method characterized by the following features. [Explanation of Symbols]

[0137] 10 External circuit 100 2-wire transmitter 101 Sensor 102 Signal Processing Circuit 103 Reference voltage processing circuit 104 Comparator Circuit 105 Shunt Regulator Circuit 106 Constant current circuit 130,160 LPF 110 Reference voltage output section 120 AD conversion circuit 121,122 Comparators 201 Shutdown Circuit 202 Switch C1, C2 Capacitors D1, D2 diodes Eb Power supply voltage L1, L2 transmission lines L3 Line L10, L11, L30 diagnostic lines L20 communication line P1, P2, P3 Reference voltage sources Q1 Buffer amplifier Q2,Q5,Q6 error amplifier Q3, Q4, Q7, Q9 Transistors Q8 Comparator R is the sensing resistor. R1 Feedback resistor R2,R3,R4,R5,R6,R7,R8,R9,R10,R11,R12,R13,R14,R41 Resistor SW1 Switch

Claims

1. An error amplifier that generates a current signal which is a constant current, A signal processing circuit inputs an input voltage to the error amplifier to generate the current signal having a target current value, and detects a fault by comparing the voltage value of the voltage output from the error amplifier with a determination voltage value. A constant current device characterized by being equipped with

2. The constant current device according to claim 1, characterized in that, when the signal processing circuit detects a fault, it controls the input voltage so that the current value of the current signal falls outside a predetermined normal range, thereby notifying the occurrence of a fault.

3. A first transistor arranged on the current signal transmission line, A second transistor adjusts the current value of the current flowed by the first transistor by the current it flows, A feedback resistor placed on the transmission line, Based on the voltage derived from the potential difference across the feedback resistor, a first resistor and a second resistor adjust the input voltage so that the current value of the current signal approaches the target current value. Furthermore, The error amplifier generates the current signal flowing on the transmission line by controlling the current flowing through the second transistor based on the input voltage. The constant current device according to feature 1.

4. The system further includes an AD conversion circuit that converts the voltage output from the error amplifier into a digital signal. The signal processing circuit detects the occurrence of a fault based on the digital signal input from the AD conversion circuit. The constant current device according to feature 1.

5. The constant current device according to claim 1, characterized in that the signal processing circuit converts the voltage output from the error amplifier into a digital signal and detects a fault based on the converted digital signal.

6. A first comparator that determines whether the voltage output from the error amplifier is above an upper threshold, The system further comprises a second comparator that determines whether the voltage output from the error amplifier is less than a lower threshold, The signal processing circuit detects a fault based on the determination results of the first comparator and the second comparator. The constant current device according to feature 1.

7. The constant current device according to claim 1, characterized in that the signal processing circuit receives an input of an analog signal voltage output from the error amplifier, compares the voltage value of the acquired analog signal voltage with a predetermined threshold voltage value, and detects a fault if the voltage value of the voltage is equal to or greater than the threshold voltage value.

8. The system further includes a shutdown circuit that, upon receiving notification of a fault detection from the signal processing circuit, cuts off the power supply to the signal processing circuit and stops its operation, thereby notifying of the fault. The signal processing circuit, upon detecting a fault, notifies the shutdown circuit of the fault detection. The constant current device according to feature 1.

9. The system further comprises a third transistor arranged in parallel with the second transistor, which adjusts the current value of the current flowing through the first transistor by the current it flows through itself. When the signal processing circuit detects a fault based on the voltage value of the voltage output from the error amplifier due to a failure of the second transistor, it controls the current flowing through the third transistor to control the input voltage so that the current value of the current signal becomes a predetermined value, thereby notifying the circuit of the fault. The constant current device according to feature 3.

10. A fault detection method for a constant current circuit equipped with an error amplifier that generates a current signal which is a constant current, The signal processing circuit, The process involves inputting an input voltage to the error amplifier to generate the current signal having a target current value, and then comparing the voltage value of the voltage output from the error amplifier with a determination voltage value to detect a fault. A fault detection method characterized by the following features.

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