High-precision pwm conversion 4-20ma current output circuit

Abstract

The application provides a high-precision PWM conversion 4-20mA current output circuit, which comprises: sequentially connected input-output isolation sub-circuit, switching sub-circuit, integration sub-circuit and amplification output sub-circuit; wherein the resistance in each sub-circuit is the optimal resistance value obtained through multiple test experiments, and the amplifier used in the integration sub-circuit and the amplification output sub-circuit is an LF444CMX operational amplifier. The circuit provided by the application selects a specific resistance value for the resistance in the circuit and selects a specific model for the amplifier in the circuit, so that the purpose of high-precision conversion of a PWM signal into 4-20mA current is achieved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of static starting frequency converters of combustion engines, and in particular to a high-precision PWM conversion 4-20mA current output circuit. BACKGROUND

[0002] A PWM signal (pulse width modulation signal) is a signal for controlling and driving a target device by changing the ratio of high-level time and low-level time, namely the duty cycle, and belongs to a digital signal. In life, for example, motor speed regulation and light brightness can be controlled by changing the PWM duty cycle. The PWM is stable and easy to produce, but in industry, analog quantities are generally used for control, such as voltage and current for control production. However, the current PWM conversion circuit has low precision, and the application provides a circuit which can convert a PWM digital signal into a 4-20mA current analog signal with ultra-high precision, realizing ultra-high precision industrial control.

[0003] Similar technical solutions:

[0004] CN102789253A discloses a PWM-controlled 4-20mA current output circuit. The controllable PWM signal is connected with a low-pass filter module, the low-pass filter module, a driving module and an operational amplifier negative feedback module are connected, the driving module is connected with a protection module, and a power supply provides power supply for each module.

[0005] CN208226862U is a utility model, which aims to provide a PWM voltage signal isolation conversion to 4-20mA current signal circuit, which is based on pulse width modulation to realize low-cost and accurate current transmitter. The technical scheme of the utility model provides a PWM voltage signal isolation conversion to 4-20mA current signal circuit, characterized in that it comprises an optical coupler U1, an optical coupler U2 and a three-terminal adjustable voltage regulator. The utility model realizes a low-cost and accurate current transmitter based on pulse width modulation. The double-channel optical coupler provides signal isolation for the PWM input, and the PWM signal after isolation is used to chop the 1.25V reference voltage of the three-terminal adjustable voltage regulator, so as to control the output current of the three-terminal adjustable voltage regulator.

[0006] CN 109683650 B is a voltage to 4-20mA current circuit. The application discloses a voltage to 4-20mA current circuit, and relates to the technical field of voltage to current, which comprises an operational amplifier U1, resistors R11, R12, R13, R14, R15, R16, a diode D1 and a triode Q1; one end of the resistor R11 is connected with an input voltage V0, and the other end is connected with the inverting input end of the operational amplifier U1; one end of the resistor R12 is connected with the non-inverting input end of the operational amplifier U1, and the other end is grounded; one end of the resistor R13 is connected with the inverting input end of the operational amplifier U1, and the other end is connected with the anode of the diode D1; one end of the resistor R14 is connected with the non-inverting input end of the operational amplifier U1, and the other end is connected with the emitter of the triode Q1; one end of the resistor R15 is connected with the output end of the operational amplifier U1, and the other end is connected with the base of the triode Q1; one end of the resistor R16 is connected with the emitter of the triode Q1, and the other end is connected with the anode of the diode D1.

[0007] However, most of the existing PWM to 4-20mA current uses GP8102 chip, which has low precision and poor linearity and following property in certain environment. The above prior art all has the problem of low conversion current precision.

[0008] How to solve the problem that there is no high-precision circuit for converting voltage analog quantity of different sizes into 4-20mA current in the prior art is still a problem to be solved by those skilled in the art. SUMMARY

[0009] The application provides a high-precision PWM conversion 4-20mA current output circuit to solve the problem that there is no high-precision circuit for converting voltage analog quantity of different sizes into 4-20mA current in the prior art.

[0010] The application provides a high-precision PWM conversion 4-20mA current output circuit, which comprises an input-output isolation subcircuit, a switching subcircuit, an integration subcircuit and an amplification output subcircuit connected in sequence.

[0011] The resistors in each subcircuit are optimal performance resistors obtained through multiple test experiments, and the amplifiers used in the integration subcircuit and the amplification output subcircuit are LF444CMX operational amplifiers.

[0012] The application provides a high-precision PWM conversion 4-20mA current output circuit, and the input-output isolation subcircuit specifically comprises a triode receiving type photoelectric coupler, a 20V power supply, a resistor R1 and a resistor R2,

[0013] The first port of the triode receiving type photoelectric coupler is connected with 5V voltage, the PWM signal is connected with the second port of the triode receiving type photoelectric coupler through the resistor R1, the third port of the triode receiving type photoelectric coupler is connected with 20V power supply through the resistor R2, the fourth port of the triode receiving type photoelectric coupler is grounded, and the signal between the resistor R2 and the third port of the triode receiving type photoelectric coupler is input to the switch sub-circuit.

[0014] The triode receiving type photoelectric coupler is EF357NA photoelectric coupler.

[0015] The switch sub-circuit comprises a MOS tube, a diode, a resistor R3, 20V voltage and 15V voltage.

[0016] The right side of the MOS tube is connected with the diode, the gate of the MOS tube is connected with the output signal of the input-output isolation sub-circuit, the source of the MOS tube is connected with 15V voltage through the resistor R3, the drain of the MOS tube is connected with 20V voltage, and the signal between the resistor R3 and the source of the MOS tube is input to the integration sub-circuit.

[0017] The model of the MOS tube is 2SK425.

[0018] The integration sub-circuit comprises a capacitor C1, a capacitor C2, an LF444CMX operational amplifier and 24V voltage.

[0019] The output of the switch sub-circuit is connected with the non-inverting input port of the LF444CMX operational amplifier, the output of the switch sub-circuit is connected with the output port of the LF444CMX operational amplifier through the capacitor C1, the output port of the LF444CMX operational amplifier is connected with the inverting input port of the LF444CMX operational amplifier, the first power supply port of the LF444CMX operational amplifier is connected with 24V voltage, the signal between the first power supply port and 24V voltage is grounded through the capacitor C2, and the second power supply port of the LF444CMX operational amplifier is grounded.

[0020] The amplification output sub-circuit comprises a capacitor C3, a capacitor C4, a capacitor C5, a capacitor C6, a resistor R4, a resistor R5, a resistor R6, a diode M7, a triode, an LF444CMX operational amplifier and 24V voltage.

[0021] The output of the integral sub-circuit is connected to the positive input port of the LF444CMX operational amplifier through a resistor R4, the signal between the resistor R4 and the positive input port is connected to a 20V voltage through a capacitor C4, the signal between the capacitor C4 and the 20V voltage is connected to the emitter of a triode through a resistor R6, the signal between the resistor R4 and the positive input port is connected to the emitter of the triode and the negative input port of the LF444CMX operational amplifier through a capacitor C3 respectively, the output port of the LF444CMX operational amplifier is connected to the base of the triode through a resistor R5, the first power port of the LF444CMX operational amplifier is connected to a 24V voltage, the signal between the first power port and the 24V voltage is connected to the ground through a capacitor C6, the second power port of the LF444CMX operational amplifier is connected to the ground, the collector of the triode is connected to a diode M7 and one end of a capacitor C5 in sequence, the other end of the capacitor C5 is connected to the ground, and the signal between the diode and the capacitor C5 is taken as the output of the amplification output sub-circuit.

[0022] According to the application, a high-precision PWM conversion 4-20mA current output circuit is provided, and the model of the triode is SS8550Y2.

[0023] According to the application, a high-precision PWM conversion 4-20mA current output circuit is provided, and the model of the triode is SS8550Y2. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0025] Figure 1 The structural schematic diagram of the high-precision PWM conversion 4-20mA current output circuit provided by the present application is shown in the figure.

[0026] Figure 2 The structural schematic diagram of the input-output isolation sub-circuit provided by the present application is shown in the figure.

[0027] Figure 3This is a schematic diagram of the structure of the switch sub-circuit provided by the present invention;

[0028] Figure 4 A schematic diagram of the integrator circuit provided by the present invention;

[0029] Figure 5 A schematic diagram of the structure of the amplified output sub-circuit provided by the present invention;

[0030] Figure 6 A detailed diagram of the high-precision PWM converter 4-20mA current output circuit provided by the present invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0032] Currently, there is no high-precision circuit in the technology to convert analog voltage signals of varying magnitudes into 4-20mA current. The following section combines... Figure 1 This invention describes a high-precision PWM converter for 4-20mA current output. Figure 1 The structural diagram of the high-precision PWM conversion 4-20mA current output circuit provided by this invention is as follows: Figure 1 As shown, the circuit includes:

[0033] The input / output isolation sub-circuit, the switching sub-circuit, the integrating sub-circuit, and the amplifying output sub-circuit are connected in sequence.

[0034] The resistors in each sub-circuit are all the optimal resistance values ​​obtained from multiple test experiments, and the amplifiers used in the integrator sub-circuit and the amplified output sub-circuit are both LF444CMX operational amplifiers.

[0035] Specifically, such as Figure 1 As shown, the high-precision PWM converter for 4-20mA current output includes four sub-modules: an input-output isolation sub-circuit, a switching sub-circuit, an integrating sub-circuit, and an amplified output sub-circuit. The connection order is as follows: the PWM signal is input to the input terminal of the input-output isolation sub-circuit; the output terminal of the input-output isolation sub-circuit is connected to the input terminal of the switching sub-circuit; the output terminal of the switching sub-circuit is connected to the input terminal of the integrating sub-circuit; the output terminal of the integrating sub-circuit is connected to the input terminal of the amplified output sub-circuit; and the output port of the amplified output sub-circuit outputs the converted current signal.

[0036] The resistance in each sub-circuit is the optimal resistance value obtained through multiple test experiments, all the resistances in the circuit are the resistance values selected through accurate and strict calculation and experiment and multiple tests, and the size ratio of each resistance is continuously adjusted to finally achieve the requirements of super-high precision output of output current, high followability and linearity to the PWM signal.

[0037] The high input impedance operational amplifier LF444CMX is also selected for the integral sub-circuit and the amplification output sub-circuit. The main reason for selecting the LF444CMX is its extremely high input impedance and high gain. While having high gain, the power supply current drawn by the LF444CMX is far less than that of other available operational amplifiers (such as LM148), and the power supply current drawn by the LF444CMX is only one fourth of that of the LM148. In addition, due to the input device of the LF444, the input bias and offset current thereof is only one ten-thousandth (1 / 10000) of that of the LM148.

[0038] The high-precision PWM conversion 4-20mA current output circuit provided by the embodiment of the present application comprises: sequentially connected input-output isolation sub-circuit, switching sub-circuit, integral sub-circuit and amplification output sub-circuit; wherein the resistance in each sub-circuit is the optimal resistance value obtained through multiple test experiments, and the amplifier used in the integral sub-circuit and the amplification output sub-circuit is the LF444CMX operational amplifier. The circuit provided by the present application selects a specific resistance value for the resistance in the circuit and selects a specific model for the amplifier in the circuit, so as to achieve the purpose of high-precision conversion of the PWM signal into 4-20mA current.

[0039] Based on the above embodiment, the input-output isolation sub-circuit comprises: a triode receiving type photoelectric coupler, a 20V power supply, a resistance R1 and a resistance R2,

[0040] The first port of the triode receiving type photoelectric coupler is connected to a 5V voltage, the PWM signal is connected to the second port of the triode receiving type photoelectric coupler through the resistance R1, the third port of the triode receiving type photoelectric coupler is connected to the 20V power supply through the resistance R2, the fourth port of the triode receiving type photoelectric coupler is grounded, and a signal is led out between the resistance R2 and the third port of the triode receiving type photoelectric coupler and input to the switching sub-circuit.

[0041] Specifically, Figure 2 The structure diagram of the input-output isolation sub-circuit provided by the present application is as follows: Figure 2As shown, optocoupler isolation is used to prevent interference from the output to the input and improve output accuracy. PWM is a signal with different duty cycles. The 5V voltage is connected to optocoupler port 1, and the PWM signal is connected to optocoupler port 2 through resistor R1. Its high and low levels are compared with the 5V signal to control the on / off state of the optocoupler. Usually, ports 1 and 2 of the optocoupler are directly connected to voltage signals without resistor R1. Here, adding R1 not only serves as a current limiter but also significantly affects the current output of the entire circuit. The value of R1 was determined through multiple tests and experiments to effectively improve current control and output accuracy. The collector of the output side, i.e., port 3, is connected to 20V through resistor R2, and the emitter is grounded. A signal is led out between R2 and port 3 into circuit 3.

[0042] Based on the above embodiments, in the high-precision PWM conversion 4-20mA current output circuit, the transistor receiving optocoupler is an EF357NA optocoupler.

[0043] Specifically, preferably, the transistor receiving optocoupler is selected as the EF357NA optocoupler to further improve the accuracy of the circuit.

[0044] Based on the above embodiments, in this high-precision PWM conversion 4-20mA current output circuit, the switching sub-circuit includes: a MOSFET, a diode, a resistor R3, a 20V voltage, and a 15V voltage.

[0045] In this circuit, a diode is connected to the right side of the MOSFET, the gate of the MOSFET is connected to the output signal of the input-output isolation sub-circuit, the source of the MOSFET is connected to a 15V voltage through a resistor R3, the drain of the MOSFET is connected to a 20V voltage, and a signal is led out between the resistor R3 and the source of the MOSFET to the integrator circuit.

[0046] Specifically, Figure 3 A schematic diagram of the switching sub-circuit provided by the present invention is shown below. Figure 3 As shown, a diode is connected to the right side of the MOSFET. Due to the unidirectional conductivity of the diode, the circuit is open when the MOSFET is not conducting. After the signal from the input / output isolation circuit arrives, the voltage at port 5 increases, causing the MOSFET to conduct, and the voltage at node 6 increases, allowing the signal to enter the integrator circuit.

[0047] Based on the above embodiments, in this high-precision PWM conversion 4-20mA current output circuit, the MOSFET is model 2SK425.

[0048] Specifically, preferably, the MOSFET is a 2SK425 to further improve the accuracy of the circuit.

[0049] Based on the above embodiment, the high-precision PWM conversion 4-20mA current output circuit, the integral sub-circuit includes: capacitor C1, capacitor C2, LF444CMX operational amplifier and 24V voltage, wherein,

[0050] The output of the switch sub-circuit is connected to the non-inverting input port of the LF444CMX operational amplifier, the output of the switch sub-circuit is connected to the output port of the LF444CMX operational amplifier through the capacitor C1, the output port of the LF444CMX operational amplifier is connected to the inverting input port thereof, the first power supply port of the LF444CMX operational amplifier is connected to the 24V voltage, a signal is led out between the first power supply port and the 24V voltage and grounded through the capacitor C2, and the second power supply port of the LF444CMX operational amplifier is grounded.

[0051] Specifically, Figure 4 The structure diagram of the integral sub-circuit provided by the application is shown in Figure 4 As shown, the signal flows into the 7th node, which is still a digital signal of a certain frequency controlled by the input PWM signal, and is integrated into a continuous analog signal by the integral circuit composed of the LF444CMX high-input-impedance operational amplifier and the capacitor, and then input to the next-stage amplification and output sub-circuit through the 8th node. The LF444CMX is also selected in the amplification and output sub-circuit and the subsequent amplification and output sub-circuit, which is different from the prior art. The LF444CMX low-power operational amplifier provides the same AC characteristics as the industrial standard LM148, while greatly improving the DC characteristics of the LM148. The amplifier has the same bandwidth, conversion rate and gain as the LM148, and the power supply current is only one-fourth of the LM148. It has the characteristics of high input impedance, high conversion rate and high gain. In addition to the well-matched high-voltage JFET, the input device of the LF444 reduces the input bias and offset current of the LM148 by 10000 times, further improving the accuracy of the final output current.

[0052] Based on the above embodiment, the high-precision PWM conversion 4-20mA current output circuit, the amplification and output sub-circuit includes: capacitor C3, capacitor C4, capacitor C5, capacitor C6, resistor R4, resistor R5, resistor R6, diode M7, triode, LF444CMX operational amplifier and 24V voltage, wherein,

[0053] The output of the integral sub-circuit is connected to the positive input port of the LF444CMX operational amplifier through a resistor R4, the signal between the resistor R4 and the positive input port is connected to a 20V voltage through a capacitor C4, the signal between the capacitor C4 and the 20V voltage is connected to the emitter of a triode through a resistor R6, the signal between the resistor R4 and the positive input port is connected to the emitter of the triode and the negative input port of the LF444CMX operational amplifier through a capacitor C3 respectively, the output port of the LF444CMX operational amplifier is connected to the base of the triode through a resistor R5, the first power port of the LF444CMX operational amplifier is connected to a 24V voltage, the signal between the first power port and the 24V voltage is connected to the ground through a capacitor C6, the second power port of the LF444CMX operational amplifier is connected to the ground, the collector of the triode is connected to a diode M7 and one end of a capacitor C5 in sequence, the other end of the capacitor C5 is connected to the ground, and the signal between the diode and the capacitor C5 is taken as the output of the amplification output sub-circuit.

[0054] Specifically, Figure 5 The structure diagram of the amplification output sub-circuit provided by the present application is shown as follows, Figure 5 The amplification output sub-circuit is the last output circuit, the analog signal is input from the 9th node, is amplified by the amplification circuit and is output from the amplifier end 10 to enter the output circuit. The 20V controls the on-off of the SS8550. Reviewing the complete circuit, the same voltage stabilizing power supply should be connected to the 20V of the whole circuit, so that the output is obtained only when the whole circuit is running, thereby protecting the external circuit. In the normal operation, the node 11 outputs the required signal, the M7 ensures that the external circuit will not be input to the present circuit, thereby preventing the damage of the present circuit, the C5 plays a filtering role to filter out fluctuations and output a stable current signal. Meanwhile, a resistor R6 is added to the emitter of the triode. The resistor R6 mainly controls the output of the current, and the resistance value of the resistor R6 is selected through a large number of tests and tests. The appropriate resistance value is selected to work together with the previous resistor, thereby improving the output precision and the following property and linearity of the current output to the voltage control.

[0055] Based on the above embodiment, in the high-precision PWM conversion 4-20mA current output circuit, the model of the triode is SS8550Y2.

[0056] Specifically, preferably, the model of the triode is SS8550Y2, which further improves the accuracy of the circuit.

[0057] Based on the above embodiment, the present application also provides another high-precision PWM conversion 4-20mA current output circuit, Figure 6 The overall detail view of the high-precision PWM conversion 4-20mA current output circuit provided by the present application is shown as follows, Figure 6As shown, 20V voltage regulator as the driving power supply of the circuit, 24V power supply for amplifier power supply, ground section can be common ground, PWM signal for input signal, control output, 5V for 5V voltage signal responsible for comparison with PWM, 15V voltage regulator control MOS tube on-off, play the role of circuit switch protection.

[0058] Finally, it should be noted that: the above examples are used to illustrate the technical solutions of the present application, rather than limit them; Although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; And these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A high-precision PWM conversion 4-20 mA current output circuit, characterized by, The circuit comprises: sequentially connected input-output isolation sub-circuit, switch sub-circuit, integral sub-circuit and amplification output sub-circuit; The input-output isolation sub-circuit specifically comprises: triode receiving type photoelectric coupler, 20V power supply, resistor R1 and resistor R2, wherein the first port of the triode receiving type photoelectric coupler is connected with 5V voltage, the second port of the triode receiving type photoelectric coupler is connected with PWM signal through resistor R1, the third port of the triode receiving type photoelectric coupler is connected with 20V power supply through resistor R2, the fourth port of the triode receiving type photoelectric coupler is grounded, and a signal is led out between resistor R2 and the third port of the triode receiving type photoelectric coupler and input to the switch sub-circuit; The switch sub-circuit comprises: MOS tube, diode, resistor R3, 20V voltage and 15V voltage, wherein the right side of the MOS tube is connected with the diode, the gate of the MOS tube is connected with the output signal of the input-output isolation sub-circuit, the source of the MOS tube is connected with 15V voltage through resistor R3, the drain of the MOS tube is connected with 20V voltage, and a signal is led out between resistor R3 and the source of the MOS tube and input to the integral sub-circuit; The integral sub-circuit comprises: capacitor C1, capacitor C2, LF444CMX operational amplifier and 24V voltage, wherein, the output of the switch sub-circuit is connected with the non-inverting input port of the LF444CMX operational amplifier, the output of the switch sub-circuit is connected with the output port of the LF444CMX operational amplifier through capacitor C1, the output port of the LF444CMX operational amplifier is connected with the inverting input port thereof, the first power supply port of the LF444CMX operational amplifier is connected with 24V voltage, a signal is led out between the first power supply port and 24V voltage, passes through capacitor C2 and is grounded, and the second power supply port of the LF444CMX operational amplifier is grounded; The amplification output sub-circuit comprises: capacitor C3, capacitor C4, capacitor C5, capacitor C6, resistor R4, resistor R5, resistor R6, diode M7, triode, LF444CMX operational amplifier and 24V voltage, wherein, The output of the integral sub-circuit is connected to the positive input port of the LF444CMX operational amplifier through a resistor R4, the signal between the resistor R4 and the positive input port is connected to a 20V voltage through a capacitor C4, the signal between the capacitor C4 and the 20V voltage is connected to the emitter of a triode through a resistor R6, the signal between the resistor R4 and the positive input port is connected to the emitter of the triode and the negative input port of the LF444CMX operational amplifier through a capacitor C3 respectively, the output port of the LF444CMX operational amplifier is connected to the base of the triode through a resistor R5, the first power port of the LF444CMX operational amplifier is connected to a 24V voltage, the signal between the first power port and the 24V voltage is grounded through a capacitor C6, the second power port of the LF444CMX operational amplifier is grounded, the collector of the triode is connected to a diode M7 and one end of a capacitor C5 in sequence, the other end of the capacitor C5 is grounded, and the signal between the diode and the capacitor C5 is taken as the output of the amplification output sub-circuit. The resistors in each sub-circuit are the optimal resistance values obtained through multiple test experiments, and the amplifiers used in the integral sub-circuit and the amplification output sub-circuit are LF444CMX operational amplifiers.

2. The high-precision PWM conversion 4-20 mA current output circuit according to claim 1, characterized by, The triode receiving type photoelectric coupler is an EF357NA optical coupler.

3. The high-precision PWM conversion 4-20 mA current output circuit according to claim 1, characterized by, The model of the MOS tube is 2SK425.

4. The high precision PWM conversion 4-20 mA current output circuit according to claim 1, characterized by, The model of the triode is SS8550Y2.

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