Due to the strong anti-interference ability of current signals, 4-20mA current signals are widely used in industry to transmit analog signals. Therefore, current type transmitters are mainly widely used at present. According to the number of external connections of current type transmitters, they can generally be divided into current type three wire transmitters and current type two wire transmitters. Current type three wire transmitters generally have three external wires, namely two power lines and one current output line; Current type two-wire transmitters are generally connected with two external wires, both of which are power lines, and the power current is the current to be output. The current type three wire transmitter has obvious disadvantages. It requires three wires to transmit signals, and in order to reduce signal interference, shielded wires are usually added to the cables. Therefore, when long-distance transmission is required, the cable cost is high and the installation is relatively complex, which greatly limits its application in the industrial field. In contrast, current type two-wire transmitters only require simple twisted pair cables for signal transmission, and twisted pair cables are less susceptible to interference and do not require additional shielded wires. When transmitting over long distances, current type transmittersTwo wire transmitterCompared with current type two-wire transmitters, current type two-wire transmitters have outstanding cost advantages, so the research on current type two-wire transmitters at this stage is of great significance.
At present, there have been some studies on current type two-wire transmitters for physical quantities such as temperature and humidity both domestically and internationally, while current type transmitters for light intensity have been developedTwo wire transmitterThere is relatively little research on it. Due to the differences between light intensity and physical quantities such as temperature and humidity, it is often not possible to directly apply current type two-wire transmitter circuits for physical quantities such as temperature and humidity to current type two-wire light intensity transmitters. In addition, some existing designed light intensity transmitters generally have problems with low accuracy, poor linearity, unstable performance, and inability to output standard 4-20mA current signals. Therefore, a current type two-wire light intensity transmitter has been designed here, which has the characteristics of high accuracy, good linearity, and low power consumption. It can stably and reliably output standard 4-20mA current signals, effectively solving the problems introduced earlier.
1. Composition of transmitter system
As shown in Figure 1, the current type two-wire light intensity transmitter circuit mainly includes four parts: light intensity to voltage conversion circuit, voltage range conversion circuit, voltage to current conversion circuit, and voltage regulator power generation circuit.
The light intensity to voltage conversion circuit mainly converts the extremely weak current signal generated by the silicon photovoltaic cell into a 0-5V output voltage that can be processed later; The voltage range conversion circuit converts the 0-5V voltage signal into a 0.4-2V voltage signal; The voltage to current circuit converts a voltage of 0.4-2V into a current signal of 4-20mA; The voltage regulator circuit actually utilizes the generated 4-20mA current to provide stable power supply voltage for the front-end illumination intensity to voltage conversion circuit and voltage range conversion circuit, thereby minimizing the impact of external power supply voltage fluctuations to the greatest extent possibletransmitterThe impact on performance.
2. Transmitter Circuit Design
2.1 Light intensity to voltage conversion circuit
As shown in Figure 2, the silicon photovoltaic cell converts the light intensity into an extremely weak current, which is proportional to the light intensity. It can be seen from the virtual interruption of the in-phase and in-phase input terminals of operational amplifier UC1 that the current generated by the silicon photovoltaic cell mainly flows through feedback resistor R1, thus forming an output voltage at the output terminal of operational amplifier UC1. The output voltage signal is isolated and amplified after passing through operational amplifier UC2, so an output voltage can be obtained at the output terminal of operational amplifier UC2, which is proportional to the intensity of light. By adjusting the resistance values of resistors R1, R2, and R3, the output voltage can be adjusted to the standard range of 0-5V.
The formula is derived as follows:
From the virtual short and virtual break of the operational amplifier, it can be inferred that:
In the formula, I is the current generated by the silicon photovoltaic cell, and V1 is the output voltage.
By adjusting the resistance values of R1, R2, and R3, a standard output voltage of 0-5V can be obtained.
2.2 Voltage Range Conversion Circuit
As shown in Figure 3, the above-mentioned light intensity to voltage conversion circuit obtains a standard output voltage of 0-5V. This voltage can be converted into a 0-1.6V voltage through a proportional amplification circuit composed of operational amplifier UC3 and resistors R4, R5, R6, R7. After passing through the voltage divider circuit, a voltage can be obtained at the sliding end of the adjustable potentiometer W1. This voltage, together with the previously obtained 0-1.6V voltage, can be added in the same phase by the operational amplifier UC4 and resistors R9, R10, R11, R12, R13 to obtain a voltage of 0.4-2V at the output end of the operational amplifier UC4.
The specific formula is derived as follows:
From the virtual short and virtual break, it can be inferred that:
Choosing appropriate resistance values for R5, R6, and R7 can result in a voltage range of 0-1.6V.
According to the in-phase summation circuit, when R9 ∥ R10 ∥ R11=R12 ∥ R13,
In the formula, Vref is the voltage division value obtained by the sliding end of the adjustable potentiometer.
By selecting appropriate resistance values for R8, R9, R10, R11, R12, and R13 and adjusting the adjustable potentiometer, Vout2=0.4-2V can be obtained. 2.3 Voltage to Current Circuit
As shown in Figure 4, careful analysis reveals that the output terminal of operational amplifier UD1 forms a negative feedback branch through resistor R17, transistor Q1, resistor R18, resistor R19, and resistor R15, which are fed back to the in-phase input terminal of operational amplifier UD1. From the characteristics of virtual short and virtual break of the operational amplifier, it can be known that when the resistance values of resistor R14 and resistor R15 are equal, the voltage drop across resistor R19 is exactly equal to the output voltage of the voltage range conversion circuit, which is 0.4-2V. Therefore, if R19 is taken as 100 Ω and R15>>R19, the total current in the final circuit is approximately equal to the current flowing through resistor R19, that is, 4-20mA.
The specific formula is derived as follows:
From the virtual short and virtual break, it can be inferred that:
If R15 is much larger than R19 and R19=100, then I=4-20mA.
2.4 Stable voltage power supply generation circuit
As shown in Figure 5, the current source stabilizes the input current of the regulated power supply circuit, thereby ensuring the stability of its output voltage, that is, the supply voltage to the front-end is stable. The voltage regulator diode D1 will generate a reference voltage of 2.5V at the positive input of the operational amplifier UD2, and then amplify the reference voltage through an amplification circuit composed of the operational amplifier UD2, resistors R20 and R21, resulting in a voltage of approximately 12V at the output of the operational amplifier UD2.
The specific formula is derived as follows:
From the virtual short and virtual break of the operational amplifier,
Because VDD=2.5V, selecting appropriate resistance values for resistors R20 and R21 can obtain the required output voltage value VDD.
3. Experiment and Results
In the above current type two-wire light intensity transmitter circuit, OPA481 is selected for operational amplifiers UC1, UC2, UC3, and UC4, and MC33172 is selected for operational amplifiers UD1 and UD2. Operational amplifiers are the main power consuming components of circuits. OPA481 and MC33172 are both high-precision and low-power single power operational amplifiers. Choosing them can maximize the conversion accuracy while meeting the requirement of a total power consumption of less than 4mA (because the circuit is powered by an external power supply, the minimum output current is 4mA, and the prerequisite for normal circuit operation is that the total power consumption of the circuit must be less than 4mA). In addition, LM234 is selected for current source D3, LM285-2.5 is selected for voltage regulator diode D1, 1N437 is selected for diode D2, and 2N3904 can be selected for transistor Q1. Generally, precision resistors with an accuracy of 1% are used for resistors. The silicon photovoltaic cell selected is SPS3030 produced by Nantong Dahua Company. This silicon photovoltaic cell has good conversion linearity and is minimally affected by temperature. Choosing it can improve the conversion accuracy of the transmitter circuit. Due to the use of a regulated power supply in the circuit to supply power to the front-end operational amplifier circuit, the impact of external power supply voltage fluctuations on the transmitter circuit can be minimized to the greatest extent possible.
Prepare the PCB board according to the above circuit, and then use the selected components to connect the circuit. When the light intensity changes from 0 to 200klux (unit of light intensity), measure the current at the output terminal of the transmitter. The experimental results are shown in Figure 6.
In Figure 6, the horizontal axis represents the light intensity, measured in klux; The vertical axis represents the output current, measured in mA. The experimental results show that the circuit effectively achieves linear conversion from 0-200klux light intensity signal to 4-20mA current signal. It has high conversion accuracy (with an error of about 1%), good linearity (approximately a straight line), and can stably and reliably convert light intensity into 4-20mA current output, ultimately achieving the expected goal.
4. Conclusion
The article provides a detailed introduction to the design of a current type two-wire light intensity transmitter. Experiments have shown that the designed transmitter has high accuracy, good linearity, low power consumption, and stable performance, and has good application prospects in industry.