Design and application of two-wire transducer circuit

 

In the article “Design and Application of Two-Wire Transducer Circuit”, analyzes in detail the key points, difficulties and technical points of two-wire transducer circuit design. In this article, she shows readers a practical circuit of a two-wire transducer with simple circuit, low power consumption and strong anti-interference ability, which provides reference for everyone to design pressure transducer and temperature transducer circuit.

 

 

In industry, it is generally necessary to measure various non-electrical physical quantities, such as temperature, pressure, speed, angle, etc., which need to be converted into analog electrical signals before they can be transmitted to the control room or display device hundreds of meters away. This device that converts physical quantities into electrical signals is called a transducer. The most widely used method in industry is to use 4-20mA current to transmit analog quantities.

 

 

The reason for using current signals is that they are not easily interfered with. In addition, the internal resistance of the current source is infinite, and the wire resistance in series in the loop does not affect the accuracy, and it can be transmitted for hundreds of meters on ordinary twisted pair cables. The upper limit of 20mA is due to the explosion-proof requirements: the spark energy caused by the on-off current of 20mA is not enough to ignite gas. The reason why the lower limit is not 0mA is to detect line breakage: it will not be lower than 4mA during normal operation. When the transmission line is disconnected due to a fault, the loop current drops to 0. 2mA is often used as the line break alarm value.

 

 

The current type transducer converts the physical quantity into 4-20mA current output, which must be powered by an external power supply. The most typical is that the transducer requires two power lines, plus two current output lines, a total of 4 lines, which is called a four-wire transducer. Of course, the current output can share a line with the power supply (common VCC or GND), which can save one line, and is called a three-wire transducer.

 

 

In fact, you may have noticed that the 4-20mA current itself can power the transducer. The transducer is equivalent to a special load in the circuit. The special thing is that the power consumption current of the transducer varies between 4-20mA according to the sensor output. The display instrument only needs to be connected in series in the circuit. This type of transducer only needs 2 external wires, so it is called a two-wire transmitter. The lower limit of the industrial current loop standard is 4mA, so as long as it is within the range, the transducer has at least 4mA power supply. This makes the design of a two-wire sensor possible.

 

 

 

In industrial applications, the measurement point is generally on-site, while the display device or control device is generally in the control room or control cabinet. The distance between the two may be tens to hundreds of meters. Calculated at a distance of 100 meters, saving 2 wires means a cost reduction of nearly 100 yuan! Therefore, a two-wire sensor must be the first choice in the application.

 

 

 

 

Structure and principle of two-wire transducer
The principle of two-wire transducer is to use 4-20mA signal to provide power for itself. If the power consumption of the transducer itself is greater than 4mA, it will be impossible to output the lower limit 4mA value. Therefore, it is generally required that the power consumption of the two-wire transducer itself (including all circuits including sensors) is not more than 3.5mA. This is one of the fundamental design principles of two-wire transducers.

Figure 1 Two-wire transducer structure

From the overall structure, the two-wire transducer consists of three parts: sensor, conditioning circuit, and two-wire V/I converter. The sensor converts physical quantities such as temperature and pressure into electrical parameters, and the conditioning circuit amplifies, conditions, and converts the weak or nonlinear electrical signal output by the sensor into a linear voltage output. The two-wire V/I conversion circuit controls the overall power consumption current according to the output of the signal conditioning circuit; at the same time, the voltage is obtained from the loop and stabilized for use by the conditioning circuit and sensor. In addition to the V/I conversion circuit, each part of the circuit has its own power consumption current. The core design idea of the two-wire transducer is to include all currents in the feedback loop of the V/I conversion. As shown in Figure 1, the sampling resistor RS is connected in series at the low end of the circuit, and all current flows back to the negative pole of the power supply through the RS resistor. The feedback signal obtained from RS contains the power consumption of all circuits. In the two-wire transducer, the total power consumption of all circuits cannot be greater than 3.5mA, so the low power consumption of the circuit becomes the main design difficulty. The following will analyze the principles and design points of each part of the circuit one by one.

 

 

2. Two-wire V/I converter
V/I converter is a circuit that can control the output current with a voltage signal. The difference between the two-wire V/I converter and the general V/I conversion circuit is that the voltage signal does not directly control the output current, but controls the power consumption current of the entire circuit itself. At the same time, a stable voltage must be extracted from the current loop to power the conditioning circuit and the sensor.

Figure 2 Two-wire V/I converter circuit

If point A is higher than 0V for some reason, the output of the op amp OP1 increases, the voltage across Re increases, and the current through Re increases. This is equivalent to the overall power consumption increasing, the current through the sampling resistor Rs also increases, and the voltage at point B becomes lower (more negative). The result is that the voltage at point A is pulled down through R2. Conversely, if point A is lower than 0V for some reason, it will also be raised back to 0V by negative feedback. In short, the result of negative feedback is that the op amp OP1 is virtual short, and the voltage at point A = 0V. The following is an analysis of the control principle of V0 on the total power consumption: Assuming that the output voltage of the conditioning circuit is V0, the current I1 flowing through R1 = V0/R1. The op amp input terminal cannot absorb current, so I1 flows through R2, and the voltage at point B VB = -I1R2 = -V0×R2/R1. When R1=R2, VB=-V0. There are only two resistors, Rs and R2, between the negative power supply and the entire transmitter circuit, so all the current flows through Rs and R2. The upper end of R2 is the virtual ground (0V), and the upper end of Rs is GND. Therefore, the voltages at both ends of R2 and Rs are exactly the same, both equal to VB. It is equivalent to connecting Rs and R2 in parallel as current sampling resistors. Therefore, the total current of the circuit is: Is=V0/(Rs//R2) If R2>>Rs, Is=V0/Rs Therefore, in Figure 2, Rs=100 ohms is taken. When the conditioning circuit outputs 0.4-2V, the total current consumption is 4-20mA. It doesn’t matter if R2>>Rs cannot be satisfied. Rs and R2 in parallel (Rs//R2) are fixed values. Is and V0 are still linearly related, and the error proportional coefficient can be eliminated during calibration.

 

 

 

In addition to the correct circuit, the normal operation of the circuit also requires two conditions: First, the power consumption itself should be as small as possible, and the saved current should also be supplied to the conditioning circuit and the transducer. Secondly, the op amp is required to work with a single power supply, that is, in the absence of a negative power supply, the input terminal can still accept 0V input and work normally. LM358/324 is the most common and cheapest single-power supply op amp, consuming 400uA/per op amp, which is basically acceptable. When powered by a single power supply, the input terminal can work normally from -0.3V to Vcc-1.5V. If you replace it with a precision amplifier such as OP07, it will not work in this circuit because the input is not allowed to be as low as 0V.

 

 

R5 and U1 form a reference source to generate a stable reference voltage of 2.5V. LM385 is a low-cost micro-power reference that can work above 20uA. The curve given in the manual is flattest near 100uA, so R5 controls the current to about 100uA. OP2 forms a unidirectional amplifier to amplify the reference and power the conditioning circuit and sensor. Because wide input voltage and low power regulators are rare and costly; using the reference amplification as a regulated power supply is a cheap solution. This part of the circuit can also choose a ready-made integrated circuit. For example, XTR115/116/105, etc., have better accuracy and stability than homemade ones, and their own power consumption is also lower (which means that more current can be left for the conditioning circuit, and the conditioning part is easier to design). But the cost is more than 10 times higher than the above solution.

 

 

3. Design of two-wire pressure transducer conditioning circuit
The output signals of pressure bridge and weighing sensor are weak and belong to mV level signals. This kind of small signal generally requires differential amplifier to amplify it in the first stage. Generally, differential amplifier with low offset and low temperature drift is selected. In addition, low power consumption is also necessary in two-wire applications. AD623 is a commonly used low-power precision differential amplifier, which is often used in differential output pre-stage amplification. AD623 has a maximum offset of 200uV and a temperature drift of 1uV/degree, which ensures sufficient accuracy in general pressure transmission applications. R0 superimposes 0.4V on the REF pin (pin 5) of AD623. When the pressure = 0, adjust R0 to make the output 4mA, and then adjust RG to output 20.00mA to complete the calibration. When designing the circuit, it should be noted that the pressure bridge sensor is equivalent to a kilo-ohm resistor, and the power consumption is generally large. Appropriately reducing the excitation voltage of the pressure bridge can reduce the power consumption current. However, the output amplitude also decreases, and the gain of AD623 needs to be increased. The sensor shown in Figure 3 is powered by constant voltage. In practical applications, most semiconductor pressure sensors require constant current power supply to obtain better temperature characteristics. A constant current source can be used to provide excitation for them.

Figure 3 Signal conditioning circuit of pressure transducer

 

 

 

4. Consideration of stability and safety
The industrial environment is harsh and has high reliability requirements. Therefore, certain protection and stability enhancement measures need to be considered in the design of two-wire transducers.

Figure 4 Protection circuit of two-wire transducer

 

 

Power supply protection
Reverse power supply, overvoltage and surge are common power supply problems in industry. Reverse power supply is the most common error when installing and wiring equipment. A diode in series at the input port can prevent damage to the circuit when the power supply is reversed. If a full-bridge rectifier is added to the input end, it can still work normally even if the power supply is reversed. In order to prevent lightning strikes, electrostatic discharge, surges and other energies from damaging the transducer, a TVS tube can be installed at the transducer entrance to absorb the energy of instantaneous overvoltage. Generally, the TVS voltage value is slightly lower than the limit voltage of the op amp to play a protective role. If there is a possibility of lightning strike, the TVS may not have enough absorption capacity, and the varistor is also necessary, but the leakage of the varistor itself will cause certain errors.

 

During the operation of the overcurrent protection device, errors such as sensor disconnection and short circuit may occur. Or the input quantity itself is likely to exceed the range. The transducer must ensure that the output will not rise without limit under any circumstances, otherwise it may damage the transmitter itself, the power supply, or the remote display instrument. In Figure 4, Rb and Z1 constitute the overcurrent protection circuit. No matter what causes the OP1 output to be greater than 6.2V (1N4735 is a 6.2V voltage regulator), it will be clamped by Z1, and the base of Q1 cannot be higher than 6.2V. Therefore, the voltage on Re cannot be higher than 6.2-0.6=5.6V, so the total current will not be greater than Ue/Re = 5.6V/200=28mA.

 

 

 

Wide voltage adaptability
Generally, two-wire transducers can adapt to a wide range of voltage changes without affecting accuracy. This can be applied to various power supplies and can adapt to large load resistances. The most sensitive part to the power supply is the reference source, and the reference source is also the main component that determines the accuracy. In Figure 4, the reference is limited by R5. When the power supply voltage changes, the current on R5 also changes, which has a great impact on the stability of the reference. In Figure 4, the constant current source LM334 is used to power the reference. When the voltage changes over a large range, the current remains basically unchanged, ensuring the stability of the reference.

 

 

Decoupling capacitor
In general circuit design, there will be decoupling capacitors at the power supply end of each integrated circuit. When the two-wire transmitter is powered on, the charging of these capacitors will cause a large current in an instant, which may damage the remote instrument. Therefore, each decoupling capacitor is generally not more than 10μF, and the total decoupling capacitor should not exceed 50μF. A 10μF capacitor is required at the entrance to ensure that the circuit does not oscillate under long-line inductive loads.

 

 

Aiming at the requirements of two-wire 4-20mA transmitter on power consumption, anti-interference ability, temperature drift, etc., the application circuit is designed. A pressure sensor is applied in this circuit, which is a pressure transducer. If the pressure sensor is replaced by a temperature sensor, it is a temperature transducer.

 

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