Thread Content
original: Production and use of simple 4-20mA current signal source http://yunrun.com.cn/tech/2571.html The 4-20mA signal source is usually provided by a process calibrator, but it is expensive. When you just want to use a 4-20mA current signal to check instruments and determine the location of the fault during on-site maintenance, using a high-end process calibrator is a bit overkill, especially for some small and medium-sized enterprises. Due to the limited number of instruments, if the instrument technician can make a simple current signal source by himself, it not only meets the needs but also exercises his hands-on ability. Why not do it. The 4-20mA current signal source production plan uses a three-terminal voltage regulator to produce the current signal source. The circuit is simple, low-priced, and easy to obtain materials. Both fixed or adjustable three-terminal voltage regulators can be used to produce the current signal source. LM317 is a three-terminal adjustable positive voltage regulator. For the appearance and pins of LM317, Changhui Instruments recommends referring to the product technical information. If an adjustable resistor is connected between the adjusting terminal and the output terminal, an adjustable precision current stabilizer can be formed. The current stabilizing circuit recommended in the LM317 product information is shown in Figure 1. LM317 is a three-terminal floating voltage regulator. When working, LM317 establishes and maintains a nominal reference voltage Vref of 1.25V between the output and the regulating terminal, and the current IAdj of the regulating terminal is designed and controlled below 100μA and made constant. Then the output current of this circuit is: Where Vref is the nominal reference voltage, which is 1.25V ； IAdj is the adjustment terminal current ； R1 is the programming resistor. http://yunrun.com.cn/upload/201906/26/201906260030255104.png Figure 1 The current stabilizing circuit recommended in the LM317 product information. The manufacturer recommends that the minimum stable current of this circuit is 10mA. However, the minimum stable operating current of LM317 is generally 1.5mA. Different manufacturers and models of LM317 have different minimum stable operating currents. When the output current of LM317 is greater than the minimum stable operating current, LM317 can work normally. Through practical application, LM317 can meet the usage of 4-20mA. 4-20mA current signal source production method. The steady current source made with LM317 is shown in Figure 2. The input Vin terminal of LM317 is connected to the positive pole of the power supply. The output Vout terminal is connected to a resistor to become a steady current output. The adjustment terminal is directly connected to the steady current output terminal Iout. The load RL is connected between the output terminal Iout and the negative pole of the power supply. Since the nominal reference voltage Vref of LM317 is 1.25V, and this voltage remains constant, this voltage is between the Vout terminal and the adjustment terminal ADj, that is, the two ends of the resistors R1 and RP1. If the resistance value is fixed and the voltage is also fixed, the current flowing through the load RL can be constant. That is, the current Iout is: The resistance values of R1 and RP1 can be determined by using the above formula. When the output current is 4mA: When the output current is 20mA: http://yunrun.com.cn/upload/201906/26/201906260031147806.png Figure 2 Schematic diagram of the LM317 current stabilizing source circuit. When the variation range of resistors R1 and RP1 is 312.5-62.5Ω, the output current is 4-20mA. The above calculation is an ideal state. Due to the discrete nature of electronic component parameters, certain adjustability must be taken into consideration when selecting the resistance value. Therefore, by selecting a 51Ω fixed resistor and connecting a 360Ω potentiometer in series, the 4-20mA current can be adjusted. The 51Ω resistor is wound with manganese-copper resistance wire, and the 360Ω potentiometer is an X-type multi-turn wire-wound potentiometer. After actual debugging, the working power supply of this current stabilizer can work normally within the range of 9-24V. For the convenience of on-site portability, I also tried reducing the power supply to 7.5V, but found that it could not be adjusted to 20mA. According to the information of LM317, its output voltage changes and has certain voltage difference requirements. Moreover, when outputting 20mA, its output voltage must be greater than or equal to 5V. From the formula, we know that the output current increases when the resistance decreases, and decreases when the resistance increases. but people * The inertial adjustment direction is that when the potentiometer is rotated clockwise, the output increases, and when the potentiometer is rotated counterclockwise, the output decreases. Therefore, when welding the pins of the potentiometer, pay attention to this relationship to meet the operation requirements. * used. The power supply of the signal source takes the form of self-preparation or external connection. You can use a 6F22 type 9V laminated battery. If there is a power supply on site, use an external power supply, but the power switch S must be turned off. In order to prevent misoperation, a 1N4001 diode V is connected in series to the laminated battery circuit for protection. How to use the 4-20mA current signal source. This current source can be used to replace the output current of the transmitter. It can be used on-site to check whether the current signal transmission from the transmitter to the control room is normal, whether the digital display instrument has an indication, or whether it can change according to the increase or decrease of the current signal. ； But the specific value of the current output signal is not necessarily required, as long as the output signal can increase or decrease the change. When checking the digital display instrument in the laboratory, it can be used as a current signal source. Before use, connect the relevant connections, turn the potentiometer counterclockwise to 4mA, connect the digital display instrument to be inspected to the output terminal, and connect the input end of the instrument to be inspected in series to a standard ammeter, or in parallel to a digital voltmeter through a 250Ω resistor. ； After turning on the working power supply, adjust the potentiometer RP1 to obtain the required current signal value. The output current value should be based on the indication of a standard ammeter or digital voltmeter. source ； Changhui Instruments http://yunrun.com.cn/