Current type temperature and humidity sensor interface processing method Knife Gate Valve,Knife Gate,Knife Edge Gate Valve,Knife Edge Valve Haogong Valve Co Ltd , https://www.haogongvalve.com
   The output range of the current output type sensor is usually 0 to 20 mA and 4 to 20 mA . When the current type sensor outputs the minimum current and the maximum current, it represents the minimum and maximum rated output values ​​calibrated by the current type sensor.
The following is an example of a current-type temperature and humidity sensor with a measurement range of 0 to 100A . For the output current 0 ~ 20mA transmitter 0mA corresponding input value 0A, output 4 ~ 20mA transmitter 4mA corresponding to an input current value 0A, 20mA current 100A corresponds to two sensor values. Welcome to the company's official website http://
For the transmitter output 0 ~ 20mA , we only need to select the appropriate step-down resistor in the circuit design, directly convert the 0-5V or 0-10V voltage on the resistor to the digital signal at the input interface of the A/D converter. Yes, circuit debugging and data processing are relatively simple. However, the disadvantage is that it is impossible to discriminate the damage of the transmitter, and it is impossible to distinguish the open and short circuit of the transmitter output.
For transmitters with 4 to 20 mA output , circuit debugging and data processing are cumbersome. However, this kind of transmitter can judge whether the circuit is faulty, whether the transmitter is faulty when the transmitter line is not connected , when the short circuit or when it is damaged, can it detect the current in the normal range (the minimum value is 4 mA at normal time ) . Damaged , so it is more widely used.
Since the 4 to 20 mA transmitter output 4 mA , the voltage on the sampling resistor is not equal to 0 , and the digital quantity directly converted by the analog-to-digital conversion circuit is not 0. The single-chip microcomputer cannot be directly utilized, and the calculation by the formula is too complicated. Therefore, the general processing method is to eliminate the voltage drop generated by the 4 mA on the sampling resistor through the hardware circuit , and then perform A/D conversion. This type of hardware circuit is the first to push the RCV420, which is a precision I/V conversion circuit .
There is also an LM258 self-contained I/V conversion circuit that uses a 4 to 20 mA current generated by a two-wire current sensor to form a current loop with 24V and a sampling resistor, resulting in a 1-5V drop across the sampling resistor. and inputs this voltage to pin 3 of amplifier LM258. Resistance voltage dividing circuit for generating a fixed voltage value at pin 2 of the integrated circuit LM258, 4mA for canceling the current generated in the voltage drop on sampling resistor. When the two-wire current-type sensor is a minimum value 4mA, LM258 3 feet and 2 feet difference voltage is substantially 0V. The LM258 and its connected resistor form an adjustable voltage amplifying circuit, which amplifies the voltage value of the two-wire current sensor current on the sampling resistor and outputs it to the analog / digital conversion circuit through the 1 pin of the LM258 for the CPU of the microcontroller to read in. The 4-20 mA current of the two-wire current sensor is displayed on the LCD/LED screen by a data processing method in the form of a 0-100 A value.