Now a lot of sensors are using current signal output, such as pressure sensors, temperature sensors, Angle sensor, displacement sensor etc. Current output type transducer output range of commonly used have 0 ~ 20 ma and 4 ~ 20 ma two, minimum current transmitter output current and the maximum current, represent the current transducer calibration by the minimum and maximum rated output value.

Output current sensor has many advantages, the current signal output what equipment we should follow-up connection? For 0 ~ 20 ma transmitter, we need to choose the appropriate step-down on circuit design resistance, in the A/D converter input interface directly to the resistance of 0 to 5 v or 0 to 10 v voltage can be converted to digital signal, circuit debugging and data processing are relatively simple. But the disadvantage is unable to distinguish transmitter damage, unable to distinguish between transmitter output open circuit and short circuit. For 4 ~ 20 ma output of transmitter, the circuit debugging and data processing are quite tedious. But this kind of transmitter can when transmitter circuit impassability, short circuit or damaged by can detect the normal range of current, to judge whether circuit malfunction, whether the damage in the transmitter, so get more extensive common use. Due to 4 ~ 20 ma ma transmitter output 4, on the sampling resistor voltage is not equal to zero, directly by the analog to digital conversion circuit converted digital quantity is not 0, MCU can't direct use, through the formula to calculate too complicated. So the general processing method is by hardware circuit to the resulting voltage drop is 4 ma on the sampling resistance, and then to A/D conversion. Indisputably RCV420, this kind of hardware circuit is a kind of precision I/V transform circuit, and the application LM258 since I/V transform circuit, the circuit produced by two wire current transducer with 24 V and 4 ~ 20 ma current sampling resistance to form current loop, thus on the sampling resistor to produce a 1-5 V voltage, and voltage input to the amplifier LM258 3 feet. Resistor voltage divider circuit used in integrated circuit LM258 2 feet have a fixed voltage value, used to offset on the sampling resistor 4 ma current produced by the pressure drop. So when two wire current transducer for minimum 4 ma, LM258 3 feet and 2 feet basic to 0 v voltage difference. LM258 in connection with the resistance of the structure can adjust the voltage amplification circuit, the two wire current transmitter current on the sampling resistor voltage amplification and output through LM258 1 foot to analog/digital conversion circuit, for single chip microcomputer CPU reads, through the data processing method two wire current transducer 4-20 ma current on LCD/LED screen displayed in the form of a value of 0-100 - a.

Sensor output current has a lot of advantages, the future will be more sensors using current signal output. So we must know how to deal with the current signal, so as to help us better use of the sensor.