The difference between two-wire system (two-wire system), three-wire system and four-wire system

The terms two-wire system (two-wire system), three-wire system, and four-wire system generally refer to the wiring method of thermal resistors or sensors. The number of wires connected is called the number of wires. The number of wires for thermal resistors and sensors is different, but the most widely used is the three-wire system. Let's introduce the differences between the three connection methods of thermal resistors and sensors.

Let's first talk about the three connection methods of thermal resistors:

Two-wire system: The method of connecting a wire at each end of the thermal resistor to lead out the resistance signal is called the two-wire system. This lead method is very simple, but since the connecting wire must have a lead resistance r, the size of r is related to the material and length of the wire, so this lead method is only suitable for occasions with low measurement accuracy.

Three-wire system: The method of connecting a lead at one end of the root of the thermal resistor and two leads at the other end is called the three-wire system. This method is usually used in conjunction with a bridge, which can better eliminate the influence of lead resistance and is the most commonly used in industrial process control.

Four-wire system: The method of connecting two wires at both ends of the root of the thermal resistor is called a four-wire system. Two of the leads provide a constant current I for the thermal resistor, converting R into a voltage signal U, and then leading U to the secondary instrument through the other two leads. This lead method can completely eliminate the resistance influence of the lead, and is mainly used for high-precision temperature detection.

Let's talk about the three connection methods of the sensor:

Two-wire system: The sensor has only two wires, one for the power supply, and the other is the signal line connected to the other pole of the power supply after passing through the instrument. Wiring is the simplest.

Three-wire system: The sensor leads out three wires, namely the positive pole of the power supply, the negative pole of the power supply, and the signal output. The three-wire connection method is the most commonly used.

Four-wire system: The sensor has four wires, two for the power supply and two for the signal line.

It should be noted that the power lead of the sensor is divided into positive and negative poles, and the signal line is divided into NPN and PNP. It cannot be connected incorrectly, which may cause damage to the sensor.