Thermal resistance three-wire system wiring method

Two leads are connected at one end of the thermal resistance sensing element, and one lead is connected to the other end. The form of the lead is called a three-wire system. It can eliminate the influence of the inner lead, and the measurement accuracy is higher than the two-wire system, which is often used in the case where the temperature measurement range is narrow, the wire is too long, or the temperature in the wire wiring is liable to change.

The three-wire lead method is often used in conjunction with the bridge circuit. The two wires are connected to the two bridge arms of the bridge, and the other wire is connected to the power supply of the bridge, eliminating the influence of the lead resistance change. That is, the three-wire lead method can reduce or eliminate measurement errors caused by variations in lead resistance.

In production, thermal resistance temperature meters are mostly measured with an unbalanced bridge. The measuring circuit principle is as shown in Figure 1. Since the resistance value of the copper wire connecting the thermal resistor to the bridge changes with the change of the ambient temperature, if only the connecting wire is connected to a bridge arm, when the ambient temperature changes When connecting the wire resistance.

The change value will be superimposed with the resistance change value of the RTD, resulting in an additional error. Therefore, in the industry, the three-wire wiring method is generally adopted, and the wires 2 and 3 are respectively connected to the two bridge arms of the bridge, when the resistance of the wires changes. Can offset each other to reduce the impact on the value of the instrument. However, the error reduction is limited. For an unbalanced bridge, full compensation can only be obtained at the beginning of the meter scale, and the above additional error is maximum at full scale.

For the unbalanced bridge, the additional temperature error of the power supply lead should also be considered. When there is a current flowing through the superheat resistor to connect the power supply wire 1, there will be a certain voltage drop. When the ambient temperature changes, the upper and lower branch voltages of the bridge It will also change, which will bring some additional temperature error to the meter.

The three-wire wiring method must be used in conjunction with the corresponding wire-made RTD components to achieve true three-wire wiring. However, in reality, many manufacturers use thermal resistance, and most of the thermal resistance components in the protection tube have only two leads, that is, the thermal resistance component is two-wire, although three connection wires are used from the protection tube junction box to the display instrument. But this can only be regarded as a two-wire thermal resistance wiring method, or a three-wire thermal resistance two-wire wiring method.