Resistive temperature sensor use precautions - Solutions - Huaqiang

inductance

(1) The resistance value must pass the current, but the current will cause the resistance to heat up and increase the resistance. In order to avoid the error caused by this factor is too large, you should try to use a small current through the resistor. Of course, the current is too small so that the voltage drop across the resistor is too small and can cause difficulties in measurement. It is generally believed that the current through the resistor should not exceed 6 mA, because the heat resistance of the Pt100 thermal resistance at 850 ° C is only 139 ohms, and the heating power is only 14 mW, which does not cause significant errors.

(2) When the resistance is measured by the bridge method, the output diagonal of the bridge has a common mode voltage. If the negative terminal of the bridge is the ground, the two output wires of the bridge have voltage to the ground, and the differential mode voltage between the two wires reflects the degree of imbalance. This puts certain requirements on the design of the electronic circuit, or requirements for the grounding point.

(3) When the resistance is measured by the potential difference, if a plurality of thermistors share the constant current source, there is also a common mode voltage. A plurality of thermal resistors are connected by a four-wire method, and the current lines can be connected in series, and are powered by the same constant current source. The two voltage lines connected to the potentiometer are respectively switched to the respective thermal resistors, so that multiple points of inspection can be performed. However, there are common-mode voltages on the two voltage lines, and the values ​​of the common-mode voltages are not equal. If you connect to the computer interface, you must pay attention to this feature. See Figure 2.4.5.

In the figure, 1, 2, 3, ..., n represent different measuring points, and all of them are measured by thermal resistance. The indexing numbers can be the same or different. Powered by the same constant current source, the current is I. Each of the two voltage lines is connected to the A/D conversion part of the computer via a multiplexer. A common integrated multiplexer can be used here, but two lines must be switched at a time. According to the solid-state connection method, the common mode voltage is sequentially increased (relative to the negative terminal of the constant current source).

(4) The thermal resistance of the same indexing number can be measured in series to measure the average temperature of multiple points. At this time, the two-wire connection method is used, and the more the number of thermal resistors connected in series, the larger the total RO. The thermal resistance does not encounter parallel conditions because it has no practical significance.