Three-wire PT100 temperature transmitter

I. Introduction

Some time ago, I saw a document "Design of Three-wire PT100 Thermal Resistor Temperature Measurement Circuit", author: Liu Wei and Li Jing (the relevant article is attached in the attachment). It introduces the use of operational amplifiers to build a constant current source to power the PT100 thermal resistor, and the use of three-wire measurement principles to eliminate errors introduced by wires.

2. System Principle

The connection between the PT100 sensor and the field instrument will be long, and the wire resistance of the wiring will introduce measurement errors. Therefore, in industry, a three-wire system is often used to eliminate errors introduced by the wires. The three-wire measurement principle is shown in Figure 1.

During measurement, the wire resistances are rL1, rL2, and rL3, and the three wires are of the same specifications and lengths. Therefore, RT is the resistance of PT100, and the measurement circuit at U1 and U2 points at the measurement end adopts a high-impedance input circuit. To measure the resistance of RT, add a constant current I to the U1 terminal.

Then the voltage U1 is:

U1=I×(rL1+RT+rL2)=I×(RT+2rL) (1)

Since the measurement terminal of U2 is a high-impedance input terminal, no current flows through the wire, so:

U2=I×rL3=I×rL (2)

Subtract 2 times the formula (2) from the formula (1) to get:

U1-2U2=I×(RT+2rL)-2×I×rL=I×RL=Uab (3)

Therefore: RT= (U1-2U2)/I (4)

In formula (4), the influence of the wire resistance rL on the measurement has been eliminated. It can be seen that the measurement only needs to provide a constant current I and measure U1-2U2.

3. Design of three-wire measurement circuit

Based on the above analysis of the three-wire principle, the thermal resistance three-wire measurement circuit is designed as shown in Figure 2. The circuit consists of a constant current source circuit and a differential amplifier circuit. The constant current source circuit is mainly composed of the voltage reference chip LM358-2.5, the high-precision operational amplifier KTA2333, and the transistors Q1 and Q2.

The constant current source circuit uses the integrated voltage reference chip LM385-2.5 to provide the reference voltage Ud=2.5V, so the voltage of the non-inverting terminal ③ of the op amp is 5V-Ud; according to the virtual short characteristics of the op amp, it can be concluded that the inverting terminal ② of U1.1 The voltage of the pin is also 5V-Ud. That is, the emitter voltage of transistor Q1 is 5V-Ud, and the voltage at both ends of current sampling resistor R2 is 5V, and the other end is connected to the emitter of Q1, so the voltage applied to both ends of R2 is actually: 5V-(5V-Ud )=Ud. Therefore, the current flowing through R2 is I=Ud/R1=2.5V/2.7K≈0.926mA. According to the virtual break characteristics of the op amp, no current flows between R2 and the inverting terminal ② pin of U1.1, so all the current on the resistor R2 flows into the emitter of the transistor Q1, and the IC1= of the composite transistors Q1 and Q2 IE1-IB2, where IC1=β1β2IB2. Since the β values ​​of Q1 and Q2 are generally above 100, IC1>10 000×IB2, so it can be approximately considered that IC1=IE1, the error is less than 0.01%, which can be ignored, so the transistor Q1 The collector current is the emitter current, and the influence of the power supply voltage +5V is eliminated during the operation. The error is only related to the resistor R2 and the voltage reference U2. Therefore, the R2 resistor should choose a metal film resistor with smaller temperature drift.

The differential amplifier circuit is mainly composed of operational amplifier U1.2 and resistors R3, R5~R9. Its input and output transfer functions are as follows:

Uo = (R9+R8)/R9 * [ (R6 + R7)/(R3 + R5) * R5/R6 * U1 - R7/R6*U2 ] = 11 * (U1 - 2U2)

RT = (U1-2U2)/I，I = Ud/R2

So, RT = Uo/11/I = (Uo * 2700 )/ (11 * 2.5)

The signal differentially amplified by the operational amplifier U1.2 is low-pass filtered by R10 and C4 and then sent to the AD converter for digital measurement. According to the measured RT value, it can be obtained by searching the PT100 index table and performing interpolation operations. temperature value.

In order to improve the measurement accuracy, the commonly used KTA2333 low-noise and low-temperature drift precision operational amplifier is selected as the operational amplifier in the circuit. Its input offset voltage is less than 10μV, input bias current is ±100pA, input offset voltage is ±120pV, and offset voltage drift is only 0.05μV/℃.

4. Introduction to other functions

The microcontroller uses STC's STC8H3K32S2-45I-LQFP32, which has a 12-bit ADC and can meet the sampling accuracy requirements of this solution. And add TL431 to the circuit. After adjusting the potentiometer to an accurate 2.5V, the power supply can be calculated after sampling by the microcontroller, thereby accurately calculating the voltage value after PT100 transmission.

The display part uses TM1650, which can drive a 4-digit 8-segment digital tube for temperature display.

The communication part uses MAX485, and the temperature value is printed in real time through RS485.

The power supply part uses the XL2009 step-down chip, and the input can be powered by a wide voltage of 8V ~ 36V.

5. Debugging process

1. First check the power supply part: whether the +5V power supply is normal

2. Verify the circuit accuracy of the PT100 resistance detection part

First test the current resistance of PT100 which is 111.0R

Connect the PT100 to the circuit board, measure the output voltage amplified by the op amp to be 1.13V, substitute it into the formula RT = Uo/11/I = (Uo * 2700 )/ (11 * 2.5), and calculate the resistance value to be 110.9R.

Checking the PT100 index table, the error is about 0.25°C, and the accuracy of the hardware part can meet the needs.

3. Debugging part of the program

This part refers to the attached program code. (The compilation environment is based on Keil C51 V9.00 + TKStudio V4.5.1)

6. Results display