PIC18F258-based automatic thermal resistance tester Research and Development

As a common thermistor temperature sensor is widely used in consumer electronics, industrial control, communications, aviation and other fields. Thermal resistance in the production process, the resistance of the performance test and calibration link is critical. This paper presents a thermal resistance can be achieved on the performance of automated testing and analysis equipment - thermal resistance tester, it can at the same time 15 thermistor performance test, the test process, the user can host machine interface to control the test temperature range and temperature interval. By measurement data, the user can draw the Excel spreadsheet program all the heat resistance temperature curve, which can easily identify the defective resistor. The instrument on the thermal resistance reduces the average time to detect and improve factory productivity.

The basic principle of measurement

Thermal resistance of the test need to be conducted into the incubator, the tester also needs to incubator temperature was measured, the temperature can be determined according to different resistance of the thermistor. In the thermostat temperature measurement process, the use of platinum resistance PT1000 as a sensor, the temperature into voltage analog-digital conversion. Analog-digital conversion process mainly uses low-noise programmable 24-bit ADC, its internal use of Σ-Δ converter technology, A / D data collected up to 14 digits or so, making the resolution of the temperature accuracy of 0.05 degree. In this case, the thermal resistance of the temperature during the test interval is relatively small, precise measurements for the thermal resistance has laid a good foundation, but also further expand the use of this instrument.

Incubator temperature acquisition

In the thermal resistance measurement process, accurate acquisition of temperature incubator is essential. Accurate measurement of the thermal resistance is a prerequisite to high-precision, high-resolution detection of temperature. Collection of temperature on the thermostat from the circuit shown in Figure 1, Graph.

PIC18F258-based automatic thermal resistance tester Research and Development

In the circuits, the voltage source of voltage produced by the AD AD780 provides precision voltage source, its output voltage is 2.5V ± 1mV, temperature coefficient of 5ppm. Precision voltage source for the accurate measurement of the introduction provides a good basis for the temperature. Use of platinum resistance temperature sensor PT1000, it is in the range of 0 ~ 100 ℃ good degree of linearity, high sensitivity, temperature coefficient of 0.4%, suitable for temperature measurement using surface or slit. Resistance R0 selected resistance 1.5kΩ ± 0.05% for the high-precision resistors, the temperature coefficient of 5ppm. PT1000 platinum resistance at the temperature between -50 ℃ ~ 100 ℃ good linearity. Therefore, the tester's temperature range between -50 ℃ ~ 100 ℃.

Thermistor resistance measurement

Measuring thermistor resistance of the circuit and the principle of measuring the temperature incubator is similar to Figure 2. The figure, the voltage source is formed by the LM317 adjustable voltage source, the size of 3.8V, R1 to R19 as 2kΩ high precision resistors, temperature coefficient (TCR) is 15PPM. Lower resistance to the unknown RX1 to RX19 thermistor. Link the two resistance points AIN1 to AIN19 AD7731 voltage was sent to the voltage input. Department Present thermistor resistance changes with temperature when, AINX point voltage is also changing, it can be measured by the change of resistance.

PIC18F258-based automatic thermal resistance tester Research and Development

Lower machine part


Thermal Resistance Tester hardware includes six modules, the hardware block diagram in Figure 3. The module features are outlined as follows:

PIC18F258-based automatic thermal resistance tester Research and Development

* Power supply circuit: +5 V power supply for analog and digital +5 V and +8 V power supply

* Power Circuit: +2.5 V for the AD7731 to provide high-precision reference voltage, the data acquisition circuit to provide +5 V voltage source.

* Data acquisition circuit: the resistance value into the appropriate voltage transmitted to the pin for the data acquisition AD7731 ADC. The actual data acquisition circuit shown in Figure 4.

PIC18F258-based automatic thermal resistance tester Research and Development

In the figure, the left part of the data acquisition circuit for the resistance, it middle of the protection circuit common to the thermistor resistance into a voltage. On the right is the thermostat temperature measurement circuit, it will PT1000 2.5V voltage through platinum resistance and a resistance to high-precision resistor 1.5kΩ after the partial pressure of the input into the AD.

* D converter circuit: from 4 AD7731 chips, on the acquisition of 20 analog inputs, collecting the results into the microcontroller. ADI AD7731 is developed by the United States with low noise, high pass rate and other characteristics of the Σ-Δ ADC. It can receive input signals from the sensors, for measuring wide dynamic range with low-frequency signal can be widely used in strain measurement, temperature measurement, pressure measurement and industrial process control and other fields. AD7731 chip in the test apparatus in the main task is to collect data, and data transfer to the microcontroller. Specific connections in Figure 5.

PIC18F258-based automatic thermal resistance tester Research and Development

Analog input and reference input are differential mode signal, added to the voltage on the analog modulator are common-mode voltage, while the AD7731 a good common-mode rejection ratio can remove the input common-mode noise. Digital filter can effectively remove the circuit board supply voltage noise. Therefore, AD7731 chip precision than traditional converters with more anti-jamming capability. However, due to the resolution of AD7731 is very high, so in the design of PCB board, we should pay great attention to shielding design and component layout of the location of relationship.

· PIC MCU and Serial Communication Module: Control AD7731 chip for data acquisition, and data received through the RS232 serial port to send to the host computer. Circuit in the microcontroller and serial communication circuit shown in Figure 6.

PIC18F258-based automatic thermal resistance tester Research and Development

The figure, MAX232 chip is responsible for the completion of TTL level to RS232 level conversion to PC, SCM, and the normal communication between. In this tester, RS232 serial communication speed using 9600bps, 8 data bits, 1 stop bit, no parity.

Software components

Thermistor Tester lower computer software to complete two major tasks:

* AD7731 chip through the SPI bus control data collection, and read into the data collected within the machine.

* Serial communication module is configured correctly, according to the agreement rate of the data sent to the host computer.

The specific operational procedures shown in Figure 7. Which, SPI and USART module belong to the internal MCU, only the relevant registers of module can be configured, and the AD7731 chip control is indirectly through SPI module.

PIC18F258-based automatic thermal resistance tester Research and Development

PC parts

Microsoft produced Visual Studio 6.0 is a common programming tools, thermal resistance tester PC software written using the tool. PC programs main function is to receive from the lower computer through RS232 sent to the A / D conversion data, processes, and the results after treatment to complete the Excel form for users to deal with. In the data processing using KALMAN digital filter to further improve the temperature measurement accuracy. System data through OLE Automation to Excel worksheet completed, if the user requests the data further, they are free to process the data. Program can meet the different needs of users.

Tester simple PC interface, the user is simple. The main interface shown in Figure 8.

PIC18F258-based automatic thermal resistance tester Research and Development

The upper part of the main interface has four buttons control are: start, stop, set and help.

Start button: Click after the start of the Microsoft communication control can be generated by the processing OnComm message, start Excel template file mytest.xlt, used for display data.

Stop button: Stop on OnComm message handler to refresh the bottom status bar of the display data.

Settings button: to achieve the calibration of temperature measuring range of measurement interval setting, within the parameters set tester. In the setting of internal parameters, first enter a password to prevent misuse of the calibration data.

In the parameter setting interface can be temperature, measurement range, within the parameters set. After the data is written to the file, start the program again set parameters will be automatically read.

Help button: Start help file. In the help file in detail the operation of the software instructions. Including hardware, connectivity, operating rules.


As sensor technology advances in the theory of thermal resistance and application development has made remarkable achievements. As the final link of the production process of the thermal resistance test, to improve their detection efficiency, is bound to shorten production time and lower production costs, so that the thermistor can be used more widely.

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