Intelligent low resistance measuring instrument based on PIC microcontroller-EEWORLD

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introduction

Low resistance testers can be divided into two categories according to the size of their test current: one type has a larger test current, which is mainly used for measuring the DC low resistance of products such as connectors, switches, and conductors; the other type has a very small test current (generally around 1mA), which is used for measuring the DC low resistance of components such as connectors and switches in electric detonators, ignition tools, or other dangerous and explosive occasions. This type of measuring instrument has very high requirements for safety performance, and multiple protection circuits must be added. Safety and reliability must also be considered when designing and wiring PCBs. Since 1989, we have continuously improved and perfected circuit design, and designed and produced four models of low resistance measuring instruments. Here we introduce the latest DZC-4 intelligent resistance measuring instrument.

1. Overall plan and technical indicators

The instrument can be divided into five parts: power supply, precision constant current source, precision voltage amplifier, A/D converter, single chip controller. See Figure 1 Main technical indicators:

1. Test range: 0~20Ω, 0~200Ω, 0~2kΩ (three ranges automatically switched);

2. Maximum resolution: 0.001Ω;

3. Test current: 0.5mA;

4. Test accuracy: ±(0.2%+2);

5. Power consumption of the whole machine: <30mA.

2. Hardware Design

1. Power supply: The whole machine is powered by 6 No. 5 NiMH batteries (7.2V), and a universal DC/DC converter is used to convert the battery voltage into a stable ±5V DC voltage. This part also has a battery voltage monitoring circuit and a charging circuit. A fully charged battery can provide continuous use of the instrument for about 50 hours.

2. The constant current source consists of a precision reference voltage source and a high-performance operational amplifier, which provides accurate test current to the resistor under test. We choose the test current as 0.5mA.

In addition, the precision current source is the weak link of the entire instrument's safety performance. It is necessary to consider the damage caused by various internal and external factors to the instrument, thereby affecting the safety performance. There are two main measures: a) Limiting the current Use the constant current characteristics of the junction field effect tube to limit the test current ITEST, generally taking IDSS≈2*ITEST. To increase reliability, two field effect tubes are connected in series.

b) The voltage is limited by connecting a voltage-limiting diode in parallel to the test terminal.

3. The precision amplifier uses a chopper-stabilized zero op amp to amplify the test signal in phase, because the full-scale voltage of the A/D converter is 2V, and the test current is 0.5mA, corresponding to the in-phase amplification factors of 20, 200, and 2k ranges of 200, 20, and 2k. Two analog electronic switches SW1 and SW2 controlled by a single-chip microcomputer are connected to the negative feedback loop to switch between different amplification factors of the three ranges: when the range is 20Ω, SW1 and SW2 are both closed, when 200Ω, SW1 is opened, and when 2KΩ, SW1 and SW2 are both opened. The instrument should be calibrated from the low range, otherwise all ranges cannot be calibrated. In addition, Ri, R1, R2, and R3 should use precision resistors and precision potentiometers to minimize the influence of temperature.

4. The A/D converter (see Figure 2) uses a 4 1/2-bit ICL7135 chip, which is connected to a voltmeter mode with a full-scale range of 2V. The ICL7135 provides data to the microcontroller in the form of a 5-bit BCD code, and provides over-range (OV) and under-range (UN) signals to the microcontroller for automatic range switching. The clock frequency of the ICL7135 comes from the Q5 terminal of T5 (CD4060), with a frequency of 125KHz, which is exactly an integer multiple of the power frequency of 50Hz, and can improve the instrument's ability to resist power frequency interference. The A/D conversion frequency is approximately: 3.3 times/second. The COUT terminal of T5 also provides a 4MHz clock frequency to the microcontroller.

5. Single-chip microcomputer controller (see Figure 2) The functions of this part include: data acquisition, processing, display, range switching, voltage monitoring, etc. The display module of this machine has two 74LSl64 chips, which are used for the bit drive and segment drive of the LED digital tube respectively, with a total of 5 digital displays. Pin 25 of the single-chip microcomputer T2 is used to turn off the display of the display module to prevent the digital tube from displaying garbled characters during data transmission. Pin 24 of T2 is the battery undervoltage detection input, and pins 23 and 22 of T2 control the analog electronic switch of the precision amplifier to produce the required amplification factor.

3. Software Design

The biggest feature of this instrument is that it uses software to achieve automatic zero adjustment and range conversion. It eliminates the need for high potential energy and range switching switches. The method of software zero adjustment is: after turning on the machine, the single-chip microcomputer performs self-test. If the system works normally, read the A/D conversion results. After reading 5 A/D conversion results in succession, determine whether they are all less than 0.2 Ω. Otherwise, it is considered that the operator has not reliably short-circuited the test rod, and the instrument continues to display the zero adjustment prompt. If 5 consecutive values are all less than 0.2Ω, then find the minimum value among them as the initial value, and subtract the initial value from each subsequent measurement result.

The single-chip microcomputer automatically switches the range according to the over-range and under-range signals of the A/D conversion chip 1CL7135. The range switching needs to complete three tasks: first, switch the magnification of the precision amplifier, then adjust the number of effective digits of the initial value, and finally adjust the position of the decimal point. The PIC16C57 single-chip microcomputer has no interrupt function, so the query method is used to communicate with the A/D converter, and the data is displayed using the gap between A/D conversions.

The PIC microcontroller and the 74LSl64 in the display module use serial data communication, and only one bit of data is displayed at a time. To prevent the display from flickering, the refresh rate should be greater than 30Hz.

The program flow chart is shown in Figure 3.

IV. Conclusion

The instrument and the resistor to be measured adopt the four-wire connection method, which can eliminate the influence of the connector resistance. In addition, since the instrument has a high resolution, the test fixture is required to be silver-plated. This is very important, otherwise it will cause the test results to drift.

The test results of mass production of the instrument show that the technical indicators of the instrument meet the design requirements and can meet the needs of production and scientific research. If the instrument is slightly improved, it can be used as a highly sensitive DC voltmeter and ammeter.

Keywords：PIC16C57 Reference address：Intelligent low resistance measuring instrument based on PIC microcontroller

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