Design principle of automatic capacitance inductance tester
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1. Capacitance Inductance Measurement Principle The oscillation principle of the LC oscillator is used. The LC oscillator selects the L or C parameter as a fixed value. With the combination of LCs, the oscillator starts to oscillate, the inductor is fixed when measuring the capacitance, and the capacitance is fixed when measuring the inductance. By calculating the frequency of the LC oscillator, the value of the capacitor or inductor to be measured can be calculated.
2. Circuit Working Principle 2.1 The capacitive and inductive circuit block diagram design is shown in Figure 1. The block diagram includes an input switching section, an oscillation section, a frequency division section, a microcontroller section, a display section, and a keyboard section. This system is controlled by the STC89C51 microcontroller as the core, the input switching part of the use of double-pole double-throw relay to complete the line switching of the capacitor or inductor to be measured, the oscillation circuit works in the amplified resonant state, the frequency of high-frequency tube 9018 collector output, due to the frequency High, so need to be divided by the signal, and because the voltage amplitude of the output is large, there is no need to add a driver at this point. With the 74LS393 digital frequency divider chip, the crossover ends are cascaded to achieve 100 frequency division, and the final signal enters the single-chip microcomputer. The frequency is calculated by the SCM and the algorithm is designed to determine the unknown capacitance or inductance parameters. Figure 1 shows the overall framework of the system.
2.2 Automatic capacitance inductance tester input switching circuit The input switching circuit is implemented using a double-pole double-throw relay. It is mainly responsible for input switching of capacitors and inductors. When the capacitor is connected, the system passes the relay K2, as shown in Figure 2. Connect the MCU, the fixed end of K2 is directly connected to pins IO3 and IO4 of the MCU, the normally open node is connected to the pin of the capacitor or inductor to be measured, and the two pins are initially set to a logic high level of 5V, one is logic Low level 0V, when K2 is energized, the fixed end and the normally closed end are connected because IO3 and IO4 are 5V and 0V, respectively. The capacitor is open to DC, so the IO3 and IO4 levels remain in their original state. If it is an inductor, 5V IO will be pulled low by 0V because the inductor is equivalent to the conductor for DC. Both IOs are 0V. Therefore, when there is no short-circuit together, the microcontroller determines the capacitance and selects the method of measuring the capacitance. At this time, the MCU sets the other IO1 pin to switch the other double-pole double-throw switch K1. The switch is connected to the capacitor. Parallel C2, as shown in Figure 2. When the short circuit is together, the SCM determines the inductance, and the SCM selects the method of measuring the inductance. At this time, the single-chip double-pole double-throw switch K1 switch is set to the lower side through the setting of the IO1 pin, that is, in parallel with the inductance L.
2.3 Principle of Oscillating Circuit of Automatic Capacitance Inductance Tester The oscillating circuit uses an LC oscillation circuit. The frequency of the oscillation is determined by L and C. Oscillating tube with 9018, Rb1 and Rb2 as the base bias, Rc is a current limiting resistor, capacitor C1, C2 and inductor L constitute a positive feedback frequency selection network, the feedback signal from the capacitor C2 ends. This circuit is also called a capacitive 3-point oscillation circuit. The input signal and feedback signal are in phase. In the measurement process, when the inductance is measured, the input circuit automatically connects the inductance Lx to be measured to both ends of L in parallel. When measuring capacitance, the input circuit automatically connects the capacitor Cx to be measured across both ends of C1.
2.4 Frequency dividing circuit principle The frequency dividing circuit adopts 74LS393 digital frequency dividing chip, the frequency dividing end cascades to achieve 100 frequency dividing, the collector output oscillation signal of the high frequency tube 9018, then divides the frequency oscillator output signal 100 frequency, the frequency will drop Within the scope of the microcontroller measurement.
2.5 Automatic Capacitance Inductance Tester SCM Realizes Calculation of Capacitance and Inductance When connecting the capacitor or inductor to be measured, the system will automatically judge and determine the algorithm according to the judgment result. When it is judged that it is a capacitor, the system counts the calculation method of the capacitor. The calculation method of the capacitor uses the formula to obtain the value of Cx from the measured frequency and the known L and C2. When the inductance is judged, the system enters the calculation method of the inductance, and the inductance calculation method uses the formula to calculate the value of Lx according to the measured frequency and the known C1, C2, and L.
3, the algorithm design system power initialization and clear screen, MCU initialization is completed, enter the keyboard scan program, when you want to measure capacitance or inductance, select the measurement button, the system automatically determine and perform capacitance or inductance measurements. When it is judged as capacitance, the system selects the calculation method of the capacitance. When the inductance is judged, the system selects the calculation method of the inductance. The measurement results are displayed on the LCD screen after the calculation is completed. The following is a specific program flow chart, as shown in Figure 3.
4. Automatic Measurement Data and Analysis of Automatic Capacitance Inductor Tester 4.1 Methods to Improve Measurement Accuracy This system is used for capacitance and inductance measurement. Due to the thermal stability of the device and external interference to the circuit, the measurement results will be The jump occurs because the junction capacitance of the transistor changes with the temperature, which affects the measurement result. This is also the key reason for the instability of the capacitor three-point oscillation circuit. Based on the above reasons, the measurement accuracy can be improved by using multiple measurements to average the measurement process.
4.2 The actual measurement circuit fixed parameters are as follows: Rb1 = 10kΩ, Rb2 = 10kΩ, Rc = 4kΩ, Re = 4.7kΩ, Cb = 1μF, Ce = 0.1μF, select different capacitors were tested 3 times, get Table 1. Choose different inductors to test 3 times, and get Table 2. The table shows that the measured value and the nominal value are close to each other, indicating that the system design scheme is correct and meets the requirements for the accuracy of electronic component parameters used in general laboratory and engineering design.
Automatic capacitance inductance tester This system uses a combination of microcontroller and oscillator start-up to calculate capacitance and inductance values. The system has more intelligent measurement methods and simple and respectable customers:
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