Power supply 1 DC resistance measurement principle In the power system, the direct current resistance of the inductive load is measured quickly and accurately, which is conducive to on-site detection, fault diagnosis and real-time maintenance of the power equipment. For example, measuring the DC resistance of the transformer winding is the transformer storage,% and! "The resistance is the standard resistance and the DC resistance of the inductive load under test; 4 is the inductance of the inductive load under test."
When measuring, close the button S1 and set the resistance! Short circuit, so that all the voltage is added to the load under test, forcing it to have a greater rate of current rise, until it reaches a predetermined current value and then disconnect the button S1, the current will quickly stabilize, set the stable power to /, from the ,"/!",%n: Larger DC current is conducive to the rapidity of inductive load DC resistance measurement; DC current with high stability is conducive to the accuracy of inductive load DC resistance measurement. Therefore, to study a high-capacity constant current source with high stability and strong load capacity directly determines the accuracy, rapidity, and reliability of the inductive load DC resistance measurement. 2.2. The high-precision, low-drift, dynamic zero-calibration CMOS-type chopper-stabilized monolithic integrated operational amplifier developed in the early 1980s is called the fourth-generation operational amplifier 6. Its input offset voltage. s is equivalent to only one thousandth of the general-purpose operational amplifier F007, s = 1.0! V, the temperature drift of offset voltage is 0.01! V/°C, monthly drift below 1! V, input bias current is about 100pA, and there is a very high common-mode rejection CM-RR! 130dB), open loop gain P140dB). In is the input reference voltage; VTR is the power amplifier part; 2 is the current limiting resistor of the VTR.
The working principle of the constant current source is briefly described as follows: After the reference source is divided down, the current is amplified by the current amplifier circuit VTR through the adder N>1 and feedback amplifier Na2, or /n). In order to stabilize the output current, in addition to the introduction of deep negative feedback at each stage, the output current sample is fed back to the adder N>1 via the voltage follower Nab to form a large feedback, which further enhances the stability of the output current and enables constant current. The source has a high degree of stability in the output current over a wide range of load variations.
2.1 The stability mechanism of the constant current source current The output voltage of the adder NA1: the output voltage 2 of the inverting amplifier NA2 is 2=-%1, and the follower NA3 is /n7"2-"3)/6. Solving the following equations is available: Since the operational amplifier NA3 constitutes a follower circuit, its input impedance is very high! 1012! / The current flowing through the standard resistor 6 flows to the inductive load under test. Equation 1) shows that the current is independent of the inductive load being measured; the current is linear with the input voltage “mâ€; the stability of the input voltage “6â€, ripple size, temperature drift characteristics, and quiescent current size all directly affect the output current! The standard resistor 6 and the resistor 1 that makes up the constant current source circuit directly affect the output current! .
In the wiring, when the grounding points of the three operational amplifiers are not properly arranged, the interference signal formed on the ground line will be connected to the nearby circuit, resulting in unstable operation of the constant current source. The grounding points of the reference source, adder, and feedback amplifier are put together and connected to the power supply ground. As shown in the figure, the interference can be effectively suppressed. 2.2 The amplifier drive technology is used to measure the DC resistance of large-scale power equipments. To improve the accuracy and rapidity of measurement, the test instrument is required to have a large constant current output capability of 0.110A), a very small ripple, and a high degree of stability, so that it can be used for inductive loads such as large transformer windings of 110 kV or more. Accurate and reliable measurement of DC resistance. The maximum current that can be stably output by the constant current source studied in this paper is 10A, and it is used to measure the DC resistance of inductive loads of 100 or more levels. Since the output current of a general integrated operational amplifier is relatively small, it is not possible to drive a large power tube directly. Therefore, in order to obtain a 10A constant current source, it is necessary to adopt a flow extension technique. This article uses a pre-driver-level approach to achieve the purpose of flow expansion.
The constant current source of the power amplifier circuit is replaced by the current drive circuit VTR. The sum is the DC resistance and inductance of the inductive load under test. The M11032NPN 120V, 50A) is used as the power amplifier. The following briefly describes the working principle of the constant current source power amplifier.
When the input voltage from Na2 is positive, it is a power amplifier that outputs a positive voltage. At this time, the control transistors VTR1, VTR3, VTR4 turn on, VTR2, VTR5, VTRh turn off; when the input voltage from Na2 is negative, it is A power amplifier outputting a negative voltage, at this time, the adjusting tubes VTR2, VTR5, VTRh are turned on, and VTR1, VTR3, VTR1 are turned off.
Due to the large power dissipation and output current of the power tube, a power tube is often insufficient. For this reason, a method of paralleling the power tubes with similar parameters is adopted. If only the three of them are simply connected in parallel, the current flowing through each power tube will be very different, causing one of the power tubes to be overburdened and damaged. 8. To avoid the characteristics of the power tube is not exactly the same The current distribution caused by uneven, need to adopt the measure of equalizing flow. That is, a balanced resistor 58 is connected to the emitter of each power tube. The voltage drop on it is proportional to the current flowing through it and forms a negative feedback of the current. The currents of several parallel power tubes are relatively close. When the balance resistance increases, the effect of the current negative feedback is stronger, and the effect of the current sharing is better; however, the power consumed in the balance resistance is also greater. Therefore, these two aspects must be taken into account when designing. Practice shows that it is reasonable to take the voltage drop across the balance resistor to 0.11.0V.
The adjusting tube, especially the Darlington tube M11032, is the power tube with the largest power loss in the constant current source circuit. The junction temperature of the transistor increases as the dissipated power increases, and the lifetime of the transistor decreases exponentially as the junction temperature increases. Therefore, adjusting the heat dissipation of the tube is an important factor that determines the reliability and stability of the constant current source.
3 constant current source error analysis to ensure measurement accuracy and reliability. The switch of the gear can be controlled by the single chip microcomputer. 10. For the input reference voltage! In, specially selected PMI company produces REF02P chip to produce the reference voltage from it! In), it is a +5V precision voltage reference/temperature sensor, in the range of -55~1255, the output voltage maximum ripple coefficient is typically 0.05. 7. It can effectively improve the stability, reliability, drift and constant current source. Noise and other indicators.
At the same time, "1 generally chooses resistors with higher precision, such as 0.017 precision resistors. Standard resistors" must be custom-made and ensure sufficient heat dissipation.
4 Constant current source characteristics analysis The entire constant current source can be simplified into the closed-loop system control block diagram shown! 1 is the base voltage of the amplifier VTR).
Open loop voltage amplification for 3 op amps, "be is the resistance of the amplifier's VTR.
Assuming that the base current of the amplifier VTR is centered, the operational amplifier and the power transistor VTR constitute a deep negative feedback of the current, while the operational amplifier N'1 and Na are the negative feedback of the voltage, +"%), /0=/+ The approximate expression of the output current can be simplified to 1112 after repeated testing, the constant current source performance indicators are as follows: 12V DC fan); switching power supply full scale output voltage 10V; preheating time "3min with 20m" or 200m" Hot, because the current is 10A); 5 using a DC comparator to verify the flow of large current instrument, the measurement accuracy of up to 10-0. It is the measured DC generated by the magnetic potential and another easy to measure DC The magnetic potential is compared in the iron core. When the two magnetic potentials are balanced, the magnetic flux in the iron core is zero, and the magnetic potential balances 13: flow; +1, +2 are the number of turns of the primary coil and the secondary coil, respectively. The constant current source check wiring is as shown.
The ZZC-H type tester in the figure is an intelligent test instrument using the inductive load DC resistance manufactured by the constant current source developed in this article. The following abbreviation tester has been passed through the appraisal/DC comparator of Hubei Provincial Institute of Metrology and Measurement. The primary and secondary coils were removed and the other coils were omitted. When verifying, use 0.01 standard resistance instead of measured object "%, % represents the terminal voltage of the DC resistance of the measured object.
Since power equipment such as transformer windings have large inductance and low resistance, some transformers can have inductances of up to hundreds or even thousands of hens, and their resistance is only in the order of 10-4 to 10-2′′, for tiny resistors. The precision measurement, the influence of the lead resistance cannot be ignored, must take effective measures to overcome.The tester adopts the four-terminal connection method, the purpose is to reduce the influence of the wiring on the measured object.
When a constant current source is used to measure small resistors in large-scale power equipment, a constant current source is required to output a large stable current for quick and accurate measurement. The use of a DC comparator can accurately verify the stability of a large current output from a constant current source, such as 10A. The following is a brief description of the calibration principle.
When the tester generates a constant current (n or /%) through the standard resistance "n output to the comparator primary coil, 1 = according to the principle of the DC comparator can obtain the comparator secondary coil current is (2 = (% +1)/+2, the exact number of the voltage displayed by the resistance being measured "should be assumed to be within the test accuracy range of the tester in addition to the constant current source. Using the DC comparator can be very convenient. The accuracy of the large current output from the constant current source is verified, and the calibration method has high accuracy, high reliability, and simple operation.
The deep negative feedback and the sampling feedback of the output current can improve the working performance of the constant current source and have good practicality and versatility. The 10A constant current source developed by using the basic ideas described in this paper has been applied to large-scale power equipment DC resistance tester.
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