Power Supply 1 DC Resistance Measurement Principle In the power system, the DC resistance of the inductive load is measured quickly and accurately, which is beneficial for on-site detection, fault diagnosis and real-time maintenance of the power equipment. For example, measuring the DC resistance of a transformer winding is the transformer storage, % and! "The standard resistance and the DC resistance of the inductive load being measured; 4 is the inductance of the inductive load being measured."
In particular, when closing the button S1, the resistor will be! Short circuit, so that all voltage is applied to the load under test, forcing it to have a large current rising speed. When the predetermined current value is reached and the button S1 is turned off, the current will be stabilized very quickly, and the stable power supply is /, , "/!", %n: Large DC current is beneficial to the rapid measurement of inductive load DC resistance; DC current with high stability is beneficial to the accuracy of inductive load DC resistance measurement. Therefore, the study of a large-capacity constant current source with high stability and strong load capacity directly determines the accuracy, rapidity and reliability of inductive load DC resistance measurement. 2. 2 Constant current source working principle company at 20 A high-precision, low-drift, dynamic-zero CMOS chopper-stabilized monolithic integrated operational amplifier developed in the early 1980s, called the fourth-generation operational amplifier 6. Its input offset voltage. s is only one thousandth of the general-purpose operational amplifier F007, s=1.0! V, the offset voltage drift is 0.01! V/°C, drifting less than 1 per month! V, input bias current is about 100pA, and has a very high common mode rejection CM-RR! 130dB), open loop gain P140dB). In is the input reference voltage; VTR is the power amplifier component; 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, the current is amplified by the current amplifier driving circuit VTR or /n by the adder N>1 and the feedback amplifier Na2. In order to stabilize the output current, in addition to the deep negative feedback introduced in each link, the output current sampling 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 makes the constant current The source has a high stability in the output current over a large range of load variations.
2.1 Stabilization mechanism of constant current source current Output voltage of adder NA1: output voltage 2 of inverting amplifier NA2. is 2=-%1, follower NA3 is /n7"2-"3)/6, simultaneous Solve the following equations: Because the operational amplifier NA3 constitutes a follower circuit, its input impedance is very high! 1012! / Then the current flowing through the standard resistor 6 flows all the way to the measured inductive load. Equation 1) shows that the current is independent of the inductive load being measured; the current is linear with the input voltage "m; the stability of the input voltage", the ripple size, the temperature drift characteristic, and the quiescent current 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! .
When wiring, when the grounding points of the three operational amplifiers are not properly arranged, the interference signals formed on the ground lines will be stringed into nearby circuits, resulting in unstable operation of the constant current source. Collect the grounding points of the reference source, adder, and feedback amplifier together, and connect the point to the power ground. As shown, the interference can be effectively suppressed. 2.2 Power amplifier driving technology measures the DC resistance of large power equipment. Improve the accuracy and speed of measurement, require the test instrument to have a large constant current output capability of 0.110A), very small ripple and high stability, so that it can be used for inductive loads such as large transformer windings above 110kV. Accurate and reliable measurement of DC resistance. The constant current source studied in this paper can stabilize the output current with a maximum current of 10A and is used to measure the DC resistance of inductive loads above 100. Since the output current of a general integrated operational amplifier is relatively small, it is not possible to directly drive a large power tube. Therefore, in order to obtain a 10A constant current source, a current spreading technique must be adopted. In this paper, the pre-driver-level method is adopted to achieve the purpose of current expansion.
A power amplifier circuit representing a constant current source, in place of the current drive circuit VTR. The DC resistance and inductance of the inductive load are measured, and M11032NPN type 120V, 50A) is used as the power amplifier tube. The following is a brief introduction to the working principle of a 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 regulating tubes VTR1, VTR3, and VTR4 are turned on, and VTR2, VTR5, and VTRh are turned off; when the input voltage from Na2 is negative, A power amplifier that outputs a negative voltage. At this time, the regulating tubes VTR2, VTR5, and VTRh are turned on, and VTR1, VTR3, and VTR1 are turned off.
Due to the large power dissipation and output current of the power tube, a power tube is often not enough. For this reason, a method in which power tubes with similar parameters are connected in parallel is adopted. If only the three poles of their respective poles are simply connected in parallel, the current flowing through the power tubes will be greatly different, causing one of the power tubes to be overburdened and damaged. 8. To avoid the characteristics of the power tubes being different. The current distribution is uneven, and the current sharing measure is adopted, that is, a balance resistor 58 is connected to the emitter of each power tube, and the voltage drop on the power tube is proportional to the current flowing, thereby forming a current negative feedback, thereby The currents of several power tubes connected in parallel 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 by the balance resistor is larger. Therefore, these two aspects must be taken into account in the design. Practice has shown that it is reasonable to take the voltage drop across the balance resistor between 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 power dissipation increases, and the lifetime of the transistor decreases exponentially as the junction temperature increases. Therefore, the heat dissipation problem of the adjustment tube is an important factor determining the reliability and stability of the constant current source.
3 Constant current source error analysis ensures measurement accuracy and reliability. The switching of the gear position can be completed by the single-chip control, 10. For the input reference voltage! In, specially selected REF02P chip produced by PMI company to generate the reference voltage! In), it is a +5V precision voltage reference/temperature sensor. In the range of -55~1255, the maximum ripple coefficient of the output voltage is 0.05. 7. It can effectively improve the stability, reliability, drift and stability of the constant current source. Indicators such as noise.
At the same time, "1 generally selects a higher precision resistor, such as 0.017 precision resistor. Standard resistor" must be customized, while ensuring sufficient heat dissipation.
4 Constant current source characteristic analysis can simplify the entire constant current source into the closed-loop system control block diagram shown! 1 is the base voltage of the amplifier VTR).
The open-loop voltage amplification of the three op amps, "be is the be junction resistance of the amplifier 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 amplifiers N'1 and Na are the deep negative feedback of the voltage, +"%), /0=/+ , the approximate expression of the output current can be simplified to 1112 after repeated testing, constant current source performance indicators are as follows: using 12V DC fan); switching power supply full-scale output voltage 10V; warm-up time "3min with 20m" or 200m" file pre- Heat, because the current is 10A); 5 The instrument that uses DC comparator to verify large current flow, its measurement accuracy can reach 10-0. It is the magnetic potential generated by the measured DC and another DC that is easy to measure. 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, the magnetic potential balance is 13: the flow; +1, +2 are the primary coil and the secondary coil turns, respectively. The wiring method for constant current source verification is as shown.
In the figure, the ZZC-H type tester is an intelligent test instrument for inductive load DC resistance experimentally produced by the constant current source studied in this paper. The following is a test instrument, which has been approved by the Hubei Institute of Metrology and Testing Institute/DC comparator. The primary coil and the secondary coil are eliminated, and other coils are omitted. When the calibration is performed, the 0.01-level standard resistance is used instead of the measured object "%, and % 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 hundreds to even thousands of hens, and their resistance is only 10-4~10-2", for small resistors. The precision measurement, the influence of lead resistance can not be ignored, must be overcome by effective measures. The tester uses four-terminal connection method, the purpose is to reduce the impact of the wiring on the object under test.
When measuring a small-resistance large-scale power device with a constant current source, a constant current source is required to output a large stable current for fast and accurate measurement. The DC comparator can accurately verify the stability of the large current output of the constant current source, such as 10A. The working principle of the calibration is briefly described below.
When the constant current (n or /%) generated inside the tester is output to the primary coil of the comparator through the standard resistor "n", 1 = the secondary coil current of the comparator can be obtained according to the working principle of the DC comparator (2= (%+1)/+2, the exact number of the voltage of the measured resistance is "should be assumed that the tester is in the range of the test accuracy except for the constant current source. It is convenient to use the DC comparator. The accuracy of the large current output by 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 practicability and versatility. The 10A constant current source developed by the basic idea described in this paper has been applied to the DC resistance tester for large power equipment.
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