Using the DC four-probe method to measure the resistivity of semiconductors
1. Testing principle:
When four metal probes are arranged in a straight line, and When a certain pressure is pressed on the semiconductor material, a current I passes between probes 1 and 4, and a potential difference V is generated between probes 2 and 3 (as shown in the figure).
According to the formula, Calculate the resistivity of the material:
Among them, C is the probe coefficient (cm) of the four probes, and its size depends on the arrangement method and needle pitch of the four probes.
2. Instrument operation:
(1) Preparation before test:
1. Insert the power plug into the power socket on the back of the instrument, and put the power switch in the off position;
2. Set the working mode switch to the "short circuit" position and the current switch to the pop-up position;
3. Connect the shielded wire plug of the manual test frame to the input socket of the electrical box;
4. Perform certain treatments on the test samples (such as sandblasting, cleaning, etc.);
5. Adjust the indoor temperature and humidity to meet the test requirements.
(2) Test:
First put the power switch to the on position, and the measurement selection switch to "short circuit", a digital display will appear, and power on for half an hour to warm up.
1 , place the sample, press down the probe, place the measurement selection switch in the "measurement" position, and place the polarity switch above the switch;
2. Select the appropriate voltage range and current range, and the digital display is basically "0000", if there is a number at the end, you can turn the zero adjustment knob to display "0000";
3. Set the working mode switch to "I adjustment", press the current switch, and turn Turn the dynamic current adjustment knob so that the digital display is "1000", which is the full-scale value of each current range;
4. Then press the polarity switch so that the digital display is also 1000±1. Exit the current switch and set the working mode switch to 1 or 6.28 (position 1 when the probe spacing is 1.59mm, and 6.28 when the probe spacing is 1mm);
(After adjusting the current, follow the above steps There is no need to repeat it in future measurements; as long as the current switch is pressed after adjustment, the measured value can be read directly from the digital display.)
5, if the digital display goes out and only "1" remains, it means If the voltage value exceeds this range, you can turn the voltage range switch to a higher level;
6. After reading, turn the polarity switch to the other side to read the measured value with negative polarity. The average of the measured values ??is the resistivity value of the sample at that location.
3. Notes:
1. When pressing down the probe, the pressure should be moderate to avoid damaging the probe. ;
2. Since the surface resistance of the sample may be unevenly distributed, several more points should be measured on a sample and then the average value should be taken;
3. The actual resistivity of the sample is still It is related to its thickness. You also need to check the thickness correction coefficient in the appendix and make corrections.
1. When measuring the capacitive load resistance, what is the relationship between the output short-circuit current of the insulation resistance tester and the measurement data? Why?
The size of the short-circuit current output by the insulation resistance tester can reflect the size of the internal resistance of the high-voltage source output by the megohmmeter. When the product under test has capacitance, at the beginning of the test process, the high-voltage source in the insulation resistance tester charges the capacitor through its internal resistance, and gradually charges the voltage to the output rated high voltage value of the insulation resistance tester. Obviously, if the capacitance value of the test sample is large, or the internal resistance of the high-voltage source is large, the charging process will take longer. Its length can be determined by the product of R and C load (in seconds). Please note that the current charging the capacitor and the current flowing through the insulation resistance of the product under test flow into the insulation resistance tester together during the test. The current measured by the insulation resistance tester not only contains the insulation resistance component, but also adds the capacitor charging current component. At this time, the measured resistance value will be smaller.
For example: an insulation resistance tester with a rated voltage of 5000V, if its short-circuit output current is 80μA (produced by Japan's ***Tekashi), its internal resistance is 5000V/80μA=62MΩ
For example: the capacity of the sample is 0.15μF, then the time constant τ=62MΩ×0.15μF≈9 (seconds), that is, at 18 seconds, the charging current on the capacitor is still 11.3μA.
It can be seen that the equivalent resistance formed only by the charging current is 5000V/11.3μA = 442MΩ. If the normal insulation is 1000MΩ, the measured insulation value displayed is only 306MΩ. This test value can no longer reflect the true condition of the insulation value, and the test value mainly changes with the change of capacitive load capacity, that is, if the capacity is small, the test resistance value is large; if the capacity is large, the test resistance value is small.
Therefore, in order to ensure accurate test values ??of R15s and R60s, a large-capacity insulation resistance tester with fast charging speed should be selected. Relevant regulations in my country require that the output short-circuit current of an insulation resistance tester should be greater than 0.5mA, 1 mA, 2 mA, or 5 mA. In situations with high requirements, an insulation resistance tester with a larger output short-circuit current should be selected as much as possible.
2. Why when measuring insulation, it is not only required to measure the simple resistance value, but also the absorption ratio and polarization index. What is the significance?
In the insulation test, the insulation resistance value at a certain moment cannot fully reflect the insulation performance of the sample. This is due to the following two reasons. On the one hand, the volume of insulation materials with the same performance When the volume is large, the insulation resistance is small, and when the volume is small, the insulation resistance is large. On the other hand, insulating materials have charge absorption ratio processes and polarization processes after high voltage is applied. Therefore, the power system requires that the absorption ratio - that is, the ratio of R60s and R15s, and the polarization index - that is, the ratio of R10min and R1min should be measured during the insulation test of the main transformer, cable, motor, etc., and use this data to determine the insulation condition The pros and cons.
3. In a high-voltage and high-resistance test environment, why is the instrument required to be connected to the "G" terminal?
When a higher rated voltage is applied to both ends of the product under test and the insulation resistance is high, the surface of the product under test will be wet and the leakage caused by contamination will be larger, and the indication error will be larger. The "G" end of the instrument bypasses the leakage current on the surface of the product being tested, so that the leakage current does not pass through the test circuit of the instrument and eliminates errors caused by leakage current.
4. When calibrating the rated output DC high voltage at both ends of "L" and "E" of some types of insulation instruments, use the DCV setting of the pointer multimeter to measure the voltage at both ends of L and E. Why does the voltage drop a lot, while the digital multimeter does not? ?
Use an ordinary pointer multimeter to directly measure the rated DC voltage output by the insulation resistance tester at the "L" and "E" ends. The measurement result is much smaller than the nominal rated voltage value (exceeds error range), but not with a digital multimeter. This is because the internal resistance of the analog multimeter is small, while the internal resistance of the digital multimeter is relatively large. The internal resistance of the pointer multimeter is small, and the output voltage of the L-E terminal of the insulation resistance tester is much lower than the output voltage during normal operation. However, it is wrong to use a multimeter to directly measure the output voltage of an insulation resistance tester. You should use an electrostatic high voltage meter with a large internal resistance or use a voltage divider or other method with a load resistance large enough to measure it.
5. Can I use a megohmmeter to directly measure the charged product under test? What is the impact on the results and why?
For personal safety and normal testing, in principle, it is not allowed to measure charged test items. If you want to measure the charged test item, it will not cause damage to the instrument (in a short time), but the test result is Inaccurate, because after being charged, the product under test is connected to other samples, so the results obtained cannot truly reflect the actual data, but the resistance value in parallel or series with other samples.
6. Why can an electronic insulation resistance tester produce a higher DC high voltage when powered by several batteries?
This is based on the principle of DC conversion and is processed by a boost circuit to increase the lower supply voltage to a higher output. DC voltage, the high voltage generated is higher but the output power is smaller. (For example, a few batteries in an electric baton can produce tens of thousands of volts of high voltage)
7. When measuring insulation resistance with an insulation resistance tester, what factors can cause inaccurate measurement data? Why?
A) The battery voltage is insufficient. The battery voltage is too low, causing the circuit to not work properly, so the measured readings are inaccurate.
B) The test wire connection method is incorrect. By mistake, the "L", "G", and "E" terminals are connected incorrectly, or the "G", "L" terminals, and the "G" and "E" terminals are connected to both ends of the product under test.
C) The "G" terminal is not connected. The product under test may suffer from errors caused by current leakage due to factors such as contamination and moisture, resulting in inaccurate testing. At this time, the "G" terminal must be connected to prevent errors caused by leakage current.
D) Too much interference. If the product under test is subject to excessive electromagnetic interference from the environment, the meter reading will jump. Or the pointer moves. causing inaccurate readings.
E) Human reading error. When measuring with a pointer type insulation resistance tester, the indication value is inaccurate due to artificial viewing angle error or scale error.
F) Instrument error. The instrument itself has too large an error and needs to be recalibrated.
8. When measuring a capacitive load in the field of a high-resistance insulation meter (such as a main transformer), the pointer shows that the resistance value suddenly drops in a certain range (not a slow and small swing within the maximum value range during normal testing) and swings back and forth rapidly. What is the reason?
This phenomenon is mainly caused by discharge and sparking in a certain part of the test system. The insulation meter is charging to the capacitive test object. When the capacitive test object is charged to a certain voltage, if there is breakdown discharge and ignition in any part of the test line inside the meter or the test object, the above phenomenon will occur. Judgment method: (1) Do not connect the instrument test socket to the test line, turn on the power and high voltage, and see if there is sparking in the instrument (if there is sparking, you can hear the sound of discharge and sparking). (2) Connect the L, G, and E test wires without connecting to the product under test. Leave the end clamp of the L test wire floating in the air. Turn on the high voltage to see if there is sparking in the test wire. If there is sparking, check: a) Whether the L and G test wire core wires (L end) are too close to the exposed wire (G end), causing arcing and sparking. b) Poor contact between the L-end core wire plug and the shielding ring of the test socket or the test clamp and the product under test causes sparking. c) There is a broken circuit between the test lead, the plug and the clip, causing gap discharge. (3) Connect the product under test and check whether there is discharge or spark near the contact point between the terminal clamp and the test product. (4) Eliminate the above reasons, connect the product under test, and turn on the high voltage. If the instrument still has the above phenomenon, it means that the insulation breakdown of the product under test causes partial discharge or arcing.
9. Why are there differences in the measured values ??of different insulation resistance testers?
Due to the non-ideal voltage source of the high-voltage insulation resistance tester test power supply, different internal resistance Ri, different series resistance Rm of the measurement loop, different dynamic measurement accuracy, and unreasonable or errors in on-site measurement operations, etc., the differences are different. There will be differences in the measurement results of the same tested product by different models of insulation resistance testers. During actual measurement, the particularity of the insulation test conditions of the insulation resistance tester should be combined to minimize various possible measurement errors: (1) When different types of insulation meters measure the same sample, the same voltage level and wiring method should be used. For example, when measuring the insulation of the high-voltage winding of a power transformer, when the winding lead-out is always connected to the L terminal of the insulation resistance tester, there is a direct method in which the E terminal is connected to the low-voltage winding and shell, while the G terminal is left floating; The low-voltage winding, and the G terminal button is connected to the shell shielding method (low potential shielding); the G terminal button is connected to the surface of the high-voltage winding bushing, and the E terminal button is connected to the low-voltage winding first, and then connected or not connected to the shell respectively. Two casing shielding methods (high potential shielding). The E terminal is connected to the shell, while the G terminal is connected to the low voltage winding and other wiring methods.
Insulation resistance testers with different structures and standards have different G-terminal button potentials, and the position of the G-terminal button on the casing surface should also change accordingly. (KD2677 is a low potential shield, that is, the G terminal button is low potential). (2) Different models of insulation resistance testers have different measuring ranges and different scale methods for indication values, different scale resolutions, and different levels of measurement accuracy, which will cause differences in the indication values. In order to ensure accurate measurement of electrical equipment, avoid using meggers with low accuracy and inconvenience to use. (3) Most of the test samples contain capacitive components and have dielectric polarization phenomena. Even if the test conditions are the same, it is difficult to obtain ideal data repeatability. (4) When measuring, the temperature of the insulating medium and the oil temperature should be consistent with the ambient temperature, and the difference is generally allowed to be ±5. (5) The measurement value should be read as soon as possible within the allowable time difference range of the specific time period. In order to ensure that the measurement error is no higher than ±5, the reading time of R60S is allowed to have an error of ±3S, while the time of reading R15S should not differ by ±1S. (6) The high-voltage test power supply is not an ideal voltage source. When under heavy load (the insulation resistance of the product under test is small), the output voltage is lower than its rated value, which will cause the measurement accuracy of the single-branch direct reading measurement insulation resistance tester to deteriorate. Decreased due to change in conversion coefficient. This change varies depending on the load characteristics of the power supply tested by the insulation resistance tester. (7) For insulation resistance testers with different dynamic test capacity indicators, there are differences in the establishment process of the test voltage on the test product (and the sampling resistor) and the charging ability of the test product, and the measurement results will also be different. Use a lower dynamic tester than When measuring the insulation resistance tester that tests the threshold value of the capacity index, due to the inertial network of the instrument (including the mechanical inertia of the pointer instrument), the response speed of the indication value is slow, and it is too late to correctly reflect the change of the actual insulation resistance value of the sample over time. , especially in the initial stage of the test, the capacitor charging current does not completely decay to zero, which will cause large errors (too small) in the R15S and absorption ratio reading values. (8) The polarization condition of the insulation medium of the sample is related to the magnitude of the applied test voltage. Because the test voltage cannot reach the rated value quickly, or the test voltage applied to the test sample is different due to the different test power load characteristics of the insulation resistance tester, the initial polarization condition of the test sample is different, resulting in different absorption currents, which makes the insulating resistance measurement inaccurate. The indication values ??are different. (9) The test high voltage of some foreign insulation resistance testers is continuously adjustable, and is adjusted from zero to the rated value after starting up. The uncertainty of the starting time of the insulation resistance tester reading and the uncertainty of the time when the high voltage reaches the rated value make the initial polarization of the test sample different, which will also cause the difference between the indicated values. (10) Different insulation resistance testers have different sensitivity and resistance to on-site interference, and the reading values ??of the same test product will be different. (11) Differences between indications are caused by conventional measurement errors of random fluctuations in data and method errors of insulation resistance testers. (12) Insufficient dielectric discharge is one of the important reasons for the differences in repeated measurement results. According to the corresponding and reversible characteristics of the charging and absorption current of the sample and its reverse discharge current, if the same sample needs to be measured again for the second time, the short-circuit discharge interval time of the sample after the first measurement should generally be longer than the measurement time. Discharge the accumulated absorbed charge so that the insulation medium of the sample can fully return to its original non-polarized state, otherwise the accuracy of the second measurement data will be affected. In order to ensure that there is no residual charge on the test object, the measuring end should be short-circuited to the ground before each test, sometimes even taking nearly 1 hour, and the connections with irrelevant equipment should be removed. In short, for insulation measurements of the same sample in different periods, the same test voltage level and wiring method should be used, and the same model or insulation resistance meter with similar performance should be used as much as possible to ensure the comparability of the measurement data. (13) Finally, special emphasis should be placed on the selection of instruments with low dynamic measurement accuracy and low high-voltage test power supply capacity, because the capacitor charging current has not completely decayed to zero, and the value indicated by the instrument cannot accurately follow the apparent insulation resistance of the sample in real time. The change in value means that the resistance value of R15S is low and a large error occurs, resulting in a falsely high absorption ratio of the sample, which should be paid special attention to by the tester. This may also be the main reason why the absorption ratio readings of various types of high-voltage insulation resistance testers differ when measuring the same sample.
This also shows that the absorption comparison index is not as scientific and objective as the polarization index.
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