Multimeter test IC, can not detect anything, unless you know the internal structure of the IC!
Test whether the integrated circuit is burned or short circuit requires professional knowledge, generally do not have this knowledge and skills, you can carefully observe the appearance of the IC, make some simple judgments: IC surface has burned traces, such as bulge, crack, discoloration, etc. Whether the IC pin is broken or unwelded. Whether the IC has a burnt mushy smell. Whether the IC peripheral components are abnormal. The following is the detection method of integrated circuits using multimeters as detection tools:
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First, no road detection
This method is carried out when the IC is not welded into the circuit, under normal circumstances, a multimeter can be used to measure the positive and reverse resistance values between each pin corresponding to the ground pin, and compare with the intact IC.
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Second, on the road detection
This is a multimeter to detect the DC resistance of each IC pin in the circuit (IC in the circuit), AC and DC voltage to ground and the total working current detection method. This method overcomes the limitation of substitutable IC and the trouble of disassembling IC by substitution test method, and is the most common and practical method for detecting IC.
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1. In line DC resistance detection method
This is a multimeter ohm gear, directly on the circuit board to measure the positive and negative DC resistance of the IC pins and peripheral components, and compared with the normal data, to find and determine the fault method. When measuring, pay attention to the following three points:
(1) Disconnect the power supply before measurement to avoid damage to the meter and components during the test.
(2) The internal voltage of the multimeter electric block shall not be greater than 6V, and the range is best to use R×100 or R×1k.
(3) When measuring IC pin parameters, pay attention to the measurement conditions, such as the model under test, the position of the sliding arm of the potentiometer related to the IC, etc., but also consider the quality of the peripheral circuit components.
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2. Dc working voltage measurement method
This is a way to measure the DC supply voltage and the working voltage of peripheral components by using a multimeter DC voltage block in the case of power-on. The DC voltage value of each IC pin to ground is detected and compared with the normal value, and then the fault range is compressed to find the damaged component. When measuring, pay attention to the following eight points:
(1) The multimeter should have a large enough internal resistance, at least more than 10 times the resistance of the measured circuit, so as not to cause large measurement errors.
(2) The potentiometer is usually rotated to the middle position, if it is a TV, the signal source should use a standard color bar signal generator.
(3) The stylus or probe should take anti-slip measures. Because any instant short circuit is easy to damage the IC. The following methods can be taken to prevent the sliding of the pen: take a bicycle valve core set on the nib of the watch, and grow the nib of the watch about 0.5mm, which can make the nib of the watch well in contact with the test point, and can effectively prevent slipping, even if it hits the adjacent point, it will not short circuit.
(4) When a certain pin voltage is not consistent with the normal value, it should be analyzed according to whether the pin voltage has an important impact on the normal operation of the IC and the corresponding changes in other pin voltages to determine the IC's quality.
(5)IC pin voltage will be affected by peripheral components. When the peripheral components have leakage, short circuit, open circuit or variable value, or the peripheral circuit is connected to a potentiometer with variable resistance value, the position of the potentiometer sliding arm is different, and the pin voltage will change.
(6) If the IC each pin voltage is normal, the IC is generally considered normal; If the IC part of the pin voltage is abnormal, you should start from the maximum deviation from the normal value, check the peripheral components have no fault, if there is no fault, the IC is likely to be damaged.
(7) For dynamic receiving devices, such as television sets, the IC pin voltage is different when there is no signal. If it is found that the pin voltage should not change but change greatly, and the change with the signal size and different positions of the adjustable element does not change, the IC damage can be determined.
(8) For devices with multiple working modes, such as video recorders, the IC pin voltage is also different under different working modes.
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3. Ac operating voltage measurement method
In order to master the AC signal changes of the IC, you can use a multimeter with a dB jack to approximate the AC operating voltage of the IC. When testing, set the multimeter in AC voltage gear and insert the front pen into the dB jack. For a multimeter without a dB jack, connect a 0.1 to 0.5 μF straight isolating capacitor to the front pen in series. This method is suitable for ics with relatively low operating frequency, such as video amplifying stage and field scanning circuit of TV sets. Since the natural frequencies of these circuits are different and the waveforms are different, the measured data is approximate and can only be used for reference.
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4. Total current measurement method
This method is a method to judge the quality of IC by detecting the total current of the IC power supply line. Since the vast majority of the IC is directly coupled, when the IC is damaged (such as a certain PN junction breakdown or open circuit), it will cause post-stage saturation and cutoff, so that the total current changes. Therefore, the IC can be judged by measuring the total current. It is also possible to measure the voltage drop of the resistance in the power supply path and calculate the total current value using Ohm's law.
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The above detection methods have advantages and disadvantages, and it is best to combine various methods and use them flexibly in practical applications.