How to use a multimeter as a testing tool to test integrated circuits?
Integrated circuit replacement is easy, but disassembly is troublesome. Before disassembly, you should accurately determine whether the integrated circuit is indeed damaged and the extent of the damage, and avoid blind disassembly. The measurement of DC resistance, voltage, AC voltage and total current are four detection methods for online detection of integrated circuits using a multimeter.
(1) Not online detection:
This method is performed when the IC is not soldered into the circuit. Generally, a multimeter can be used to measure the forward and reverse resistance values between each pin corresponding to the ground pin, and compare it with the intact IC.
(2)Online detection:
This is a detection method that uses a multimeter to detect the DC resistance of each IC pin online (the IC is in the circuit), the AC and DC voltage to ground, and the total operating current. This method overcomes the limitations of the substitution test method, which requires replaceable ICs and the trouble of disassembling ICs. It is the most commonly used and practical method for testing ICs.
①Online DC resistance detection method: This is a method that uses a multimeter ohm block to directly measure the forward and reverse DC resistance values of each pin of the IC and peripheral components on the circuit board, and compares it with the normal data to find and determine the fault. Please pay attention to the following three points when measuring:
a. Disconnect the power supply before measuring to avoid damaging the meter and components during testing.
b. The internal voltage of the multimeter's electrical barrier must not be greater than 6 V. It is best to use the R×100 or R×1 K range for the measurement range.
c. 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., and also consider the quality of the peripheral circuit components.
② DC working voltage measurement method: This is a method of measuring the DC supply voltage and the working voltage of peripheral components with a multimeter DC voltage block when power is on; detect the DC voltage value of each pin of the IC to ground, and compare it with the normal voltage. Compare the values to compress the fault scope and find out the damaged components. Pay attention to the following eight points when measuring:
a. The multimeter must have a large enough internal resistance, at least 10 times greater than the resistance of the circuit being measured, to avoid causing large measurement errors.
b. Usually, turn each potentiometer to the middle position. If it is a TV, the signal source should use a standard color bar signal generator.
c. Anti-slip measures should be taken for the test leads or probes. Any instantaneous short circuit can easily damage the IC. The following method can be used to prevent the test lead from sliding: take a piece of bicycle valve core and put it on the test lead tip, and extend the test lead tip by about 0.5 mm. This can not only make the test lead tip well in contact with the tested point, but also effectively prevent slipping, even if There will be no short circuit even if it hits adjacent points.
d. When the measured voltage of a certain pin does not match the normal value, the quality of the IC should be judged by analyzing whether the pin voltage has any significant impact on the normal operation of the IC and the corresponding changes in the voltages of other pins.
e. IC pin voltage will be affected by peripheral components. When the peripheral components leak, short circuit, open circuit or change value, or the peripheral circuit is connected to a potentiometer with variable resistance, the position of the potentiometer sliding arm will change, which will cause the pin voltage to change.
f. If the voltage of each pin of the IC is normal, the IC is generally considered normal; if the voltage of some pins of the IC is abnormal, you should start from the point where the voltage deviates the most from the normal value and check whether the peripheral components are faulty. If there is no fault, the IC is likely to be damaged.
g. For dynamic receiving devices, such as TVs, the voltage of each pin of the IC is different when there is a signal or not. If it is found that the pin voltage should not change but changes greatly, and it should change with the signal size and the position of the adjustable component but does not change, it can be determined that the IC is damaged.
h. For devices with multiple working modes, such as video recorders, the voltages of each pin of the IC are also different in different working modes.
③ AC working voltage measurement method: In order to grasp the changes in the AC signal of the IC, a multimeter with a dB jack can be used to approximately measure the AC working voltage of the IC. When testing, put the multimeter in the AC voltage setting, and insert the positive test lead into the dB jack; for multimeters without dB jacks, a 0.1 to 0.5 μF DC blocking capacitor needs to be connected in series to the positive test lead. This method is suitable for ICs with relatively low operating frequencies, such as TV video amplification stages, field scanning circuits, etc. Since these circuits have different natural frequencies and different waveforms, the measured data are approximate and can only be used for reference.
④Total current measurement method: This method is a method to judge the quality of the IC by detecting the total current of the IC power supply line. Since most of the internal ICs are directly coupled, when the IC is damaged (such as a breakdown or open circuit of a certain PN junction), it will cause saturation and cutoff of the subsequent stage, causing the total current to change. Therefore, the quality of the IC can be judged by measuring the total current. You can also measure the voltage of the resistor in the power path and calculate the total current value using Ohm's law.
