What is the method of using a multimeter as a testing tool to test integrated circuits?
The integrated circuit is easy to replace, but it is troublesome to disassemble. Before disassembly, it is necessary to accurately judge whether the integrated circuit is indeed damaged and the degree of damage, so as to avoid blind disassembly. The measurement of DC resistance, voltage, AC voltage and total current are four methods of online detection of integrated circuits with a multimeter.
(1) Offline detection:
This method is carried out when the IC is not welded 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 to detect the online (IC in the circuit) DC resistance of each pin of the IC, the AC and DC voltage to the ground, and the total operating current through a multimeter. This method overcomes the limitations of replacing the IC and the trouble of disassembling the IC in the substitution test method, and is the most commonly used and practical method for detecting ICs.
①On-line DC resistance detection method: This is a method to directly measure the forward and reverse DC resistance values of IC pins and peripheral components on the circuit board with the ohm block of a multimeter, and compare it with normal data to find and determine faults. Pay attention to the following three points when measuring:
a. Disconnect the power supply before measurement, so as not to damage the ammeter and components during the test.
b. The internal voltage of the electric barrier of the multimeter shall not be greater than 6 V, and the measuring range is preferably R×100 or R×1 K.
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 peripheral circuit components.
②DC working voltage measurement method: This is a method of measuring the DC supply voltage and the working voltage of peripheral components with the DC voltage block of a multimeter under the condition of power on; detect the DC voltage value of each pin of the IC to ground, and compare it with the normal voltage. Values are compared, and then the fault range is compressed to find out the damaged components. Pay attention to the following eight points when measuring:
a. The internal resistance of the multimeter must be large enough, at least 10 times greater than the resistance of the circuit under test, so as not to cause large measurement errors.
b. Usually turn each potentiometer to the middle position. If it is a TV, the signal source should be a standard color bar signal generator.
c. Anti-slip measures should be taken for test leads or probes. Any momentary short circuit can easily damage the IC. The following methods can be adopted to prevent the test pen from sliding: take a bicycle valve core and put it on the test pen tip, and extend the test pen tip by about 0.5 mm. It will not be short-circuited even if it hits a neighboring point.
d. When the measured voltage of a certain pin is inconsistent 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 the voltage of other pins, so as to judge whether the IC is good or bad.
e. IC pin voltage will be affected by peripheral components. When leakage, short circuit, open circuit or value change occurs in the peripheral components, or the peripheral circuit is connected to a potentiometer with variable resistance, the position of the sliding arm of the potentiometer is different, which will cause the pin voltage to change.
f. If the voltage of each pin of the IC is normal, it is generally considered that the IC is normal; if the voltage of some pins of the IC is abnormal, start from the point where the deviation from the normal value is the largest, and check whether there is any fault in the peripheral components. If there is no fault, the IC is likely to be damaged.
g. For dynamic receiving devices, such as TV sets, 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 the pin voltage that should change with the signal size and the different positions of the adjustable components does not change, it can be determined that the IC is damaged.
h. For devices with multiple working modes, such as video recorders, the voltage of each pin of the IC is also different under different working modes.
③ AC working voltage measurement method: In order to grasp the change of IC AC signal, a multimeter with dB jack can be used to approximate the IC's AC working voltage. When testing, put the multimeter in the AC voltage block, and insert the positive test lead into the dB jack; for a multimeter without a dB jack, a 0.1-0.5 μF DC blocking capacitor needs to be connected in series with the positive test lead. This method is suitable for ICs with relatively low operating frequency, such as video amplifier stages of TV sets, field scanning circuits, etc. Since these circuits have different natural frequencies and waveforms, the measured data are approximate and can only be used for reference.
④ Total current measurement method: This method is a method to judge whether the IC is good or bad by detecting the total current of the IC power supply line. Since most of the ICs are directly coupled, when the IC is damaged (such as a PN junction breakdown or open circuit), it will cause the subsequent stage to be saturated and cut off, and the total current will change. Therefore, the quality of the IC can be judged by measuring the total current. It can also be used to measure the voltage of the resistor in the power path, and use Ohm's law to calculate the total current value.
