What is the method of measuring integrated circuits with multimeter as a testing tool?
Answer: The integrated circuit can be replaced well, but it is troublesome to disassemble. Before disassembly, we should judge whether the integrated circuit is really 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 to detect integrated circuits online with multimeter.
(1) Off-line detection:
This method is carried out when the IC is not soldered into the circuit. Generally, the positive and negative resistance values between each pin corresponding to the grounding pin can be measured by multimeter and compared with the intact IC.
(2) On-line detection:
This is a method to detect the on-line DC resistance, AC/DC voltage to ground and total working current of each pin of IC by multimeter. This method overcomes the limitation that substitution test method needs replaceable IC and the trouble of disassembling IC, and is the most commonly used and practical method for detecting IC.
① On-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 a multimeter ohmmeter, and compare them 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 meter and components during the test.
B. The internal voltage of the multimeter's electric block shall not be greater than 6 V, and the range is best blocked by R×100 or R×1 K.
C. When measuring IC pin parameters, we should pay attention to the measurement conditions, such as the tested model, the position of the sliding arm of the potentiometer related to IC, and also consider the quality of peripheral circuit components.
(2) DC working voltage measurement method: this is a method of measuring DC power supply voltage and working voltage of peripheral components with a multimeter DC voltage block when power is on; Detect the DC voltage of each pin of IC to ground and compare it with the normal value, then compress the fault range and find out the damaged components. Pay attention to the following eight points when measuring:
A. The multimeter should have enough internal resistance, at least more than 10 times the resistance of the circuit to be measured, so as not to cause large measurement errors.
B. Usually turn all potentiometers to the middle position. If it is a TV set, the signal source should be a standard color bar signal generator.
C. Anti-skid measures shall be taken for pens or probes. It is easy to damage IC because of any instantaneous short circuit. The following measures can be taken to prevent the stylus from slipping: a section of bicycle valve core is sleeved on the stylus, and the stylus grows about 0.5 mm, which can not only make the stylus contact with the tested point well, but also effectively prevent the stylus from slipping, even if it hits an adjacent point, it will not be short-circuited.
D. When the measured voltage of a pin is not in conformity with the normal value, we should analyze whether the pin voltage has any important influence on the normal operation of the IC and the corresponding changes of other pin voltages, so as to judge the quality of the IC.
E. 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 with a potentiometer with variable resistance, the position of the sliding arm of the potentiometer is different, which will change the pin voltage.
F. If the voltage of each pin of the IC is normal, it is generally considered that the IC is normal; If the pin voltage of IC is abnormal, we should start with the maximum deviation from the normal value 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 a dynamic receiving device, such as a TV set, the voltage of each pin of the IC is different with or without a signal. If it is found that the pin voltage should not change, but changes greatly, which changes with the signal size and different positions of adjustable components, the IC damage can be determined.
H. For devices with multiple working modes, such as video recorders, the pin voltages of IC are different under different working modes.
③ AC working voltage measurement method: In order to master the change of AC signal of IC, the AC working voltage of IC can be approximately measured by a multimeter with dB jack. When testing, the multimeter is placed in the AC voltage block, and the positive stylus is inserted into the dB jack; For multimeter without dB jack, a 0.1 ~ 0.5 μ f DC blocking capacitor needs to be connected in series with the positive stylus. This method is suitable for IC with low working frequency, such as video amplifier stage and field scanning circuit of TV set. Because 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 the quality of IC by detecting the total current of the incoming line of IC power supply. Because most of the IC is directly coupled, when the IC is damaged (such as a PN junction breakdown or open circuit), it will cause saturation and cut-off of the later stage, which will change the total current. Therefore, the quality of IC can be judged by measuring the total current. You can also measure the voltage of the resistance in the power supply path and calculate the total current value by ohm's law.
