How to Test Integrated Circuits Using Only a Multimeter
Editor's note: Although the replacement of integrated circuits is good, disassembly is more troublesome after all. Therefore, it is necessary to accurately judge whether the integrated circuit is indeed damaged and the degree of damage before disassembly, and avoid blind disassembly. This article introduces the methods and precautions for out-of-circuit and in-circuit detection of integrated circuits using only a multimeter as a detection tool. The four methods of on-road detection described in this article (measurement of DC resistance, voltage, AC voltage and total current) are practical and commonly used detection methods in amateur maintenance. Here, I also hope that you can provide other practical (integrated circuits and components) discrimination testing experience.
1. Not on the road detection
This method is carried out when the IC is not soldered into the circuit. Generally, a multimeter can be used to measure the forward and reverse resistance values between the pins corresponding to the ground pins, and compare with the intact IC.
2. On-the-road detection
This is a detection method of detecting the DC resistance, AC and DC voltage to ground and total working current of each pin of the IC in the circuit (IC in the circuit) through a multimeter. This method overcomes the limitation of the replacement test method that requires a replaceable IC and the trouble of dismantling the IC, and is the most common and practical method for testing the IC.
1. On-circuit DC resistance detection method
This is a method of using a multimeter ohmmeter to measure the forward and reverse DC resistance values of IC pins and peripheral components directly on the circuit board, and compare them with normal data to find and determine faults. 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 electrical barrier of the multimeter shall not be greater than 6V, and the range is preferably 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., and also consider the quality of the peripheral circuit components.
2. DC working voltage measurement method
This is a method of measuring the DC power supply voltage and the working voltage of peripheral components with the DC voltage block of a multimeter under the condition of power-on; detecting the DC voltage value of each pin of the IC to the ground, and comparing it with the normal value, thereby compressing the fault range. Locate damaged components. Pay attention to the following eight points when measuring:
(1) The multimeter must have a large enough internal resistance, at least 10 times greater than the resistance of the circuit under test, so as to avoid large measurement errors.
(2) Usually, turn the potentiometers to the middle position. If it is a TV, the signal source should use a standard color bar signal generator.
(3) Anti-skid measures should be taken for the test pen or probe. The IC is easily damaged due to any momentary short circuit. The following methods can be taken to prevent the test pen from sliding: take a piece of bicycle valve core and put it on the test pen tip, and grow the test pen tip by about 0.5mm. , it will not short-circuit even if it hits an adjacent point.
(4) When the measured voltage of a pin is inconsistent with the normal value, the quality of the IC should be judged by analyzing whether the voltage of the pin has an important influence on the normal operation of the IC and the corresponding changes in the voltages of other pins.
(5) IC pin voltage will be affected by peripheral components. When there is leakage, short circuit, open circuit or change in value of peripheral components, or the peripheral circuit is connected to a potentiometer with variable resistance, the position of the potentiometer sliding arm will vary, which will cause the pin voltage to change.
(6) 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 maximum deviation from the normal value, and check whether the peripheral components are faulty. If there is no fault, the IC is likely to be damaged. .
(7) For a dynamic receiving device, such as a TV, the voltage of each pin of the IC is different when there is a signal or not. If it is found that the voltage of the pin should not change but changes greatly, and the voltage that changes with the size of the signal and the different positions of the adjustable components does not change, it can be determined that the IC is damaged.
(8) For devices with multiple working modes, such as video recorders, under different working modes, the voltage of each pin of the IC is also different.
3. AC working voltage measurement method
In order to grasp the change of 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, place the multimeter on 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 frequencies, such as video amplification stages of TV sets, field scanning circuits, etc. Since the natural frequencies and waveforms of these circuits are different, the measured data are approximate and can only be used for reference.
4. total current measurement
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), the latter stage will 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 is also possible to measure the voltage drop across the resistors in the power path and use Ohm's law to calculate the total current value.
The above detection methods have their own advantages and disadvantages. In practical applications, it is best to combine various methods and use them flexibly.
