Use a multimeter to judge the quality of capacitors
Time: 2020-08-05 Source: Yiduoduo Instrument Network Author: Yiduoduo Mall
Depending on the capacity of the electrolytic capacitor, the R×10, R×100, R×1K gears of the multimeter are usually used for testing and judgment. The red and black test leads are respectively connected to the negative pole of the capacitor (the capacitor needs to be discharged before each test), and the quality of the capacitor can be judged by the deflection of the needle. If the hands of the watch swing to the right quickly, and then slowly return to the original position to the left, the capacitor is generally good. If the hands of the watch do not turn back after swinging up, it means that the capacitor has broken down. If the watch hands gradually return to a certain position after swinging up, it means that the capacitor has leaked. If the hands of the watch cannot move, it means that the capacitor electrolyte has dried up and lost capacity.
Some leakage capacitors are not easy to accurately judge whether they are good or bad by the above method. When the withstand voltage value of the capacitor is greater than the voltage value of the battery in the multimeter, according to the characteristic that the leakage current of the electrolytic capacitor is small when it is charged forward, and the leakage current is large when it is charged reversely, you can use the R×10K gear to reverse charge the capacitor and observe Whether the place where the needle stays is stable (that is, whether the reverse leakage current is constant), and thus the quality of the capacitor can be judged with high accuracy. The black test lead is connected to the negative pole of the capacitor, and the red test lead is connected to the positive pole of the capacitor. The hands swing up quickly, and then gradually retreat to a certain place to stay still, indicating that the capacitor is good. The capacitor that is slowly moving to the right has leaked and is no longer usable. The hands of the watch generally stay and stabilize within the scale range of 50-200K.
This is a detection method by detecting the DC resistance of each pin of the IC (the IC is in the circuit), the AC and DC voltage to the ground, and the total operating current. 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.
1. In-circuit DC resistance detection method
This is a way to use the ohm block of a multimeter to directly measure the positive and negative DC resistance values of each pin of the IC and peripheral components on the circuit board, and compare it with the normal data to find and determine the fault. Pay attention to the following three points when measuring:
(1) Disconnect before measurement to avoid damage to the ammeter and components during the test.
(2) The internal voltage of the electric block 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, attention should be paid to the measurement conditions, such as the tested model, the position of the sliding arm of the potentiometer related to the IC, etc., and the quality of the peripheral circuit components should also be considered.
2. AC working voltage measurement method
In order to grasp the change of the IC AC signal, a multimeter with a dB jack can be used to perform an approximate measurement of the AC operating voltage of the IC. When testing, the multimeter is placed in the AC voltage block, and the positive test lead is inserted into the dB jack; for a multimeter without a dB jack, it is necessary to connect a 0.1-0.5μF DC block to the positive test lead in series. This method is suitable for ICs with relatively low operating frequency, such as the first amplifier stage of a TV, field scanning circuits, etc. Since these circuits have different natural frequencies and waveforms, the measured data are approximate and are for reference only.
3. 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 IC can be judged whether it is good or bad by measuring the total current. It can also be used to measure the voltage drop of the resistor in the power path, and use Ohm's law to calculate the total current value.
