A fixed capacitor is a capacitor with a fixed capacitance. A variable capacitor is a capacitor whose capacitance can be adjusted within a certain range. When the relative effective area between the pole pieces or the distance between the pieces changes, its capacitance changes accordingly. Variable capacitors are often used as tuning capacitors in radio receiver circuits.
1. Detection of fixed capacitors
1. Detect small capacitors below 10pF
Because the capacity of the fixed capacitor below 10pF is too small, measuring with a multimeter can only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, you can use the multimeter R×10k block, and use two test pens to connect the two pins of the capacitor at will, and the resistance value should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it means that the capacitor is damaged by leakage or internal breakdown.
2. Detect 10PF～0.01μF fixed capacitor
The multimeter selects R×1k block. The β value of the two triodes is above 100, and the penetration current should be small. 3DG6 and other silicon triodes can be selected to form a composite tube. The red and black test leads of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplifying effect of the composite triode, the charging and discharging process of the capacitor under test is amplified, so that the pendulum of the multimeter pointer is increased, which is convenient for observation.
3. Detection of electrolytic capacitors
A. Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, when measuring, appropriate ranges should be selected for different capacities. According to experience, in general, the capacitance between 1 and 47μF can be measured in R×1k block, and the capacitance larger than 47μF can be measured in R×100 block.
B. Connect the red test lead of the multimeter to the negative electrode and the black test lead to the positive electrode. At the moment of first contact, the multimeter pointer is deflected to the right by a large deflection (for the same electrical block, the larger the capacity, the larger the swing), and then gradually to the left Rotate until it stops at a certain position.
The resistance value at this time is the forward leakage resistance of the electrolytic capacitor, which is slightly larger than the reverse leakage resistance. The actual use experience shows that the leakage resistance of electrolytic capacitors should generally be above several hundred kΩ, otherwise, it will not work properly. In the test, if there is no charging phenomenon in the forward and reverse directions, that is, the needle does not move, it means that the capacity has disappeared or the internal circuit is open; Can no longer be used.
C. For electrolytic capacitors whose positive and negative signs are unknown, the above method of measuring leakage resistance can be used to identify them. That is, first measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads to measure a resistance value. The one with the larger resistance value in the two measurements is the forward connection method, that is, the black test lead is connected to the positive electrode, and the red test lead is connected to the negative electrode.
D. Use a multimeter to block electricity, and use the method of forward and reverse charging of the electrolytic capacitor. According to the magnitude of the pointer swinging to the right, the capacity of the electrolytic capacitor can be estimated.
Second, the detection of variable capacitors
A. Gently rotate the shaft by hand, it should feel very smooth, and it should not feel loose and tight or even stuck. When the carrier shaft is pushed forward, backward, up, down, left, right, etc., the rotating shaft should not be loose.
B. Rotate the shaft with one hand, and touch the outer edge of the moving film group with the other hand, you should not feel any looseness. A variable capacitor with poor contact between the rotating shaft and the moving plate cannot be used any longer.
C. Put the multimeter in the R×10k block, connect the two test pens to the moving piece of the variable capacitor and the lead end of the fixed piece with one hand, and slowly rotate the shaft with the other hand several times. Should be stationary at infinity.
In the process of rotating the shaft, if the pointer sometimes points to zero, there is a short-circuit point between the moving plate and the fixed plate; if a certain angle is encountered, the reading of the multimeter is not infinite, but a certain resistance value will appear, indicating that the variable There is a leakage phenomenon between the moving plate and the stator of the capacitor.
(Summarized by Easybom)
