Capacitor testing methods are mainly divided into three categories: testing of variable capacitors, testing of electrolytic capacitors, and testing of fixed capacitors.
I. Testing of Variable Capacitors
A. Gently rotate the shaft by hand; it should feel very smooth, without any intermittent tightness or jamming. When pushing the load shaft forward, backward, up, down, left, and right, the shaft should not feel loose.
B. Rotate the shaft with one hand and gently touch the outer edge of the moving plate assembly with the other hand. There should be no looseness felt. A variable capacitor with poor contact between the shaft and the moving plate should no longer be used.
C. Set the multimeter to the R×10k range. With one hand, connect the two probes to the leads of the moving and fixed plates of the variable capacitor, respectively. With the other hand, slowly rotate the shaft several times. The multimeter pointer should remain at infinity throughout the rotation. If the pointer sometimes points to zero during the rotation, it indicates a short circuit between the moving and fixed plates. If, at a certain angle, the multimeter reading is not infinite but shows a certain resistance, it indicates leakage between the moving and fixed plates of the variable capacitor.
II. Testing of Fixed Capacitors
For capacitors smaller than 10pF, due to their small capacitance, a multimeter can only qualitatively check for leakage, internal short circuits, or breakdown. When measuring, use a multimeter on the R×10k range. Connect the two probes to either lead of the capacitor; the resistance should be infinite. If the resistance is zero (the pointer swings to the right), the capacitor is either leaking, damaged, or internally broken down.
Test whether a 10pF to 0.01μF fixed capacitor is charging to determine its condition. Use the R×1k range on the multimeter. Both transistors should have a β value of 100 or higher, and their leakage current should be sufficient; a 3DG6 or similar silicon transistor can be used to form a composite transistor. Connect the red and black probes of the multimeter to the emitter (e) and collector (c) of the composite transistor, respectively. Due to the amplification effect of the composite transistor, the charging and discharging process of the capacitor under test is amplified, increasing the amplitude of the multimeter pointer swing, making it easier to observe. Note that during testing, especially with smaller capacitors, the leads of the capacitor under test should be repeatedly switched between points A and B to clearly observe the multimeter pointer swing.
For fixed capacitors above 0.01μF, the R×10k range of a multimeter can be used to directly test whether the capacitor is charging and whether there is an internal short circuit or leakage. The capacitance can be estimated based on the magnitude of the pointer swing to the right.
III. Testing of Electrolytic Capacitors
Because electrolytic capacitors have a much larger capacitance than regular fixed capacitors, the appropriate measurement range should be selected for different capacitance values. Based on experience, capacitors between 1 and 47 μF can generally be measured using the R×1k range, while capacitors larger than 47 μF can be measured using the R×100 range.
Connect the red probe of the multimeter to the negative terminal and the black probe to the positive terminal. At the instant of contact, the multimeter needle will deflect significantly to the right (for the same resistance range, the larger the capacitance, the greater the deflection), then gradually return to the left until it stops at a certain position. The resistance value at this point is the forward leakage resistance of the electrolytic capacitor, which is slightly greater than the reverse leakage resistance. Practical experience shows that the leakage resistance of an electrolytic capacitor should generally be several hundred kΩ or higher; otherwise, it will not function properly. During testing, if there is no charging in either the forward or reverse direction (i.e., the needle does not move), it indicates that the capacitance has disappeared or there is an internal open circuit. If the measured resistance is very small or zero, it indicates that the capacitor has a large leakage current or has broken down and is no longer usable.
For electrolytic capacitors with unclear positive and negative markings, the method described above for measuring leakage resistance can be used to determine their polarity. First, measure the leakage resistance arbitrarily and note its value. Then, reverse the probes and measure another resistance value. The higher resistance value indicates the correct connection, meaning the black probe is connected to the positive terminal and the red probe to the negative terminal.
D uses a multimeter in resistance mode to charge the electrolytic capacitor in both forward and reverse directions. The capacitance of the electrolytic capacitor can be estimated based on the magnitude of the pointer's rightward swing.
