Skill sharing ! How to judge whether the capacitor is good or bad?

As an indispensable part of our daily electrical products, capacitors are widely used in DC blocking, coupling, bypass, filtering, tuning loops, energy conversion, control circuits, etc. Most of the causes of damage to electrical products may be caused by damaged capacitors, so how do we test whether the capacitor is good or bad? Today qihe smt equipment take this opportunity sharing with you some methods of measuring the quality of capacitance!

How to measure the quality of mini capacity capacitor?

Detect small capacitors below 10pF, because the capacity of fixed capacitors below 10pF is too small, if you use a pointer-type multimeter to measure, you can only check qualitatively whether there is leakage, internal short circuit or breakdown.

When measuring, you can choose the R×10k gear of the multimeter, and connect the two pins of the capacitor with the metal measuring rods of the two ammeters, and the resistance value should be infinite. If the measured resistance value (the indicator swings to the right) is zero, it means that the capacitor is damaged by leakage or internal breakdown.

For fixed capacitance above 0.01μF, you can use the R×10k block of the pointer multimeter to directly test whether the capacitor has a charging process and whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right.

How to detect whether the electrolytic capacitor is good or bad?

Because the capacity of electrolytic capacitors is much larger than that of ordinary fixed capacitors, when measuring, you should choose a suitable range for different capacities. According to experience, in general, the capacitance between 1 and 47μF can be measured with R×1k gear, and the capacitance larger than 47μF can be measured with R×100 gear.

Connect the metal measuring stick of the red electric meter of the multimeter to the negative pole, and the metal measuring stick of the black electric meter to the positive pole. At the moment of first contact, the pointer of the multimeter will deflect to the right by a large degree (for the same electric resistance, the larger the capacity, the larger the swing ), and then gradually turn to the left until it stops at a certain position.

The resistance at this time is the forward leakage resistance of the electrolytic capacitor, which is slightly larger than the reverse leakage resistance. Practical use experience shows that the leakage resistance of electrolytic capacitors should generally be above several hundred kΩ, otherwise, it will not work normally.

In the test, if there is no charging phenomenon in the forward and reverse directions, that is, the hands of the watch do not move, it means that the capacity has disappeared or the internal circuit is broken; if the measured resistance value is very small or zero, it means that the capacitor has a large leakage or has been broken down. Can no longer be used.

For electrolytic capacitors with unknown positive and negative signs, the above method of measuring leakage resistance can be used to identify them. That is, first randomly measure the leakage resistance, remember its size, and then exchange the metal measuring rod of the ammeter to measure a resistance value.

The one with the larger resistance value in the two measurements is the positive connection method, that is, the metal measuring rod of the black ammeter is connected to the positive pole, and the metal measuring rod of the red ammeter is connected to the negative pole.

Use a multimeter to block electricity, use the method of forward and reverse charging the electrolytic capacitor, and estimate the capacity of the electrolytic capacitor according to the size of the pointer swinging to the right.

Measure the discharge speed of the capacitor

Use a digital multimeter on both ends of the capacitor to charge first, then open the circuit to measure the speed at which the voltage drops. Because the capacitor is an open circuit, the only thing that consumes power is the leakage current. If you measure the curve of the voltage and time, you can reverse the leakage current. Remember to use a better digital multimeter, because the input impedance of the multimeter itself is limited no matter how large it is. If it is a good quality capacitor with low leakage current, then a digital multimeter with a slightly smaller input impedance will not be accurate.

Attention! If you need to measure positive and negative back and forth when measuring a large capacity capacitor, you must short-circuit and discharge the capacitor to avoid damaging the meter head.

According to experience, in high-frequency circuits, switching power supply circuits have many small capacitors that cannot be correctly judged by ordinary multimeters, and some capacitors may increase. It is strongly recommended to measure with a dedicated digital capacitance meter.

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