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Common Fault Analysis of Filter Capacitors

Views: 6     Author: Site Editor     Publish Time: 2022-03-16      Origin: Site

The common faults of filter capacitors are breakdown, leakage, open circuit and capacity reduction, etc. Today, we will conduct a simple fault analysis of filter capacitors.

Let's talk about breakdown and leakage first. The breakdown and short circuit of the filter capacitor can cause the failure of the low-voltage fuse to blow, the rectifier tube or the semiconductor rectifier element to burn. Among them, if the input capacitor of the filter breaks down, the impact will be great.When the output capacitor of the filter is broken down, the choke or filter resistor plays a role of limiting current, so the risk of damaging the rectifier tube is slightly smaller, but excessive current will make the choke or filter resistor overheat.The leakage of electricity will make the rectifier tube work overload, the rectifier output voltage will be reduced, and in severe cases, it may also endanger the rectifier tube or semiconductor rectifier components.It is usually feasible to use ohmmeter to measure the open or short circuit fault of capacitors, but the leakage resistance value varies greatly due to the different types of electrolytic capacitors, so it is not good enough to measure the leakage situation.The other way is to try a good one.

Secondly, the fault of open circuit or capacity reduction is analyzed. The input capacitor of the filter has the function of charging to the peak value of the input ac voltage, so it has a great influence on the size of the rectified output voltage. In a general half-wave, full-wave or bridge rectifier circuit, when the capacitor is open-circuited or the capacity is reduced, the rectified output voltage will decrease a lot. Taking a TV as an example, it is likely to appear dark, small grating amplitude, poor focus, insufficient image contrast, poor synchronization, or low sound.

When the output capacitor of the filter has an open circuit or a capacity reduction fault, it has no obvious effect on the rectified output voltage, but because the filtering efficiency of the filter is not good at this time, the ripple component of the output voltage is large, so various problems will also occur.In order to obtain better image reproduction and sound quality, the DC operating voltage in the TV should be well filtered. If the ripple component is too large due to a defective filter, it will distort the raster, image or sound. In low frequency amplifiers, excessive ripple voltage can produce hum. In amplifiers, gratings appear due to excess ripple voltage, which is not too much of a description. Although the ripple voltage with very low frequency cannot directly pass through the high frequency part and the image intermediate frequency amplifier, it can also modulate the signal voltage in the high frequency part or the image intermediate frequency amplifier, so as to reach the picture tube. The characteristic at this time is that the grating itself does not have this black band, but only when it is received.

In addition to the output filter capacitor open or reduced capacity can cause poor low-voltage rectifier filter, there are many other reasons for the fault, such as the input filter capacitor open or reduced capacity, local short circuit of the filter choke, poor contact between rectifier tube and tube holder, etc. These problems will lead to full wave rectification to half wave rectification, ripple voltage will increase, and its frequency from 100 hz to 50 hz, then the filtering effect is not sufficient.

When the output capacitor has an open-circuit fault or the capacity is reduced, in addition to the above-mentioned fault phenomenon due to the increase of the ripple component in the output voltage, other forms of adverse effects will also occur. For example, the output capacitor has the function of decoupling. If it does not work, there will be parasitic coupling between the various circuits of the load, so there may be phenomena such as sound interference image or self-excitation. In addition, this kind of failure is sometimes particularly easy to affect the operation of the horizontal scanning circuit, and thus has a more serious impact on the raster than the above. Among them, there are two common effects, one is the small horizontal size of the grating, and the other is the horizontal curling of the grating, and the light is dark. Therefore, when encountering this kind of fault phenomenon, if the horizontal scanning circuit is checked for no fault, and the B+ voltage is not low, it is necessary to pay attention to whether the cause of the fault lies in the power supply part.

There are three kinds of open-circuit faults of electrolytic capacitors. One is that the shell and the bottom plate are in poor contact. At this time, it is enough to re-tighten the nut. The other is that the external lead solder tab of the electrolytic capacitor is not fixed tightly, and it can be re-fixed at this time. The third type is that the lead wire in the electrolytic capacitor is loose or broken. This kind of capacitor can also be repaired. It is necessary to disassemble the casing, then fasten the rivet, or reconnect the broken lead wire.In addition to the common faults mentioned above, the electrolytic capacitor of the low-voltage rectifier circuit may also burst. The reasons are generally high voltage of the circuit, insufficient voltage resistance of the capacitor, or the breakdown of the semiconductor rectifier element and the capacity of the low-voltage fuse is too large and so on.

Have you learned about the failure analysis of filter capacitors?


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