Bulging Capacitors & Thoughtless Design

DN Staff

June 13, 2013

3 Min Read
Bulging Capacitors & Thoughtless Design

Several years ago, I bought a used digital satellite receiver for reception of free-to-air (FTA) programs. The receiver worked great, and I mapped hundreds of television and radio stations on the various satellites. Later, some of the stations disappeared. Then, more started disappearing, until I couldn't find any stations. Since my analog receiver still worked, I concluded that the digital receiver had failed.

I opened the receiver and found that in one corner was the rf section, another corner had the digital section, and the other side housed the power supply. My first step was to check the voltages from the power supply. Each output was marked with a test point. Most of the voltages were low, indicating a failure common to all of the supplies. As I made a physical inspection of the supply, I saw a large diode. Beside the diode was a large electrolytic with a noticeable bulge on the top.

After replacing the capacitor, the receiver worked perfectly. This was not the first time I had found a defective electrolytic to be the problem. I have read blogs and articles about bad electrolytics causing failures in televisions, computer monitors, and other electronic devices. The writers of these blogs and articles wrote it off as cheap or underrated capacitors, but I was not so sure.

I did not have a schematic of the satellite receiver, but everything was laid out in an orderly fashion, so I was able to follow the circuit. The incoming line voltage was rectified by a full-wave bridge and the output was filtered with a high-voltage electrolytic capacitor. The voltage was probably about 150V. This powered an oscillator of probably tens or hundreds of kilohertz. The output of the oscillator fed a small transformer with a ferrite core. The output was rectified by the large diode and then into the large electrolytic capacitor, which I had just replaced. That voltage went to the individual voltage regulators for the analog and digital circuits.

What surprised me was the size of the rectifier diode. It must have been a three amp diode and it was well above the board to keep it cool. It was right next to the capacitor. Since there was only one diode, it was a half-wave rectifier. The size and mounting of the diode made me conclude that the supply delivered at least two amps to the various small regulators.

With a supply using a half wave rectifier, the current (in this case, two amps) is supplied to the load by the filter capacitor during the non-conducting part of the cycle. During the conducting part of the cycle, the output of the diode recharges the capacitor, and also provides power to the receiver. Thus, the diode will have an average current of two amps and the capacitor will also have an average current of two amps, although half the time it will be to charge the capacitor, and half the time to discharge the capacitor.

Since I primarily design radio frequency circuits, I am aware that capacitors are complex. They have series resistance, series inductance, and parallel resistance. When selecting the appropriate capacitor for an rf circuit, I have to consider all of these parasitic elements. In audio and power supply work, the parasitics can usually be ignored. I did some investigation of capacitor specifications and found that the series resistance for a 1,000 uf electrolytic capacitor is approximately 0.1 ohms.

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