Aluminum electrolytic capacitors have a finite life. They consist of two strips of aluminum foil coated with a paste made of aluminum oxide, water, and other chemicals. If the water dries out, the capacitorís value drops and the internal resistance increases. At 25o C, the useful life is many years, but as the temperature increases, the life drops exponentially.
If a capacitor has an internal resistance of 0.1 ohms and a current of two amps, then the power dissipated in the capacitor is P = I2R = 2*2*0.1 = 0.4 watts. That is enough to raise the temperature at least a few degrees. Add that to the heat from the diode and the surrounding parts, and it could be enough to raise the internal temperature to 40o C or more.
With time, as the capacitor operates at this high temperature, the internal resistance will increase and the capacitance decreases. Even as the capacitor deteriorates, it must still supply two amps to the load during the non-conducting part of the cycle. The series resistance increases, and internal heating increases as the capacitor deteriorates. This causes a thermal runaway until the top bulges and the steam escapes. At that point, the capacitance drops rapidly and the product using the capacitor fails to operate properly, if at all.
The design engineer of my satellite receiver probably considered only the ripple from the power supply and never considered the long-term consequences to the filter capacitor by using a half-wave rectifier. Had he used a full-wave rectifier, the average current in the capacitor would have been no more than a few hundred milliamps, causing minimal internal heating and much longer life.
I suspect that most of the failure of aluminum electrolytic capacitors is the result of excessive current caused by thoughtless design. Hopefully, designers will read this and work around the problem of high current in capacitors, or use capacitors designed to handle high current without deterioration. Of course, the use of high-quality parts that are not underrated is always a good practice!
This entry was submitted by Frank Karkota and edited by Rob Spiegel.
Frank Karkota worked with power transmitters in the range of less than 1 MHz to 5 GHz. He designed and built equipment for radio stations and eventually started a company that made commercial and consumer receivers that covered 500 kHz to almost 1 GHz.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.