POWER & ENERGY EFFICIENCY HANDBOOK
Why electrolytic capacitors blow up Electrolytic capacitors have a reputation for failing spectacularly when mistreated.
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open a common LED bulb and you’ll often find an electrolytic capacitor occupying a place in the input from the ac line. Though illumination-grade LEDs generally have lifetimes exceeding 10,000 hours, the electrolytic caps in their base may not last nearly that long. There may be a variety of reasons for such bad outcomes. Perhaps the main reason for difficulties with electrolytic caps is their poor performance when exposed to reverse voltages. Electrolytics are polarized devices that only work well when applied signals on the cap’s positive terminal exceed that on the negative terminal. The sensitivity to polarity arises because of the cap’s construction. The most common electrolytic cap is the aluminum electrolytic. Its anode electrode (+) is a pure aluminum foil with an etched surface. A thin insulating layer of aluminum oxide acts as the dielectric of the capacitor. A nonsolid electrolyte covers the rough surface of the oxide layer, serving in principle as the cathode (-). A second aluminum foil called “cathode foil” touches the electrolyte and serves as the electrical connection to the cathode. The entire assembly is rolled up to form the distinctive cylindrical shape defining electrolytics. A point to note is that applying a positive voltage to the anode material in the electrolytic bath forms an insulating aluminum oxide layer. Its
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DESIGN WORLD — EE NETWORK
10 • 2020
Leland Teschler • Executive Editor
thickness corresponds to the applied voltage. This oxide layer acts as the dielectric. After forming a dielectric oxide on the rough anode structures, a counter-electrode must match the rough insulating oxide surface. The electrolyte serves this purpose. The thickness of the dielectric is thin, typically measured in nanometers. The voltage strength of the oxide layer is quite high in the right direction. But exceeding the maximum voltage spec can make the capacitor look like an electrical short. The result can make for a noteworthy video as evidenced by the number of exploding capacitors available for viewing on YouTube. Here’s where polarity comes in: Applying a signal with the wrong polarity prevents the oxide layer from forming. The result, again, can be catastrophic failure. Of course, application circuits that are functioning properly will supply
Setting series capacitance balance resistance
Nichicon Inc. gives recommendations for balancing electrolytic capacitors wired in series so their individual leakage currents don’t cause problems.
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