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Old 6th February, 2008, 08:28 PM
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Capacitors come in a lot of different types, ceramic, mylar, polypropylene, aluminum electrolytic, tantalum electrolytic (just to name some of the most popular kinds). The problem is that 'lytics are the only designs that give you large capacitance in a manageable size: e.g. a ceramic capacitor that could give you 100 µF of storage would be about the size of a dinner plate and would weigh several pounds.

Tantalum electrolytics are usually much more capable than aluminum electrolytics, but are also MUCH more expensive; It's not uncommon for a tantalum to cost as much or more than an aluminum that is an order of magnitude bigger. Consequently, tantalums are generally only used where nothing else will do.

Now, to get at what I suspect is driving your question.

Aluminum electrolytics are not inherently bad. As with many things, you have to use parts from a reputable supplier, AND you have to understand their limitations. Even when you use parts from a reputable supplier, you still have the possibility of getting bad parts occasionally, as with anything else.

Understanding the limitations of the parts is the bigger issue. Aluminum electrolytics dry out over time; that's just the nature of the beast. Operation at high temps will make them dry out faster, as will operation at elevated current levels, both of which tend to happen in power supplies, and to a lesser extent in motherboards, especially in the CPU vreg section. Good designs by competent engineers will tend to use parts that are somewhat overrated for the application in order to ensure a reasonable design life, but that costs more money. Hence, cheap designs tend to do three things that cause them to fail prematurely:
  1. Source parts from the cheapest suppliers available: cost, not quality, becomes the overriding consideration.
  2. Use parts that are of adequate capacity and voltage for the application when they are new, without allowing for derating as the parts age.
  3. Use parts that are rated for 80C operating temps rather than 105C operating temps. Strictly speaking, an 80C part may be within tolerances for the application, but they also tend to break down more rapidly with operation at elevated temperatures. This gets back tonot allowing for derating of the parts as they age.

So, in general, don't blame electrolytics for being bad; they do what they are designed to do very well. Instead, blame the manufacturers who insist on using parts that barely meet spec when new in an effort to shave costs.

There's a reason quality PSUs cost more than bargain-basement units.
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