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July 4, 2014
News - Why You Should De-Rate Ca…
about 2 years ago
Shawn - thanks for the capacitor overview. I agree that de-rating is an important concept that does not get enough attention and you’ve covered it well.
I would like to clarify a couple items:
“electrolytic capacitors cannot be used to couple AC signals”
This is not entirely accurate. I think what you are trying to say is that reverse polarity on a polarized electrolytic can make for a bad day. We (electrical engineers) use electrolytics all the time to couple one amplifier stage to another. The capacitor serves to couple or pass the AC signal while blocking the DC portion. If there is a DC bias present (so polarity is not reversed on the electrolytic), then electrolytics work well in this kind of an application.
There have also been quite a few comments already on tantalum capacitors, but I will add that they are preferred in high reliability applications (space, military, telecommunications) and places where the solid dielectric works better than the liquid/paste in an electrolytic and they tend to be smaller than ceramics (for a similar capacitance value). A 50% voltage de-rating is more than sufficient for electrolytic, tantalum, ceramic, and many other dielectrics as long as you do not stress them in other areas (e.g. ripple current in a power supply filter or thermal environment in the case of something mounting in an outdoor environment). Experimenters should not be discouraged from using solid dielectric capacitors like tantalum, niobium, and polymers in general. They should have a place in the designer’s toolbox along side their larger, cheaper electrolytic cousins.
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