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Electrolytic Capacitor Polarity

June 16th Fact-of-the-Day

Most types of capacitors do not have a specified polarity and they can be used with either polarity. That is not true of electrolytic capacitors, because the dielectric is a thin aluminum oxide layer coated on only one of two aluminum foil plates. The aluminum oxide layer is held in place by the electric field if the polarity is correct. However, it dissolves into the electrolyte (which is usually boric acid or sodium borate in water with an additive to reduce evaporation) if the polarity is reversed. That causes an electrical short-circuit between the aluminum plate the aluminum oxide layer was on and conducting electrolyte in contact with the other plate. A large-current results under this condition, unless it is externally limited, and the electrolyte will heat to where a capacitor would explode without the safety-pressure-value provided in modern electrolytic capacitors. ©2005 Martek International All rights reserved.

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