Electronics/Capacitor Construction
Electrolytic capacitor
editFrom Wikipedia, the free encyclopedia: Electrolytic_capacitor
An electrolytic capacitor is a type of capacitor with a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, at the frequency or twice the frequency of AC input power, in rectifier output, and especially in the absence of rechargeable batteries that can provide similar low-frequency current capacity.
Electrolytic capacitors are constructed from two conducting aluminium foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. (Two popular geometries use axial leads, or two leads or lugs in one circular face of the cylinder, respectively.)
A common modelling circuit for an electrolytic capacitor has the following schematic:
R_leak
_______
o-----|_______|-----o
| |
| |
(+) | || | _______ (-)
0-----o--------||---------o-----|_______|-----oOoOoOo-----0
|| R_ESR L_ESL
C
where R_leak is the leakage resistance, R_ESR is the Equivalent Series Resistance, L_ESL the Equivalent Series Inductance (L being the conventional symbol for inductance).
R_ESR must be as small as possible since it determines the loss power when the capacitor is used to smooth voltage. Loss power scales linearly with the ripple current flowing through and quadratically with R_ESR. Low ESR capacitors are imperative for high efficiencies in power supplies.
It should be pointed out that this is only a simple model and does not include all the effects associated with real electrolytic capacitors.
Since the electrolytes evaporate, design life is most often rated in hours at a set temperature. For example, typically as 2000 hours at 105 degrees Celsius (which is the highest working temperature). Design life doubles for each 10 degrees lower, reaching 15 years at 45 degrees.
Electrolytic capacitors may explode (have weak link safety valve) when charged up with wrong polarity.
What are they made of
Aluminum electrolytic capacitors contain aluminum foil, porous paper, and an electrolyte. This is usually boric acid or sodium borate in water with some sugars or ethylene glycol added to retard evaporation. While you should not eat this, nor get it in your eyes, it is not very corrosive or dangerous. Simply wash it off your skin after you gut the old capacitor.
Wet-slug tantalum electrolytics (low voltage and so expensive that only the military can generally afford them) do contain sulfuric acid. I doubt if these are ever found in audio equipment.
It is important, however, to always be careful and not take chances. Safety glasses are always advised. With chemicals, extra precaution is always worthwhile.
External link
* Electrolytic Capacitors