Electrolytic Capacitors are generally used when very large capacitance values are required. Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode).
The dielectric is a very thin layer of oxide which is grown electro-chemically in production with the thickness of the film being less than ten microns. This insulating layer is so thin that it is possible to make capacitors with a large value of capacitance for a small physical size as the distance between the plates, d is very small.
Electrolytic Capacitor
The majority of electrolytic types of capacitors are Polarised, that is the DC voltage applied to the capacitor terminals must be of the correct polarity, i.e. positive to the positive terminal and negative to the negative terminal as an incorrect polarisation will break down the insulating oxide layer and permanent damage may result.
All polarised electrolytic capacitors have their polarity clearly marked with a negative sign to indicate the negative terminal and this polarity must be followed.
Electrolytic Capacitors are generally used in DC power supply circuits due to their large capacitance’s and small size to help reduce the ripple voltage or for coupling and decoupling applications. One main disadvantage of electrolytic capacitors is their relatively low voltage rating and due to the polarisation of electrolytic capacitors, it follows then that they must not be used on AC supplies. Electrolytic’s generally come in two basic forms; Aluminium Electrolytic Capacitors and Tantalum Electrolytic Capacitors.
Electrolytic Capacitor
1. Aluminium Electrolytic Capacitors
There are basically two types of Aluminium Electrolytic Capacitor, the plain foil type and the etched foil type. The thickness of the aluminium oxide film and high breakdown voltage give these capacitors very high capacitance values for their size.
The foil plates of the capacitor are anodized with a DC current. This anodizing process sets up the polarity of the plate material and determines which side of the plate is positive and which side is negative.
The etched foil type differs from the plain foil type in that the aluminium oxide on the anode and cathode foils has been chemically etched to increase its surface area and permittivity. This gives a smaller sized capacitor than a plain foil type of equivalent value but has the disadvantage of not being able to withstand high DC currents compared to the plain type. Also their tolerance range is quite large at up to 20%. Typical values of capacitance for an aluminium electrolytic capacitor range from 1uF up to 47,000uF.
Etched foil electrolytic’s are best used in coupling, DC blocking and by-pass circuits while plain foil types are better suited as smoothing capacitors in power supplies. But aluminium electrolytic’s are “polarised” devices so reversing the applied voltage on the leads will cause the insulating layer within the capacitor to become destroyed along with the capacitor. However, the electrolyte used within the capacitor helps heal a damaged plate if the damage is small.
Since the electrolyte has the properties to self-heal a damaged plate, it also has the ability to re-anodize the foil plate. As the anodizing process can be reversed, the electrolyte has the ability to remove the oxide coating from the foil as would happen if the capacitor was connected with a reverse polarity. Since the electrolyte has the ability to conduct electricity, if the aluminium oxide layer was removed or destroyed, the capacitor would allow current to pass from one plate to the other destroying the capacitor, “so be aware”.
2. Tantalum Electrolytic Capacitors
Tantalum Electrolytic Capacitors and Tantalum Beads, are available in both wet (foil) and dry (solid) electrolytic types with the dry or solid tantalum being the most common. Solid tantalum capacitors use manganese dioxide as their second terminal and are physically smaller than the equivalent aluminium capacitors.
The dielectric properties of tantalum oxide is also much better than those of aluminium oxide giving a lower leakage currents and better capacitance stability which makes them suitable for use in blocking, by-passing, decoupling, filtering and timing applications.
Also, Tantalum Capacitors although polarised, can tolerate being connected to a reverse voltage much more easily than the aluminium types but are rated at much lower working voltages. Solid tantalum capacitors are usually used in circuits where the AC voltage is small compared to the DC voltage.
However, some tantalum capacitor types contain two capacitors in-one, connected negative-to-negative to form a “non-polarised” capacitor for use in low voltage AC circuits as a non-polarised device. Generally, the positive lead is identified on the capacitor body by a polarity mark, with the body of a tantalum bead capacitor being an oval geometrical shape. Typical values of capacitance range from 47nF to 470uF.
Aluminium & Tantalum Electrolytic Capacitor
Electrolytic’s are widely used capacitors due to their low cost and small size but there are three easy ways to destroy an electrolytic capacitor:
- Over-voltage – excessive voltage will cause current to leak through the dielectric resulting in a short circuit condition.
- Reversed Polarity – reverse voltage will cause self-destruction of the oxide layer and failure.
- Over Temperature – excessive heat dries out the electrolytic and shortens the life of an electrolytic capacitor.