Electrolytic capacitors
- an overview, information or tutorial about the basics of the electrolytic
capacitor: its construction, properties and the uses of the electrolytic
capacitor.
Today electrolytic capacitors are used in huge quantities.
They are very cost effective and able to provide a larger capacitance per volume
than other types of capacitor. This gives them very many uses in circuits where
high currents or low frequencies are involved. Typically they are used most in
applications such as audio amplifiers of all types (hi-fi to mobile phones) and
in power supply circuits.
Like any other capacitor, it is necessary to understand the
advantages and limitations of these capacitors to enable them to be used most
effectively.
Electrolytic capacitor development
The electrolytic capacitor has been in use for many years.
Its history can be traced back to the very early days or radio around the time
when the first broadcasts of entertainment were being made. At the time, valve
wireless sets were very expensive, and they had to run from batteries. However
with the development of the indirectly heated valve or vacuum tube it became
possible to use AC mains power. While it was fine for the heaters to run from an
AC supply, the anode supply needed to be rectified and smoothed to prevent mains
hum appearing on the audio. In order to be able to use a capacitor that was not
too large Julius Lilienfield who was heavily involved in developing wireless
sets for domestic use was able to develop the electrolytic capacitor, allowing a
component with sufficiently high capacitance but reasonable size to be used in
the wireless sets
Construction of electrolytic capacitors
The plates of an electrolytic capacitor are constructed from
conducting aluminium foil. As a result they can be made very thin and they are
also flexible so that they can be packaged easily at the end of the production
process. The two plates, or foils are slightly different. One is coated with an
insulating oxide layer, and a paper spacer soaked in electrolyte is placed
between them. The foil insulated by the oxide layer is the anode while the
liquid electrolyte and the second foil act as cathode.
In order to package them the two aluminium foils with the
electrolyte soaked paper are rolled together to form a cylinder, and they are
placed into an aluminium can. In this way the electrolytic capacitor is compact
while being robust as a result of the protection afforded by the can.
There are two geometries that are used for the connection
leads or tags. One is to use axial leads, one coming from each circular face of
the cylinder. The other alternative is to use two radial leads or tags, both of
which come from the same face of the cylinder.
The lead styles give rise to the descriptions used for the
overall capacitors. Descriptions of axial and radial will be seen in the
component references.
Electrolytic capacitor properties
There are a number of parameters of importance beyond the
basic capacitance and capacitive reactance when using electrolytic capacitors.
When designing circuits using electrolytic capacitors it is necessary to take
these additional parameters into consideration for some designs, and to be aware
of them when using electrolytic capacitors.
- ESR Equivalent series resistance: Electrolytic
capacitors are often used in circuits where current levels are relatively
high. Also under some circumstances and current sourced from them needs to
have a low source impedance, for example when the capacitor is being used in
a power supply circuit as a reservoir capacitor. Under these conditions it
is necessary to consult the manufacturers datasheets to discover whether the
electrolytic capacitor chosen will meet the requirements for the circuit. If
the ESR is high, then it will not be able to deliver the required amount of
current in the circuit, without a voltage drop resulting from the ESR which
will be seen as a source resistance.
- Frequency response: One of the problems with
electrolytic capacitors is that they have a limited frequency response. It
is found that their ESR rises with frequency and this generally limits their
use to frequencies below about 100 kHz. This is particularly true for large
capacitors, and even the smaller electrolytic capacitors should not be
relied upon at high frequencies. To gain exact details it is necessary to
consult the manufacturers data for a given part.
- Leakage: Although electrolytic capacitors have much
higher levels of capacitance for a given volume than most other capacitor
technologies, they can also have a higher level of leakage. This is not a
problem for most applications, such as when they are used in power supplies.
However under some circumstances they are not suitable. For example they
should not be used around the input circuitry of an operational amplifier.
Here even a small amount of leakage can cause problems because of the high
input impedance levels of the op-amp. It is also worth noting that the
levels of leakage are considerably higher in the reverse direction.
- Ripple current: When using electrolytic capacitors in
high current applications such as the reservoir capacitor of a power supply,
it is necessary to consider the ripple current it is likely to experience.
Capacitors have a maximum ripple current they can supply. Above this they
can become too hot which will reduce their life. In extreme cases it can
cause the capacitor to fail. Accordingly it is necessary to calculate the
expected ripple current and check that it is within the manufacturers
maximum ratings.
Polarisation
Unlike many other types of capacitor, electrolytic capacitors
are polarised and must be connected within a circuit so that they only see a
voltage across them in a particular way. The capacitors themselves are marked so
that polarity can easily be seen. In addition to this it is common for the can
of the capacitor to be connected to the negative terminal.
It is absolutely necessary to ensure that any electrolytic
capacitors are connected within a circuit with the correct polarity. A reverse
bias voltage will cause the centre oxide layer forming the dielectric to be
destroyed as a result of electrochemical reduction. If this occurs a short
circuit will appear and excessive current can cause the capacitor to become very
hot. If this occurs the component may leak the electrolyte, but under some
circumstances they can explode. As this is not uncommon, it is very wise to take
precautions and ensure the capacitor is fitted correctly, especially in
applications where high current capability exists.
Electrolytic SMD capacitors
Electrolytic capacitors are now being used increasingly in
SMD designs. Their very high levels of capacitance combined with their low cost
make them particularly useful in many areas. Originally they were not used in
particularly high quantities because they were not able to withstand some of the
soldering processes. Now improved capacitor design along with the use of reflow
techniques instead of wave soldering enables electrolytic capacitors to be used
more widely in surface mount format.
Often SMD electrolytic capacitors are marked with the value
and working voltage. There are two basic methods used. One is to include their
value in microfarads (m F), and another is to use a code. Using the first method
a marking of 33 6V would indicate a 33 mF capacitor
with a working voltage of 6 volts. An alternative code system employs a letter
followed by three figures. The letter indicates the working voltage as defined
in the table below and the three figures indicate the capacitance on picofarads.
As with many other marking systems the first two figures give the significant
figures and the third, the multiplier. In this case a marking of G106 would
indicate a working voltage of 4 volts and a capacitance 0f 10 times 10^6
picofarads. This works out to be 10 mF
| Letter |
Voltage |
| e |
2.5 |
| G |
4 |
| J |
6.3 |
| A |
10 |
| C |
16 |
| D |
20 |
| E |
25 |
| V |
35 |
| H |
50 |
Voltage codes for SMD electrolytic capacitors
|
