This is only a preview of the May 1989 issue of Silicon Chip. You can view 38 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
|
l~ &J..y-tie--ea~auit&F-S-a~-e-thec~ acitor world, generally ha
"" ·
han f il ,-0.11°0 . er,.a
1
_o,
e_q :
-1~~ - - - -.2d.....
k re
..L.
_ _ _ _ _ _ _ _ _ _ _..:.:,o:.:::oM::::H.:..;1P;.;,:.RE:::A::::MP...;A;;;;Mo::..·_;;_'
UNITS DISPLAY
120'.,
Every DC power supply has one
or more electrolytic capacitors.
They are used wherever a large
amount of capacitance is required
in a small space. As already noted,
they generally require a DC voltage
to work properly but there are
special versions which can be used with high AC voltages such as
"motor start" and capacitors for
240V AC powered appliances.
Well, why are these capacitors
called "electrolytics"? The answer
is because they contain an "electrolyte", a chemical solution
through which an electric current
can pass. To properly explain · the
subject though, we'll have to back
up a bit and repeat what we said in
the first episode on capacitors.
There, we stated that every
capacitor has two electrodes or
plates which are separated by an
insulating medium known as the
dielectric. And so they have. But in
electrolytic capacitors, the method
of construction is quite different
and they involve a good deal of complex chemistry. Don't be put off
though- we'll tell you just as much
as you need to know, to avoid unnecessary confusion.
The vast majority of electrolytic
capacitors are based on aluminium
foil. Inside electrolytic capacitors
there are two aluminium foils
4
SILICON CHIP
_LJ_
~
14
It
over
ED-8IMPSON - - - - - - , t - - - - - ----'1,-J
__..
--------------+------4-.►+5V
wound together but separated by
an absorbent paper which is impregnated with a liquid electrolyte.
Now that description might not
sound all that different to the structure of a film/foil capacitor, as
described in the previous episode in
this series. But there is a radical
difference because the impregnated paper does not perform the
function of a dielectric - it is actually the negative electrode of the
capacitor!
Although there are two aluminium foils in an electrolytic capacitor, one is quite different from the
other. The foil connected to the
positive terminal of the capacitor,
known as the 'anode foil', is deeply
CATHODE
FOIL
Fig.1: inside an electrolytic capacitor.
It has two aluminium foils which are
wound together but separated by an
absorbent paper impregnated with a
liquid electrolyte.
etched to greatly increase its surface area and thereby the
capacitance. Secondly, the anode
foil has a thin coating of oxide.
Aluminium oxide is a very good insulator and it is this thin oxide
coating on the anode film which actually provides the dielectric of the
capacitor.
This oxide dielectric is very much
thinner than the film dielectric used
in plastic capacitors and so th,is is
another factor in the very high
capacitance of electrolytic capacitors.
So if the electrolyte is not the
dielectric, what is its purpose? It
actually provides the negative electrode of the capacitor. Since the
electrolyte is a liquid (more correctly, a paste), it is in intimate contact
with the deeply etched oxide surface of the anode foil and thereby
allows the enormous surface area
of the foil to fully contribute to the
total capacitance.
The other aluminium foil in the
capacitor is called the "cathode
film'' and it makes the electrical
connection from the negative terminal to the electrolyte.
The electrolyte is an organic
solution, which often used to be
glycol borate but nowadays is likely
to be more complex, depending on
the performance parameters the
CATHODE LEAD
ALUMINIUM CAN
DISC COMPOSED
OF PTFE
HARO PAPER
Fig.2: this cutaway drawing
shows all the essential
parts of an electrolytic
capacitor. The anode foil is
deeply etched to increase
its surface area and thus
the capacitance. The
impregnated paper forms
the negative electrode of
the capacitor.
ALUMINIUM FOIL ANODE
WITH ALUMINIUM OXIDE
DIELECTRIC
ALUMINIUM
i'OIL CATHODE
manufacturer is striving for. Some
of these more modern electrolytes
are dimethyl acetamine, dimethyl
formanide or butyrolactone.
To summarise, an electrolytic
capacitor has an anode film which
is deeply etched to increase its surface area and then its surface is oxidised to provide the capacitor's
dielectric. The electrolyte then provides the negative connection to the
capacitor.
Well, as you might expect, electrolytic capacitors are a great deal
more complicated than this short
description implies but this is adequate for the moment. To go deeper
would take up a great deal more
space.
The real reason for taking so
much space to describe the internal
structure of an electrolytic capacitor is to make the point that it is
quite different from other types of
capacitors.
ANODE LEAD
Range of capacitance
These vertical mount electrolytic capacitors range in value from 1250µF to
8000µF. Note the mounting clamps fitted to three of the capacitors.
Electrolytics are commonly
available over the counter in
capacitances ranging from 0.47µF
up to 10,000µF although they are
manufactured in values as small as
0.lµF and as large as 1,000,000µF
(1 Farad) for very large computer
grade capacitors.
In recent yea rs, another special
type of pola rised capacitor known
as a "super capacitor" or "double
layer capacitor" has become
available. These have extremely
large values of capacitance, up to 1
Farad, in very small cases. But
while these are electrolytics they
do not use the same oxide dielectric
principle as conventional aluminium electrolytic capacitors.
We 'll co me to double layer
capacitors later.
Polarisation and
voltage rating·
Aside from their relatively large
values of capacitance, the aspect
which distinguishes electrolytics
from other types of capacitor is the
fact that they can only be operated
with the correct polarity of DC
voltage applied to them. That is a
long-winded way of saying that they
MAY 1989
5
:,
JJ
·~•. ,
"Ii
.
i
'
·
.,
.
·1
'
'
'
.
·,:
V
;
"
'
,·.
-
-\
These are PC-mount electrolytics, designed for direct mounting on a printed
circuit board. The negative terminal is usually indicated by a minus symbol
and an arrow printed on the side of the case.
voltage ranges. Because electrolytic capacitors have become a
great deal smaller and because
their characteristics have become
much more stable and reliable, it is
unusual to find capacitors with a
rating below 25V for the smaller
values or below 16V for the larger
values.
It also used to be the case that for
long life, electrolytic capacitors
should be used in circuits at close to
the rated voltage. If this did not
happen, the capacitors would
gradually deteriorate. Nowadays
though, it is quite permissible to use
a capacitor with only a fraction of
its rated voltage applied to it.
For example, you can use a
lOOµF 16V capacitor with less than
1V applied to it. Indeed, this circuit
situation is very common. However,
it is still important that the DC
voltage across the capacitor is of
the correct polarity (ie, positive
voltage to positive terminal).
V and VW: what
do they mean?
Pigtail or axial lead electrolytics differ from PC-mount types by having a lead
at each end. On these units, plus and minus symbols are marked on the case
to indicate the positive and negative terminals.
must have a positive DC voltage at
their positive terminal.
If the capacitor is operated with
a reverse DC voltage, it will eventually fail and probably become a
short circuit.
Depending on the manufacturer,
electrolytic capacitors are made
with the following DC voltage
ratings: 6.3V, 10V, 16V, 25V, 35V,
6
SILICON CHIP
50V, 63V, 75V, 100V, 160V, 200V,
350V and 450V. However, it is also
possible to come across capacitors
rated at 40V, 80V and so on.
For a given value of capacitance,
a capacitor rated at 75V will be
much larger than one rated at 16V.
Nowadays though, most parts
wholesalers and retailers do not
stock capacitors in all the above
In SILICON CHIP and on most circuits, you will see electrolytic
capacitors specified with a value
and a voltage rating, such as 47µF
25VW. The value is straightforward enough but what is the meaning of "VW". VW stands for "volts
working". Some capacitors are
labelled "WV" which means exactly the same thing, "working volts".
In some ways the VW designation
is an anachronism, a holdover from
the days when all electrolytic
capacitors had two voltage ratings:
VW and VP. VP stands for "volts
peak " and is the surge voltage that
the capacitor can withstand for
short periods. The surge voltage is
generally 20% to 30% higher than
the rated voltage. It is related to the
voltage used to "form" the oxide
coating on the aluminium film and if
it is exceeded, the capacitor is
liable to fail within a very short
period of time.
These days most capacitors only
have their rated voltage printed on
them, together with their value and
polarity marking.
Lead types
Often, you '11 see electrolytic
capacitors referred to as PC-mount,
vertical mount, pigtail types, axial
lead or radial lead types. With the
exception of the last term, all these
are fairly descriptive. A pigtail type
is a conventional small can
capacitor with a lead at each end;
these are also known as "axial
lead".
PC-mount types are those which
have both leads coming out at one
end so that they can mount vertically on a printed circuit board. These
are also known as radial lead types.
Of course, pigtail capacitors can
be mounted on a printed circuit
board too but vertical or PC-mount
capacitors take up less board
space.
Leakage
Non-polarised or bipolar electrolytics
can be connected into circuit either
way around. They are identified by
an NP or BP label on side of the case.
Polarity marking
Since the voltage polarity across
an electrolytic capacitor is so
critical, it is important to be able to
distinguish which is the positive terminal and which is the negative terminal. On most electrolytics these
days, a minus symbol, and sometimes an arrow, is printed on one
side of the case, nearest to the
negative electrode.
On the other hand, you may come
across electrolytics where the
positive electrode is indicated with
an adjacent + symbol and there
may also be an arrow to reinforce
the message. On some pigtail electros, both the positive and negative
electrodes may be labelled.
On larger can type electrolytics,
the negative terminal may be indicated with a dab of black paint.
Alternatively, the positive terminal
may be indicated with a dab of red
paint or perhaps a + symbol
moulded into the lid.
Non-polarised electrolytics
Having made the point above
about the necessity for the DC
voltage needing to be of the correct
polarity, we will now muddy the
water by stating that with some
electrolytics, this is not a problem.
These are "bipolar" or "nonpolarised" electrolytics. They are
virtually two conventional electrolytics connected back-to-back inside a common can. They are made
with two etched and oxidised (formed) anode foils.
They can be used in circuits
where the DC voltage is indeter-
Compared to other types of
capacitors, such as plastic or film,
electrolytic capacitors have very
poor insulation. In fact, it is so poor,
relatively speaking, that instead of
expressing the insulation resistance in terms of hundreds or
thousands of megohms, it is usual to
express it as "leakage current" in
microamps or milliamps .
Small value electrolytics, say
with a value of 22µF or less, will
typically have a leakage current of
10 microamps or less at the rated
voltage. The larger can types, with
a capacitance of lO00µF or more,
will typically have a leakage current of 1 or 2 milliamps.
Tantalum electrolytics
Shown here larger than actual size,
this NEC super capacitor has a value
of .047 Farads and is rated at 5.5V.
Values of up to 1 Farad are
obtainable.
minate (ie, might be polarised one
way or the other) or where there is
no DC voltage but quite substantial
AC voltage. They are made in quite
a wide range of DC voltage ratings
although if you are buying them
over the counter you will usually
only be able to obtain·them with a
rating of 50 volts.
Non-polarised electrolytic capacitors are used where relatively
large capacitors are needed, say
up to lO0µF, and where the cost of
alternative plastic or paper capacitors would be prohibitive. A
typical application is in crossover
networks for loudspeaker systems.
On circuit diagrams, non-polarised capacitors are indicated with
the label "BP" or " NP". They are
also labelled this way on the can.
Tantalum is an alternative metal
to aluminium in electrolytic capacitors. Tantalum electros are in
values up to lO0µF and with a
restricted voltage (usually only
50V) range.
Tantalum electros can be made
in foil, wet sintered and solid types.
Tantalum foil capacitors are
similar to aluminium electrolytics
in that the foil is anodised but the
electrolyte is sulphuric acid.
Wet sintered tantalum electrolytics have a sintered tantalum
anode in an electrolyte of sulphuric
acid and ionised water or a gelled
electrolyte of sulphuric acid and
silica.
We mention tantalum foil and
wet sintered tantalum foil capacitors for the sake of completeness
but it is unlikely that such
capacitors ever become available
over the counter to enthusiasts.
Most people will only come across
the epoxy dipped solid tantalum
electrolytics which are widely
available and generally only slightly dearer than conventional aluminium electros.
The solid tantalum capacitor
again has a sintered tantalum
anode. The porous anode pellet is
impregnated with manganese
nitrate and heated to 400°C. This
decomposes the manganese nitrate
to solid manganese dioxide which
becomes the electrolyte.
When they were first introduced
to the market, about 20 years ago,
solid tantalum capacitors had a
MAY 1989
7
Table 1: Tantalum Capacitor Markings In the E12 Serles
Value
Alt value
IEC value
0.1µF
0.12µF
0.15µF
0.18µF
0.22µF
0.27µF
0.33µF
0.39µF
0.47µF
0.56µF
0.68µF
0.82µF
1.0µF
1.2µF
1.5µF
··1.8µF
2.2µF
2.7µF
3.3µF
3 .9µF
4.7µF
5.6µF
6 .8µF
8 .2µF
10µF
12µF
15µF
18µF
22µF
27µF
33µF
39µF
47µF
56µF
68µF
82µF
100µF
100nF
120nF
150nF
180nF
220nF
270nF
330nF
390nF
470nF
560nF
680nF
820nF
100n
120n
150n
180n
220n
270n
330n
390n
470n
560n
680n
820n
1µ0
-
number of advantages over aluminium electrolytics. These included
better shelf life, lower leakage,
wider operating temperature range
(up to 125°C instead of 85°C), lower
power factor and closer tolerance
on value. Now, with the general improvement of aluminium electrolytics, these improvements are
nowhere near as clear-cut.
Low leakage (LL) aluminium electrolytics are comparable with tantalums as far as leakage is concerned and their general stability is just
as good.
Aluminium electrolytics are now
also available (although not over
8
SILICON CHIP
1µ?.
1µ5
1µ8
2µ2
2µ7
3µ3
3µ9
4µ,7
5µ6
6µ8
8µ2
10µ
12µ
15µ
18µ
22µ
27µ
33µ
39µ
47µ
56µ
68µ
82µ
100µ
EIA code (108/e
tolerance)
104K
124K
154K
184K
224K
274K
334K
394K
474K
564K
684K
824K
105K
125K
155K
185K
225K
275K
335K
395K
475K
565K
685K
825K
106K
126K
156K
186K
226K
276K
336K
396K
476K
566K
686K
826K
107K
the counter at retailers) with
operating temperatures up to
125°C with voltage derating by a
third.
Substituting for
tantalum capacitors
In general, you can substitute
low leakage aluminium electros for
tantalums provided they are not being used in an oscillator or timing
circuit. In the latter case, the
designer has probably specified
tantalum not only for their low
leakage but for their closer
tolerance in capacitance value.
Identifying tantalum
capacitors
These days most tantalum
capacitors are labelled with their
capacitance value, voltage rating
and a + sign near the positive lead.
However, it is quite likely that,
unless you are really keen sighted,
you will need a magnifying glass to
read the labelling.
Some brands also use the EIA
code to indicate the capacitance
value. This is the same code as
shown in the previous episode on
film and ceramic capacitors. Table
1 shows the code for capacitors
ranging from O. lµF up to lOOµF.
Tantalums are normally made with
a tolerance of ± 10% as indicated
with a K following the 3-digit EIA
code, or ± 20% as indicated with
the letter M.
Some tantalums indicate the
polarity with a vertical line near
the positive lead, together with a
tiny + sign. This could be confusing
to the newcomer to electronics
since some older pigtail electrolytics indicated the negative end
of the can with a line around one
end.
You may also come across tantalum capacitors that are colour
coded and with the polarity shown
by a dot. You hold the capacitor
with leads hanging down and with
the dot facing towards you. The
positive lead is then the one on the
right. This is shown in the diagram
of Fig.3.
Be warned, the colour code for
tantalum capacitors does not bear
much similarity to that for resistors. So to identify tantalums,
you should examine Fig.3 closely.
Let's explain the tantalum colour
code in a little more detail. First,
hold the capacitor with the leads
hanging downwards. The first two
colours, reading from the top down,
give the first two significant figures
of the capacitance value (just as for
the resistor colour code). The colour of the spot then gives the
multiplier, so that the value is read
off in microfarads. For example, a
capacitor with the first two colours
yellow and violet, with spot colour
black, is 47µF.
The third colour on the body of
the capacitor gives the voltage
rating, as follows:
MULTIPLIER (µF)
WHITE x.01
GREY x0.1
BLACK xl
BROWN x10
RED x10D
VOLTAGE RATING
WHITE 3VW
. / YELLOW 6.3VW
BLACK 1DVW
GREEN 16VW
BLUE 20VW
GREY 25VW
PINK 35VW
PO~J~~E ORANGE 40VW
2ND SIGNIFICANT FIGURE -
\
BLACK 0
BROWN 1
RED 2
ORANGE 3
YELLOW 4
GREEN 5
BLUE 6
VIOLET 7
GREY 8
WHITE 9
CAPACITANCE
IN uF
\
POSITIV
LEAD
CAPACITANCE IN
EIA CODE
VOLTAGE
RATING
CAPACITANCE
IN uF
VOLTAGE RATING
POSITIVE
LEAD
\.
33~ -INDICATES
STRIPE
35V
POLARITY
POSITIVE
LEAD
Fig.3: tantalum capacitors are usually labelled with their value,
voltage rating and a plus (+)sign to indicate polarity. However,
some brands use the EIA code while others may be colour coded.
white ........... 3VW
yellow .......... 6.3VW
black ........... l0VW
green ........ ... 16VW
blue .. ........... 20VW
grey ............. 25VW
pink ............ . 35VW
orange ......... 40VW
As can be seen, the colour code
for tantalum capacitors bears little
resemblance to that for plastic film
capacitors, as listed last month. In
fact , the only similarity is in the
code for the first two significant
figures for the capacitance value.
It seems that colour coding of
capacitors has now fallen into
disuse, partly because the code is
so confusing and partly because of
the improved capability for printing
values on such difficult objects as
dipped tantalum capacitors.
Super capacitors
We mentioned these capacitors
earlier. These are also known as
"double layer" capacitors. They
are packaged to look like electrolytic capacitors but they do not
use the same dielectric principle as
electrolytics and they are not
used in FM tuners, TVs and VCR.s
for storing station settings and a
host of other electronic equipment
where data needs to be stored in
spite of the removal of mains
power.
The "double layer" capacitor is
the first really new capacitor to be
produced in the last 25 years or so
although the principle has been
known for over 100 years.
The main constituents are activated carbon and a sulphuric acid
solution as the electrolyte. The interface between the activated carbon and the sulphuric acid forms
the electric " double layer" .
A basic double-layer capacitor
consists of two half-cells, each consisting of an activated carbon electrode saturated with sulphuric acid
and separated by an ion permeable
membrane. The two half-cells make
up a non-polarised capacitor cell
which has a low voltage rating and
so between 8 and 15 of these cells
are connected in series to give a
practical capacitor with a voltage
rating of 5V or 10V.
Really, a double-layer capacitor
is more like a battery than a
capacitor and that is reflected in
typical applications such as low
power battery backup for microprocessors. They are not a substitute for conventional capacitors
because they cannot handle
substantial ripple current. Nor can
they deliver substantial DC current
because their internal resistance is
high.
As a substitute for batteries in
low backup power circuits though,
they are ideal.
Barrier layer capacitors
Tantalum capacitors are available in
values up to about 100~F and with
voltage ratings up to 50V. In most
cases, you can substitute low-leakage
electros for tantalums.
polarised. They can be connected
into circuit either way around.
Made by companies such as NEC,
they can have a capacitance up to 1
Farad and usually are available in
only one voltage rating: 5V.
At present, their main use is as a
replacement for lithium and mercury cells in microprocessor controlled appliances. Thus they are
Having described double layer
capacitors we should mention " barrier layer" and "boundary layer"
capacitors because they may be
thought to be similar. They are not.
In fact , barrier layer and boundary layer capacitors are a special
type of ceramic capacitor, based on
barium titanate. They have relatively high capacitance values for
a ceramic capacitor, up to 0.47,-iF,
but at low voltage ratings, typically
12V, 16V and 25V. These days they
are tending to be displaced by the
even smaller " multilayer "
ceramics.
~
MAY1989
9
|