hi this blog is going to be all about
capacitors the what why how and where of
using capacitors I've got ten minutes
let's start now okay the first type of
capacitor is the electrolytic capacitor
you've seen them before these things in
the metal cans they've basically got an
aluminium foil in them filled with a
liquid electrolyte and these things are
absolutely horrible they are all full
capacities in in almost every respect
try and avoid using them if you can but
the problem is the advantage of them is
that they come in massive capacitance
values for their size so really yeah you
really can't avoid using them for things
like power supply filtering and large
bypassing and things like that and in
power storage and some AC coupling
applications for very low frequencies
it's hard to avoid these things so
unfortunately there are necessary evil
you've almost certainly noticed this
little vent mark on the top this is
actually a pressure vent that in the
case of these things overheating and
overloading
the vent is supposed to break and
release the energy so that they don't
explode and our foil goes everywhere
well they don't always work these
suckers can explode they're dangerous be
careful one of the hugest problems with
these things is that they dry out over
time heat can kill these things the
liquid in there dries out and this
increases the equivalent series
resistance the ESR and really to measure
this you've got to have an ESR meter not
a capacitance meter an ESR meter I use
this Bob Parker one thumbs up you've got
to have something like this if you want
to test electrolytic because the life of
these things is inversely proportional
to the temperature they use that so the
higher the temperature the shorter the
life now these will have a rated life on
the datasheet at a certain temperature
but you can easily shorten that life if
you mount this thing near a heat sink or
something like that inside your product
these things need to be kept
cool but don't make them too cool
because then their their ESR just shoots
through the roof of cold temperatures
they're horrible high ESR on these
things is probably the number one
failure mode in consumer products if
you're plasma TV fails or your digital
set-top box or DVD player fails it's
probably because one of these has dried
out in the power supply odds-on now if
you want the best reliability for
electrolytic capacitors you need to
stick to the name-brand ones and check
the data sheets and use the high quality
ones
there's tons of fake ones and dodgy ones
on the market and you can get duped into
it another major problem with these
things is that they have a maximum ah
charge and discharge or ripple current
they can handle so make sure you read
the data sheets and you don't exceed
that ripple current because if you do
they can heat up internally and like I
said shorten the life if you want really
reliable designs using these things you
put two or more in parallel that way
they share the ripple current they share
the heat and they lower the ESR and it
increases your liability and system so
parallel is a really good way to go and
you can also get low ESR versions of
these things and they're good for use in
switch mode power suppliers and low
dropout voltage regulators and things
like that now the temperature variation
of electrodes is pretty horrible can be
anywhere from plus 5 to minus 40% minus
50% so just watch out for it and of
course they're polarized you have to put
them in your circuit the right way
that's a major disadvantage now they
have a maximum reverse voltage of about
1.5 volts anything over that continuous
and you're going to damage these things
permanently sometimes you're going to
need a non polarized electrolytic
capacitor and you can't actually buy
them but if you haven't got one you can
do it by putting two in series
back-to-back like this with the two
negatives together or the two positives
together it doesn't matter which way
around sometimes in high voltage
applications you need to put two
electrolytic capacitors in
Cirie's it's not recommended but if you
have to you can do it got to capacitors
like this one of them could leak very
badly and that will change the voltage
division ratio and you'll get excess
voltage across one of the capacitors and
that can quickly destroy it so what you
do is you put ballast resistors
across the two caps and you calculate
these values based on the worst-case
leakage current of one of the capacitors
and the maximum voltage that that
capacitor can handle the other thing
about electrolytic capacitors is a
phenomenon called dielectric absorption
and this means that the capacitor can
actually rebuild its charge after you've
shorted it you know 1020 percent
recovery something like that so in high
voltage applications that can actually
be dangerous so be wary of it next up we
have tantalum capacitors these things
these little resin dipped ones or the
surface mount tantalum caps and these
are another necessary evil in a lot of
cases you avoid using them if you can
the first problem with tandems is that
the tantalum material itself is actually
a fairly rare metal it requires lots of
mining and this can lead to very high
prices and shortage of tantalum ah if
you use quite a few years back there's a
huge tantalum shortage in the market
simply marketing to complete and utter
chaos and they're talking about it
happening again soon the other major
problem with tandems is that they're
famous for blowing up and bursting into
flames the damn things are flammable and
this is generally considered a bad thing
tantalum is also just like Electro's
they hate high pulse and ripple currents
so watch the datasheet and don't go
anywhere near those limits but it's not
all doom and gloom with Tanna lumps they
do have some good aspects if you use
them with inside their specs they're
actually very reliable much more
reliable than Electro's in some cases
but just like Electro's they're they're
pretty horrible caps generally all right
they're they're very low leakage though
that's another good thing about them but
you'll generally find them in just
pretty much power supply decoupling and
large value uh energy reserves things
like that
you're very common for FPGA decoupling
these days that now there are
different types of tan limbs these days
as the technology gets better organic
and polymer tatum's and look at using
those as substitutes but they're rarer
and more expensive now thankfully the
ceramic technology these days is eating
away the need to have tantalum
capacitors now another type of main
capacitor are these film capacitors
plastic film capacitors and they come in
two major types one of the main types of
plastic film capacitors are the
polyester type there are general-purpose
ones you use for bypassing AC coupling
filtering applications things like that
you can get quite stable ones their main
advantages that they have a high
capacitance per unit volume due to their
high dissipation factor you wouldn't use
polyester in high poles high current or
high frequency applications at the other
major type of cap is polypropylene now
these have much higher performance due
to their lower dissipation factor and
their higher dielectric strength you use
them for high voltage high frequency
high current pulse applications and
things like that so really grunty
capacitors but their major disadvantages
you can only get them in small values
here's a good tip polypropylene and
polyester capacitors have roughly
opposite temperature characteristics so
if you actually put the two in two types
in parallel you can get a reasonably
stable or a much more stable temperature
characteristic the other major type of
film capacitor is the polyethylene
sulfide and these are mainly used in SMD
caps because the other ones are mainly
through-hole and these are very stable
and close tolerance caps now the other
type of capacitor out there is a mica
and these are very exotic kind of things
that are used really for only ultra high
stability ultra close tolerance
applications so if you need a mica
you're going to know it I can't do this
blog without mentioning mains rated
capacitors now you can't just use any
old capacitor when you're hooking these
things directly on the mains for like
suppression these special mains Raider
ones come in two different types two
different classes Class X and class Y
now Class X
comes in two types x1 and x2 x1 are
rated for higher voltage higher pulse
applications but either can be used
generally and they are designed x-class
are designed to go directly across the
mains they're not designed for anywhere
that can be accessed by people something
for safety and things like that
that's where class y comes in class Y
capacitors are used between either of
the mains lines and protective earth and
you've got to make sure you use the
right type and these the advantage of
these capacitors is that they're
self-healing what it means is that if
you get a tiny little overload and spark
inside the capacitor basically what it
does is itself extinguishes itself and
just breaks it and you lose a tiny
amount of capacitance miniscule but it
means the thing doesn't overload or
short or explode or something like that
and you've got to make sure you use
these right type of capacitors if you
don't your products won't get approved
and they can kill someone