----- Original Message ----- 
From: "Russell McMahon" <apptech@paradise.net.nz>
Subject: Re: [EE] Why specify one kind of capacitor over another?


> Beginners - Save this as a tutorial AFTER other guys have pointed out
> shortcomings, added to it and generally hacked it about.

So I won't do huge clips as I would typically do in the interest of keeping
it together.

>
> Different dielectric materials have different characteristics.
> Some of the key parameters that can be affected are
>
> - Size
> - Cost
> - Stability with temperature or voltage or age
> - Voltage breakdown per size.
> - Repeatability of capacitance between units
> - Susceptibility to mechanical affects
> - Generation of voltages under mechanical affects
> - Polarity sensitivity (or not)
> - Electrical leakage.
> - Loss
> - ESR
> - AC impedance with frequency
> - more
>
> Many of these are inter-related, and to variable extents with different
> materials.
>
>
> Electrolytics (there are a number of types but this usually means
Aluminium
> wet electrolytic) are relatively low cost per capacitance and low size per
> capacitance. They  have the disadvantages of polarity sensitivity, limited
> lifetimes, life time decreases with increasing temperature and more. They
> are very commonly used when large capacitance is needed and low price is
> important.
>
> There is almost never a good reason to specify a 0.1 uF electrolytic. This
> capacity achieved with a cheap, small non polar capacitor of better
> electrical characteristics. if the data sheet says 0.1 uF electrolytic it
is
> probably  a designer being lazy. SOMETIMES a regulator's stability  may
> benefit from an electrolytics inferior ESR but this is unusual. usually
> using a 0.1 uF ceramic or mylar would work well.
>
> There are much better descriptions on web but here are a few comments:
> All non-polar except electrolytics.
>
> Polystyrene - Bulky. dearer. excellent accuracy repeatability. Less
common.

Polystyrene caps have an opposite temperature characteristic from most caps.
As a result, they are frequently used in frequency-determining circuits to
temperature stabilize the circuit.  When you see a polystyrene in parallel
with one or two other types of capacitors, that is what is probably going
on.  The combination is supposed to provide the opposite drift from the
inductor.

>
> Mylar (Polycarbonate) - moderate size, good stability and lifetime,
moderate
> cost.

You tend to see mylars in audio circuits.  High inductance maybe?

Paul Harden, in his excellent chapter on caps in his Data Book (as far as I
can tell, no longer available) says polyester (mylar) is pretty much the
standard for DC.  Polypropylene film good for high frequencies but with a
cost in size, and polycarbonate film almost ideal, except too expensive for
non-military applications.

>
> Tantalum electrolytic - high capacitance per size, excellent low
impedance,
> dangerously prone to very hard short circuit due to overvoltage. Risky in
> low impedance high energy circuits.

Probably should re-emphasize that overvoltage failures tend to be quite
exciting!  On the other hand, really a lot of capacitance in a tiny space.

>
>  Ceramic - CHEAP, excellent capacitance per size, bad temperature
> coefficient (varies with type),  can have dangerous voltage spike
generation
> when subject to step voltages,. Much used for decoupling. Cheap and
> cheerful.

Ceramics are available in a number of temperature characteristic curves,
although most are pretty odd shaped curves.  Low inductance is a good
feature here, too.  Downside is that they are prone to microphonics in the
disk format.

>
> Aluminium electrolytic. As above: relatively low cost per capacitance and
> low size per capacitance. Polarity sensitivity, limited lifetimes, life
time
> decreases with increasing temperature. Deteriorate even more rapidly with
> increasing temperature when unpowered. Poorish ESR decreases with
lifetime.
> Caustic innards.

These also have a fairly high inductance.  On power rails, you will often
see an electrolytic in parallel with a ceramic ... the large capacitance of
the electrolytic deals with low frequency transients, and the low inductance
of the ceramic takes care of the higher frequencies.

>
> Aluminium solid electrolytic. All the advantages of tantalum electrolytics
> and none of the disadvantages. Dearer than wet electrolytic. High
> capacitance per size. Medium price. Compact for voltage. Excellent for
> decoupling and power supplies generally.
>

Silver mica - excellent high frequency characteristics, fairly high voltage
ratings in a relatively small space.  Very long life.  Very expensive.
Generally only used in transmitter output networks where high voltage and
low loss are worth the price.

> Many more ....
>
>
>
> Someone else add to this ...
>

--McD


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