I have sometimes wondered about the use of polarized capacitors in places
where they are just used to couple an AC signal to another circuit. Since
the signal being coupled is AC, there really wouldn't be a whole lot of DC
potential across it for any length of time.

In this case, would it really matter what direction the capacitor's
polarity was facing?



On Wed, 24 Nov 2004, Mcgee, Mark wrote:

> Thanks very much for the detailed descriptions Russell.
>
> Cheers,
> Mark
>
> > -----Original Message-----
> > From: piclist-bounces@mit.edu
> > [mailto:piclist-bounces@mit.edu]On Behalf
> > Of Russell McMahon
> > Sent: 24 November 2004 12:06
> > To: Microcontroller discussion list - Public.
> > Subject: Re: [EE] Why specify one kind of capacitor over another?
> >
> >
> > > ..I was wondering why you would choose one type of
> > capacitor over another
> > > in
> > > given applications.
> > >
> > > I notice that the example cct on a datasheet for the LM317LZ vreg
> > > specifies
> > > 0.1uF electrolytic, but has onother in a different part of
> > the circuit
> > > that
> > > isn't electrolytic.  Why might that be?
> >
> > Beginners - Save this as a tutorial AFTER other guys have pointed out
> > shortcomings, added to it and generally hacked it about.
> >
> > 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.
> >
> > Mylar (Polycarbonate) - moderate size, good stability and
> > lifetime, moderate
> > cost.
> >
> > 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.
> >
> >  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.
> >
> > 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.
> >
> > 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.
> >
> > Many more ....
> >
> >
> >
> > Someone else add to this ...
> >
> >
> >     RM
> >
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
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> >
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