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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