> ..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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