AW: Paul' capacitor FAQ

[Haible Juergen]

Wow - lot of capacitor knowledge on the list.
I have nothing to add but questions:

(1) Tantal caps.
      In older circuits they often fail and cause a short between the
supply
      rails. I have always avoided them. But I am not sure if this is
still true for
      modern Tantal caps. I doubt it, because everybody uses them. So,
      can anybody tell me for sure ?

(2) Polystyrilene
      I used to think they should be marked "KS". But I recently bought
a large
      package of mixed caps that were sold as "Polystyrilene", but were
      maked differently (I think it was "KT" or something). Otherwise,
they
      looked identical. I hesitated to use them in VCOs and VCFs so far.
      What do you think?

(3) Elkos
      I never understood the real limits of polarisation completely.
      I always used unpolarized elkos (or two back to back in series)
      when I had to AC-connect two points with unknown DC offset
      in the millivolt range. Sure, that's the best method, but also
very
      expensive and needs a lot of space on the PCB.

	Now, with all the myths around, what is the truth about
polarized
	elkos to couple AC without a DC voltage, or with a tiny (10mV)
	reverse DC voltage?  *Everyone* seems to do this, and while
	I am sure that it has *some* side effects, I am not sure if
these
	side effects would bother me.

	Typical Example:
	Opamp stage with +/-10mV offset on its output -> 10uF polarized
elko ->
	10kOhm potentiometer to GND -> high impedance Opamp input.

	Questions for this Example:
	a) Will the 10mV, that are possibly in reverse, cause damge to
the elko
	     over the years ? (I could not live with that.)
	b) Will the different impedance for the positive and negative
half wave
	     of my AC signal cause significant distortion on a _10kOhm_
load ?
	     (this may be important for some, but not all applications.)
	c) Will the lack of a proper DC bias cause the cap to change its
	     capacitance, and how much ? (I can live with 50% change,
because
	     it's easy to oversize the elko by a factor of 10)
	d) Are there any other negative effects I did not mention so far
?

Lots of questions - but I know someone of the list has good answers.
(No guesses please, only facts.)

JH.