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Path: utzoo!linus!philabs!cmcl2!floyd!harpo!eagle!allegra!alice!rabbit!jj
From: jj@rabbit.UUCP
Newsgroups: net.audio
Subject: Re: Re: Warmup Period-Filter Cap's
Message-ID: <2090@rabbit.UUCP>
Date: Wed, 19-Oct-83 14:43:16 EDT
Article-I.D.: rabbit.2090
Posted: Wed Oct 19 14:43:16 1983
Date-Received: Thu, 20-Oct-83 09:13:46 EDT
References: <294@whuxk.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 45


Electrolytic capacitors do indeed require a polarization voltage when
they are first used, however, most cap's are "formed" at the factory,
and will not have outrageous leakage when you turn them on,
unless they've been out of use for a long period (a typical half
life for an electrolytic coating is 6 months.).  

If the capacitors do need to be formed, the "right" way is to limit
the leakage current, and let them form slowly, which is not
what happens when you turn on your amp. 


As an aside, whenever you turn on your amp (or anything with
semiconductor rectifiers in it) you charge the filter capacitors
VERY QUICKLY, typically a normal 35 volt supply's inrush current
will be on the order of 100 amps for one half cycle of the
AC line, and 25 amps for the next half cycle.  This surge is what
makes your lights dim, and your TV shrink when you turn on your
amplifier.   Some amplifiers do have surge limiting built in,
especially higher powered amps (they have a disproportionally 
large inrush current for reasons that I will momentarily skip.), but
most amps just use a large rectifier <they're dirt cheap> and let the
surge happen.  This surge is indeed hard on both rectifiers and
capacitors, which is a good reason not to turn you amp on and
off a lot.  (Independant of other considerations.)  


If your amplifier has been off for quite a while, I would suggest
putting a commercial (UL approved, etc) surge limiter in series, and
starting it up with the surge limiter in circuit.  That way, if the
electrolytic cap's have depolarized, you will limit the current through
the breakdown points.  <When a capacitor depolarizes (as opposed to
drys out, another, much slower mechanism) the capacitance goes 
UP and the breakdown voltage goes down.  Problems usually arise only
if the supply voltage is lower than the breakdown voltage of the
deformed cap, which is an unusual occurence.)  Capacitors usually
have a safety factor of 2-3 built in, with additional depolarization
protection in the form of a permanently anodized layer on each
foil.  Anodizing is the same process as polarization, but the
rate of loss varies with the material.
-- 
 O   o   From the pyrolagnic keyboard of
   ~              rabbit!jj
 -v-v-
 \^_^/   (pyrolagnic- from pyro<=>fire and lagnic<=>eating.)