Re: caps (was: 8250LP....)

From: silverdr_at_wfmh.org.pl
Date: Mon, 21 Aug 2017 20:55:20 +0200
Message-Id: <B6AD321D-2305-42CE-96D8-373E70E86D50@wfmh.org.pl>
> On 2017-08-21, at 17:06, Mike Stein <mhs.stein@gmail.com> wrote:
> 
>> Did they really use electrolytic caps for bypasses in the eighties?
> 
> Bypass capacitors are filters to remove any AC component from a DC signal, which has in fact always been the main application for electrolytics. 

As I mentioned before, my problem is that I got used to call "bypasses" only the caps used to provide a shortest, small inductance path to GND for power supply of components like ICs. I don't ever recall seeing electrolytics used for that particular purpose. They just don't "fit" there with their HF characteristics. Obviously when we talk about lower frequencies and filtering e. g. boards' power rails then they surely fit there with their high capacitance.

> Whether to use electrolytics (or tantalums, MLCCs etc.) or ceramics etc. depends on the frequency you want to bypass/filter; many times you will find an electrolytic bypass capacitor in parallel with ceramics, each one(s) filtering different frequencies.

True.

> Tantalums have their own issues with absorbing moisture and shorting out, often catastrophically, so in most cases (as Francesco suggests) MLCC caps are probably the best modern electrolytic replacement these days.

As already mentioned - unless one needs the higher capacitance, which is probably the primary reason for using electrolytics in the first place. Here we don't talk about designing our own circuits where we have potentially more freedom. We talk about replacing caps in some existing old circuits and I find it hard to imagine replacing a typical 2200 (or even 470) F electrolytic with a bag of parallel connected MLCCs ;-) where - BTW - even one costs more than an aluminium or tantalum of equivalent capacity. This might change in a few years but as of today we are not there yet..

-- 
SD! - http://e4aws.silverdr.com/


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