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

From: Mike Stein <mhs.stein_at_gmail.com>
Date: Mon, 21 Aug 2017 17:07:26 -0400
Message-ID: <A98DCCF5238D465DA668F43F2499D7A9@310e2>
----- Original Message ----- 
From: <silverdr@wfmh.org.pl>
To: <cbm-hackers@musoftware.de>
Sent: Monday, August 21, 2017 2:55 PM
Subject: Re: caps (was: 8250LP....)



> 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.
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How is any cap across the IC's power pins not "filtering the power rail(s)"? If you look along the IC's supply rail you will probably also come across some 10 - 100uf caps across the power rails, maybe even right beside a ceramic; are they filters or bypasses? As I said, the only difference is the frequency they're intended to 'bypass' which will determine the apppropriate type and location.

But if you want to call electrolytics and tantalums "filters" and ceramics "bypasses" that's OK; we'll know what you mean... ;-)
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>> 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..
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We did say "in most cases," specifically including the original topic under discussion; I doubt you'll find any 2200uf caps in a JU570.

The main point was that when replacing an electrolytic if it's a small enough value that a tantalum cap would replace it you're probably wiser to use an MLCC instead these days.

Large electrolytics do indeed sometimes burst, but tantalums (and the Rifa X and Y line filter types) are much more notorious for shorting and exploding in 'normal' service (proper polarity and within specs).

m

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Received on 2017-08-21 22:02:59

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