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

Everyone's experience is different, but if you read any repair discussion you'll probably find that the first thing to look for when there is a power supply issue are any shorted tantalum caps or, if there was a loud bang and a bad smell, exploded tantalums or line filters; they're usually either good or dead. 

Aluminum electrolytics usually just swell, leak and make a mess, and often change in value and ESR without failing completely.

Once again, I think we were talking about using MLCCs instead of tantalums as a replacement for an electrolytic; if the cap to be replaced is too big for an MLCC it's probably too big for a tantalum as well.

Obviously electrolytics still have their place, especially in vintage equipment.

m

----- Original Message ----- 
From: <silverdr@wfmh.org.pl>
To: <cbm-hackers@musoftware.de>
Sent: Monday, August 21, 2017 3:26 PM
Subject: Re: caps (was: 8250LP....)



> On 2017-08-21, at 20:46, Francesco Messineo <francesco.messineo@gmail.com> wrote:
> 
>>> Tantalum don't leak, but have some other problems (sometimes they
>>> explode or burn for example,
>> 
>> This might happen quickly when you "overdrive" them or put them the wrong way but exploding (to a degree) and burning is certainly a domain of leaking aluminium ones, even if used correctly :-)
> 
> hell, no. I've seen far too many tantalum capacitor start burning or
> just go "BANG" after decades
> of use, and yes, they were all installed correctly.

Means I must have been lucky. I basically had only the aluminium ones exploding on me. Maybe yours were exposed to moisture as Mike mentioned?

> [...] I'm happy that Tektronix knew how to make a proper power supply
> current limit, if not, that capacitor would be scattered everywhere
> inside the scope and probably some parts of the power supply
> regulation circuit would be dead too (it happened on
> less-carefully-designed power supplies).

Which, unfortunately are probably more common these days.

>> Unless you need the capacitances to match the bigger electrolytic ones. Especially at higher voltages. For (relatively small) bypasses they're obviously the best choice but I wouldn't be so sure to recommend ceramics as a general replacement for electrolytic caps. For (your) example if we talk about the bigger ones like those used as board power rails filters then you can quickly need to get into mF range. I just checked with a big supplier here and the biggest MLCC I can get is 22µF and is almost twice the price of a 1000µF tantalum and only about 30% cheaper than the 470µF tantalum-polymer one.
> 
> I routinely use MLCC up to 100 uF (they're on digikey.com for example)
> and you can find them up to 680uF (last time I've checked).

I never used them (digikey) but true, they seem to have higher capacitances in their offer.

> But I agree, I would never replace the big electrolytics with them. We
> started this thread talking about small leaking capacitor with a small
> footprint.
> Why one would drift to 1000 uF? They aren't small in any sense.

I read your post as if you were recommending MLCCs as a *general* replacement for "supply bypass or DC block" and felt like disagreeing with that. Especially that (my naming habit) I meant not exactly the same when reading "supply bypass". But even with that out of the way I mostly recall seeing small ceramics close to components and big (470, 1000, 2200, 4700µF) aluminium ones filtering power rails of the whole board and I couldn't imagine those being replaced with MLCCs.

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


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