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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: heating a cap
Date: Fri, 04 Oct 2024 10:28:48 -0700
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On Fri, 04 Oct 2024 18:03:36 +0100, Cursitor Doom <cd@notformail.com>
wrote:

>On Fri, 04 Oct 2024 07:43:10 -0700, john larkin <JL@gct.com> wrote:
>
>>On Fri, 4 Oct 2024 07:09:26 -0700, John Robertson <jrr@flippers.com>
>>wrote:
>>
>>>On 2024-10-03 4:36 p.m., john larkin wrote:
>>>> I got a small (under 1" long) aluminum electro cap, 220 uF 63v, and
>>>> cranked up the voltage. It started drawing a bunch of current at 105
>>>> volts, got hot, and folded back to 80 mA at 87v.
>>>> 
>>>> It got too hot to touch in a couple of minutes, after roughly 500
>>>> joules. Freeze spray let it go back up to 100 volts or so.
>>>> 
>>>> None of that seemed to damage it, so an electrolytic cap sort of has a
>>>> built-in MOV.
>>>> 
>>>
>>>Caps have vents...eventually the electrolyte with evaporate and outgas 
>>>and you are left with a slug of aluminum foil.
>>>
>>>I've seen many thousands of caps fail over the decades, you don't want 
>>>to push them above 85c (even if rated at 105c) unless you like short 
>>>lifetimes. Heck even caps that are never over 50c will dry out 
>>>eventually - 20 to 30 years in many cases. Seals aren't perfect.
>>>
>>>I assume SMD electrolytics are the same.
>>>
>>>Let's not talk about stress testing tantalum caps - "Bang!".
>>>
>>>John ;-#)#
>>
>>The usual MnO2 tantalums actually detonate, and a bit of peak current
>>will set them off.  The polymer tantalums don't detonate.
>
>"Actually detonate" - this is really just a steam explosion we're
>talking about here, though, John? It doesn't involve the breaking
>apart and re-combination of molecular bonds.

As I understand it, a current spike can heat up a tiny bit, and then
an exothermic chemical reaction takes over. MnO2 is the oxidizer and
tantalum is the fuel.

For reliable operation, make sure the available current is limited,
namely don't use tants on power busses.

LM317s and output tants seem to work well.