This morning I used an old 16V electrolytic as a pre-regulator filter for a 12V rectified AC supply. I didn't have a higher voltage rating in the size I wanted and knew it was pushing the rating.

I got the electronic answer I wanted, then the steam came out in a nicely controlled fashion.

Does anyone know the chemical composition of the "steam?"

John

 

From where the steam came out? Steam is just vapourized water but with high pressure.

 

The "steam" came out the relief scribes on the top. I used steam in quotes to signify that it was not just water steam, but rather looked like steam. I assume water is part of it, but clearly the smell and way in which it dissipated indicated other components.

I was thinking of something along the order of aluminum oxides/hydroxides plus something a bit more acrid like aluminum chloride, an oxide of sulfur, etc. I am not concerned so much about toxicity as about clean up of possible acidic residues.

John

 

As I understood the capacitor was destroyed. This is because the peak value of the voltage of the transformer's secondary is not 12V but about 17V.

 

Here's a quote from Wikipedia:

The electrolyte is usually boric acid or sodium borate in aqueous solution together with various sugars or ethylene glycol which are added to retard evaporation. Getting a suitable balance between chemical stability and low internal electrical resistance is very tricky and in fact, the exact composition of high-performance electrolyte is a closely guarded trade secret. It took many years of painstaking research before reliable devices were developed.

So, I guess, the answer to my question will not be had.

I am not too concerned about sugars, ethylene glycol, or boric acid. That might actually make a nice soldering flux.

John

 

It's nice that electrolytics have the top scored. Before that got started, it was common for the case to blow off with a big pop, sometimes with the foil unwinding like a bitty party favor.

 

There were some Asian cap manufacturers who were using overly-concentrated electrolyte solutions in their caps to boost the capacitance rating; however this led to lots of early failures.

Before using electrolytics that have been sitting around for awhile, it's a good idea to re-form them. Charge them up to their rated voltage using very low current (a few mA) over several hours.
The general formula for maximum permissible leakage current is:
Capacitance in uF * Working Voltage * 1.5 / 10,000
So, for a 1000uF cap, 16wv, max permissable leakage would be 2.4mA.

 

That is a good reminder. I learned about capacitor spoilage from my usual supplier who occasionally runs short of Panasonic caps. Apparently, it uses something like 6 months for shelf life to ensure customers have a year or two. On researching it a bit more, I found a masters thesis (2002) on the Internet that established/reconfirmed the need to reform unused electrolytics after 5 years. I wonder if that only applies to "wet" electrolytics or to tantalums too?

As for what led to this thread, I was chasing some transient noise, was in a hurry, and didn't have the right voltage cap available. I found the source of the noise -- a dumb error.

John

 

Tantalums definitely need re-forming as well. Better to do it ahead of time, than to have to replace it again after it blows.

I got "bit" by this just a couple of months ago. At least the Tant cap that went bad failed quietly. Sometimes they go out rather spectacularly.

It doesn't take much equipment to re-form them - just a variable voltage power supply, a 10k current limiting resistor, and a voltmeter to measure the drop across the resistor (thus leakage current) vs the cap voltage.

Much of the time you can re-form even really old electrolytics, as long as they haven't dried out. Just as long as you can get the leakage rate down below the max, it's OK. It might take much longer to do it that way, but it can be worth it to those who are trying to preserve antiques.

 

Tantalums definitely need re-forming as well. Better to do it ahead of time, than to have to replace it again after it blows.

That explains another thing that happened. I got a reel of AVX TAJD 22uF/25V tantalums on ebay cheap. About 4 or 5 worked fine. The next one popped as soon as voltage was applied. It was parallel to an identical one. Lot code is L221H, but I can't find that on the AVX site. If it is like some other manufacturers' codes, it might be something like the 21st week of 2002 (or older).

So now, it is time I start reconditioning them before use.

Thanks for the heads up. I didn't know the outdating applied to solid electrolytes too.

John

 

That is good to know - I have all kinds of old caps around & don't need them popping. Thought that only applied to electrolytics.